Writing Arden Syntax medical logic modules

Com/~ur.

Biol.

Med.

Vol. 24, No. 5. pp. 331-363, Iv94 Copyright @ 1994ElsevierScienceLtd Printedin GreatBritain.All rightsreserved OOIO-4825/94 $7.00+0.00

OOlO-4825(94)00022-O

WRITING ARDEN SYNTAX MEDICAL LOGIC MODULES GEORGE HRIPCSAK Center for Medical Informatics, Columbia-PresbyterianMedical Center, 161 Fort Washington Avenue, AP-1310, New York, NY 10032, U.S.A. (Received 3 June 1994; received for publication 21 July 1994)

Abstract-The Arden Syntax for Medical Logic Modules is a language for encoding medical knowledge bases that consist of independent modules. The Arden Syntax has been used to generate clinical alerts, diagnostic interpretations, management messages, and screening for research studies and quality assurance. An Arden Syntax knowledge base consists of rules called Medical Logic Modules (MLMs), which are stored as simple ASCII files that can be written on any text editor. An MLM is made of slots grouped into three categories: maintenance information, library information, and the actual medical knowledge. Most MLMs are triggered by clinical events, evaluate medical criteria, and, if appropriate, perform an action such as sending a message to a health care provider. This paper provides a detailed tutorial on how to write MLMs. Arden Syntax Standards

Medical logic modules Automated decision-support

MLM Knowledge representation Expert systems

1. INTRODUCTION The Arden

Syntax for medical logic modules (MLMs) is a language for representing medical knowledge. It was born out of a desire to share knowledge across institutions (medical centers, hospitals, office practices, health departments, etc.). The original draft standard was prepared in 1989 for a retreat held at the Arden Homestead in Harriman, New York [l]; the goal of the retreat was to investigate and encourage knowledge base sharing. The Arden Syntax was later adopted by the ASTM, a consensus standards organization, and the official standard was published in 1992 [2]. While the official standard has been available for some time, it is a technical specification document that does not serve novice users well. This paper is a tutorial for the novice user. It is organized to bring the user through the syntax in a logical manner with many examples. This tutorial does not replace the standard, but should be used in conjunction with it. Details that have been left out of this tutorial due to space are contained in the standard. A third document, which describes the rationale behind the Arden Syntax [3], will be helpful to Arden Syntax users. It explains in detail why the standard was created and how it got to look as it does today. It specifically addresses the issues of sharing MLMs, and it covers the advantages and disadvantages of using the Arden Syntax. Knowledge of a computer programming language like Pascal will also be helpful. If the reader has little experience with computers, then an introductory text on Pascal or BASIC is recommended. 2. OVERALL

APPROACH

An Arden Syntax knowledge base consists of a set of rules called Medial Logic Modules (MLMs), each of which contains sufficient logic for a single medical decision. Most MLMs produce narrative or coded messages that are relayed to the appropriate health-care provider. MLMs have been used for many purposes. An alert is a message 331

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that warns a provider of some clinically important situation, such as a medication interaction or a dangerous laboratory result. An interpretation is a non-emergent message intended to supply the provider with supportive information, such as an interpretation of liver function tests. It includes diagnosis support. A screen is a message sent to a clinical researcher or a quality assurance officer that indicates that a patient fulfills a set of criteria, either for a clinical trial, or for some quality assurance concern. Management messages are used for institutional administration, such as managing bed assignments, carrying out regulations, and automating ancillary department procedures. Sets of MLMs have been used to implement practice guidelines and care plans. Functionally, an MLM consists of four major components: evoking event, logic, action, and data mapping. Most MLMs are evoked (triggered) by some clinically relevant event, such as the admission of a patient or the storage of medical data. Thus, the evoking event sets the context in which the MLM is pertinent. When an MLM is evoked, the logic portion is evaluated. It generally consists of medical criteria and algorithms. The result of the evaluation is to conclude true or false. If true, the action is executed. Storing a message in the patient record and sending electronic mail to the health-care provider are typical actions. To define the evoking event, logic, and action, an MLM must use data mappings to convert from institution-specific entities (database entries, electronic mail addresses, etc.) to terms actually used in the MLM. The four functional components are defined in the MLM, along with many others related to maintenance and outside knowledge sources. 3.

A SIMPLE

MLM

EXAMPLE

Before embarking upon the details of the Arden Syntax, it is illustrative to look at a simple MLM that includes only the common, required elements. The MLM in Fig. 1 is typical. Its goal is to alert health-care providers of new or worsening anemia, as indicated by a blood test called the hematocrit. Whenever a hematocrit is recorded, it is compared to a threshold and to a previous value. If the hematocrit fulfils certain criteria indicative of anemia, a message is sent to the provider. As seen in Fig. 1 the MLM consists of a set of slots organized into three categories: maintenance, library, and knowledge. The first slot of the maintenance category is the title, which is a brief description of what the MLM does. The filename serves as a unique identifier that one can use to specify the MLM. The version keeps track of updates to MLMs. The institution specifies where an MLM was written, and the author is the person(s) who wrote it. The specialist slot (empty here) is reserved for future approvals of this MLM for clinical use. The date the MLM was last updated is shown. The validation level indicates that this MLM is only used for testing at present. The library category follows. The purpose slot describes what this MLM is used for. The explanation slot describes how the MLM works. The keywords slot supplies terms that can be used to search through a knowledge base of MLMs. The knowledge category contains the functional components of the MLM. The Arden Syntax type slot always says data-driven at present. The data slot maps terms used locally in the MLM to entities in the patient database. In this example, blood_count storage* is defined as an event in which a complete blood count test is stored. Hematocrz is defined as the last blood hematocrit result in the patient database, and previous hct is defined as the one before it. The evoke slot specifies that this MLM is triggered when a blood_count_storage event occurs. The logic slot contains a set of criteria. This MLM first checks to see whether the hematocrit result is a number. If it is not a number (e.g. “clotted”), then an error occurred when the blood test was performed, and the MLM terminates without sending a message (conclude false). If it is a number, then the MLM checks whether the current hematocrit is 5 units less than the previous one, or less than 30 units in total. If so, the MLM concludes true, which means that the action *The

monospaced

bold font of blood_count_storage

actually occurs within an MLM.

is used throughout

this paper

to indicate

text that

Writing Arden Syntax MLMs

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maintenance: title: Alert on low hematocrit;; filename: low_heamtocrit;; veraion: 1.00;; institution: CPMC;; author: George Hriposak, M.D. (hripcsalcucis.columbia.edu):: specialist: :; date: 1993-10-31;; validation: testing:: library: purpose: Warn provider of new or worsenmg anema.:; explanation: Whenever a . blood count result is obtained, the . . _ . nematocr;t zs cnecxea to see whether it is below 30 or at least 5 points below the previous value.;: keywords: anemia; hematocrit:; knowledge:

type: data-driven;: data: blood count storage := event T'comaprete blood count'); hematocrit := read last ('hematocrit'j: previous_hct := read last ( ('hematocrit') where it occurred before the time of hematocrft);; evoke: blood_count_storage;; logic: if hematocrit is not n&et then coxlude false: elsaif hesmtoerit <= previous_hct - 5 or hematocrit < 30 then conclude true: endif:: action: write "The patient's hematocrit ("I I hematocrit low or falling rapidly.";:

I I”) is

end: Fig. 1. A simple MLM.

slot is evaluated. In this MLM the write phrase sends a message to the health-care provider, indicating that there is new or worsening anemia. This MLM is simpler than a real MLM would be. For example, one might not want to send an “anemia” alert for a hematocrit that dropped from 52 (above normal) to 47 (normal). Many contingencies must be covered in a real MLM. 4. CATEGORIES

AND

SLOTS

4.1, General rules An MLM is an ASCII file that can be created by using any word processor. comprised of slots grouped into categories as follows:

It is


name > : name > : < slot body > ;; : < slot body > ;; 1..
A sample MLM that has all possible Arden Syntax slots is shown in Fig. 2. It begins with the word maintenance followed immediately by a colon (:). This marks the beginning of the maintenance category, and it is followed by a series of slots. Each slot has a name with a colon (title:), followed by the slot’s value (Screen for hypokalemia with digoxin therapy), and terminated with a double semicolon (;;). MLMs follow several genera1 rules. The allowable characters are printable ASCII (33 to 126), space (32), carriage return (13), linefeed (lo), and horizontal tab (9). The Arden Syntax is not case sensitive (except when quoted within character strings). The double

334

G. HRIPCSAK maintenance: title: Screen for hypokalemia with digoxin therapy:: filename: hypokalemia-and_digorin;; version: 1.06;; institution: Columbia-Presbyterian Medical Center;; author: George Hripcsak, M.D. ([email protected]);; specialist: George Eripcsak, M.D.;; data: 1993-09-17;; validation: production;; library: purpoas: Warn the health oare provider of hypokalemia in the setting of digoxin therapy.;; explanation: Whenever a serum or whole blood potassium value ia stored, it is checked for hypokalemia (less than 3.3). If hypokalemia ia present, then if tha patient has a non-zero aerum digoxin level within the ... ;: keywords: hypokalemia; digoxin; arrhythmia;; citations: 1. International Committee of Medical Journal Editors. Uniform Requiraments for Manuscripts Submitted to Biomedical Journals. NSJM 1991:324:424-S.;; links: CTIM-1.14.5;; knowledge

typm:

:

data-driven;;

d&a: K storage := avent (result, intravaxular potassium); It-:= read last (intravaaoular potaesium) where it occurred within the past 3 days: diaoxin :- read last (intravascular diaoxin level) where it occurrd within 1 week bhore tiniif K;; priority: 50;; evoke : X-storage;; logic: if K < 3.3 and digoxin > 0 then /* send an alert l/ conclude true; endif: action: write "This patient has hypokalemia (" II X 1) "1 in tha settina of diaoxin theraw. This can pbtentiate card& arrhithmias." ::' urgency: 50;; end: Fig.

2. An MLM with all optional

slots.

semicolon (;;) is special because it marks the end of a slot; it cannot appear in the body of a slot. Categories and slots must appear in the order that they are listed here. There are three categories. The maintenance category contains slots for knowledge base maintenance and change control. The library category explanatory information, keywords, and links to other knowledge sources. The knowledge category contains the actual medical logic encoded in the MLM. The phrase end: marks the end of the MLM; syntactically it appears like a category, but is has no slots. 4.2. Maintenance category The maintenance category has the following slots (see Fig. 2). The title is a brief statement of what the MLM does. It is treated as free text (that is, it is not structured in any particular way), and it often contains an institution-specific name. Thefilename is the MLM’s unique identifier. It is composed of a letter; then letters, digits, underscores (J. It can be 1 to 80 characters in length. When one is referring to an MLM, the filename rather than the title is used. Note that an MLM’s “filename” need not be the same as the name of the computer file that the MLM is stored in. For example, more than one MLM can be placed in a single computer file. The oersion contains a revision number that is incremented each time the MLM is changed. It begins with 1.00,and it always has two digits to the right of the decimal point. Minor changes to MLMs should result in an increment of 0.01, and major changes should result in an increment of 1. The imtitution names the origin of the MLM. At present, institutions are represented with free text rather than coded names. The combination of the institution with the

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Syntax

MLMs

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filename should make an MLM unique across all institutions. The artthor is the person or persons who wrote the MLM. The format is first name, middle name or initial, and last name, followed by optional suffixes separated by commas. An optional electronic mail address may follow in parentheses. Internet addresses should be used directly; bitnet addresses should be suftixcd by .bitnet. and UUCP addresses should be suftixed by .uucp. If thcrc arc several authors. then their names arc separated by semicolons. The ,spccirrlist is the person who approved the MLM in the institution using the MLM, which may bc different from the institution that authored the MLM. It has the same format as the author slot. It is always blank when an MLM is shared, because each institution must approve MLMs for its own use. The clatc slot shows the date of last change. Most often, it contains a simple date (YYYY-MM-DD), but it can also include time of day according to the IS0 8601 standard [4] (e.g. 1993-OI-3OT33: 12:34.5). The r~ufidation indicates for what the MLM has been approved in the institution. There are now four valid entries: production--approved for clinical care research-approved for clinical research testing-only for debugging, sharing expired-no longer in use 4.3.

Lihrury

category

The library category follows. The purpose slot describes the motivation for using the MLM. It contains free text, and it is generally one or two sentences. The explanation slot details how the MLM works. It is also free text, but it usually contains a whole paragraph. It can be displayed to the clinical users when they are reviewing MLM messages. The keywords are descriptive terms for indexing an MLM knowledge base. The list is delimited by semicolons. UMLS [5] terms are preferred. The citutions slot contains references to literature that supports the MLM’s medical behavior. References should follow “Vancouver style” [6]. The citations slot is optional. The links slot contains institution-specific links to other sources of information, such as electronic textbooks and education modules. The links slot is also optional. 4.4.

Knowledge

category

The knowledge category contains the MLM’s medical knowledge. The type slot is intended to allow future slot variations within the knowledge category. At present. the type is always data-driven. (This does not mean, however, that the MLM must necessarily be triggered by the storage of data; it could be triggered by any event or called by another MLM.) The dutu slot maps terms used in the MLM to the institution’s database and environment. Terms may refer to clinical data, clinical events, electronic mail addresses, etc. The purpose of the data slot is to separate those parts of the MLM that are specific to an institution from the more generic parts of the MLM (e.g. the logic). It is considered a “structured slot”, which means it follows a special syntax that is described below. The priority indicates the order in which MLMs should be invoked. It can be a number from 1 (last) to 99 (first). It is an optional slot, and it is rarely used. The erloke slot specifies how the MLM should be triggered. An MLM can be triggered by an event, by the expiration of a time interval after an event, or by a direct call from an MLM or an application program. The logic slot contains medical criteria or an algorithm. It uses terms defined in the data slot and decides whether to perform the MLM’s action. The uction slot usually generates a message that is routed to a health-care provider, but it can perform a number of other actions. Like the data slot, the evoke, logic, and action slots are structured slots, and they are described in more detail below. The urgency indicates importance of MLM’s message or action. It is a number from 1 (low) to 99 (high). It is an optional slot, and it has not been used often. The MLM ends with its end: marker. If there is more than one MLM in file, then the next one simply begins with its maintenance category.

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33h 5.

SYNTAX

OF

THE

STRUCTURED

SLOTS

The structured slots share II common syntax. although each has its own slot-specific constructs. The syntax for these slots is similar to programming languages like Pascal. It it is derived from these is also similar to the syntaxes of HELP [7] and RMRS [Xl, since . systems. A slot is composed of sfutetnettts. each of which performs some function or defines some mapping. Statements. in turn, contain expressiotts. which are detined in the next section. This part of the tutorial begins by describing simple expressions, such as those that consist of numbers. More complex expressions are defined later in the tutorial.

5.2. Expressims An expressiott is irn Arden

Syntax phrase consisting of numbers,

strings, variables,

operators. etc. When expressions are evaluated, they return a result. For example, 2+2 returns a result of 4. Two expressions are said to be equioalenr if they return the same result. A typical expression might be

vat-l + 4.67 - COS(6). Numbers follow ASTM El238 format [9]. A number contains digits, an optional decimal point, and optional digits to the right of the decimal point. A number between zero and one may drop the zero before the decimal point. Scientific notation is permitted using E. There is no distinction between integer and real data types. The following expressions contain valid numbers: 3+0.1 3.00+0.100 3.+.1

3EO+ lE-I The Boolean

values are true and false.

true OR false Variables are made of a letter; then letters, digits, underscores (3. There are no variable type declarations. One simply uses a variable in the MLM. A variable can accept multiple data types within one MLM. For example, a variable can be assigned a number, then later be assigned a Boolean value. Remember that the syntax is case insensitive,

so these two variables are the same:

abcd_e3 aBcD_E3 Comments

follow the same syntax as C:

/* this is a comment, Arden

Syntax operators

much like C */ are similar

to those of other languages.

Or, and, and not

provide logical disjunction, conjunction, and negation. The operators =, <>, >, C, >=, and <= perform familiar comparisons, and +, -, *, and / provide plus, minus, times, and divide. The ** performs exponentiation (3**2 equals 9). There are several numeric functions. The following trigonometric functions are supported: arccos, arcsin, arctan, cosine, sine, and tangent. All angles are expressed as radians. The other numeric functions are defined as follows. exp-raise e to power log-natural logarithm IoglO-logarithm in base IO int-truncate to next smaller integer

Writing

abs--absolute sqrt-square

A&n

Syntax MLMs

value;

root.

For example, exp 1 equals” 2.71828... log 10 eclucrls 2.3025... log10 10 equds I int 3.4 equals 3 int -3.4 equuls -4 abs 3.4 eqds 3.4 abs -3.4 equals 3.4 sqrt 9 e~~ff~s 3

The Arden Syntax supports several lexical variants to improve readability and familiarity. The and of are formatting tokens. The is treated as white space. Of can appear after a numeric function. Several of the numeric functions have short forms: cos (cosine), sin (sine), and tan (tangent). Thus these expressions are the same: cos 3 cosine of 3 the cosine of 3 cos of (3)

Comparison =

EQ

<> > < > = < =

NE GT LT GE LE

operators is is is is is is

also have variants:

equal not equal greater than less than greater than or equal less than or equal

The word is has the following synonyms: are, was, and were. Is can be followed by not, which reverses the logic of the clause. For example, these two expressions are the same: 2<=3 2 was not greater than 3 Operator precedence is similar to other programming higher precedence than + :

languages. For example, * has

3 + 4*6 equals 3 + (4*6) One can use parentheses to override defaults. The common operators here in order of decreasing precedence:

are arranged

1. arccos arcsin arctan cosine sine tangent exp log log10 int abs sqrt 2. ** 3. +I 4. + 5. =<>><>=<= 6. not 7. and 8. or

The following groups of expressions serve as examples. Expressions within groups are equivalent. ” In these cxprcssion

CBM 24:5-8

cxamplcs and all that l’ollow. ~U~I/S is not part ol’ the Ardcn

to the Icl’t of qrtds

is cquiwlcnt

to the cxprcssion

Syntax.

fnstcad,

to the right oC c~qtrols.

it mc;ma that the

33x

3~4 OR 26 OR false (3>4) OR (2~5) OR (false) false OR true OR false true sine 0+2 (sine 0) + 2 0+2 log(l)+2 (log (I,)+2 0+2 3>4 + 7 or not S/4 - 9
7)))

Stutements

A structured slot is made of statements, which are built from the expressions defined above. The ussignment statement is the most basic. A variable holds (stores) a value for later use in the MLM. The value is set using an assignment statement. Assignment is indicated by : = or let-be: a:= 3. let a bd3; One could then use the variable

a as follows:

b : = a; /* b is assigned 3 (equals a) */ Now the following

expressions

are equivalent:

a + 1 equals 4 b + 1 equals 4 Notice that : = (assignment) and = (test for equality) are different. Also notice that all statements end with a semicolon. An if-then statement tests whether a criterion (a Boolean expression) is true. and then evaluates a block of statements based on the result. For example, if vat-1 > 5 then varl := 5; endif;

/*nowvarl<=

591

The general form of the if-then statement allows several tests. Only associated with the first test that is true is executed. If none is true, associated with else part are executed. IF a= 1 THEN b:= 1; ELSEIF a=2 THEN b := 2; ELSEIF a = 3 THEN b:= 3; ELSE b:= 0; ENDIF; /* if a is 1, 2, or 3, then b is a; otherwise b is 0 */ 5.4.

Character strings

Character

strings are values just like numbers:

“this is a character

string”

the statements the statements

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To a user, this string would appear like this: this is a character string To place a quotation mark within a string, use ““. “this contains ,,,’one quote mark” would appear like this: this contains IIone quote mark Character strings may cross a line. A single carriage return is translated into a space. Two or more carriage returns are translated into one carriage return. The expression “the first half the second half” would appear like this: the first half the second half One can concatenate This expression

strings together using 11.It does not add intervening spaces itself.

“first half” 11“second half” would print like this: first halfsecond half When non-strings are concatenated,

they are converted to strings. For example,

“result = ” IJ3 would print like this: result = 3 5.5. Lists The Arden Syntax supports lists, A list is an ordered set of values. One can create a list using ,. Lists can be heterogeneous (e.g. strings and numbers in the same list). Character strings are not considered lists of characters. These are valid lists: 3,475 3, true, “asdf”, 7 Because , has the lowest precedence, parentheses:

list expressions

are usually surrounded

with

(3,495) Two special lists are supported. An empty list is created using 0. The , operator is used to make a single value into a list of length 1. 0 ,I Except for 0, parentheses do item surrounded by redundant Lists can then be used must list, the function is applied to

not create a list. Therefore, (1) is not a list; it is a single parentheses. like single values. When a numeric function is applied to a each element:

int(3.4, 4.5, 5.6) equals (3, 4, 5) Operators that accept two or more arguments, such as addition, apply their operation pair-wise if the lists are the same length: (.l, .2, -3) + (1, 2, 3) equals (1.1, 2.2, 3.3)

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If one argument is a single value and the other is a list, then the single value is applied to each element of the list: 2 + (.l, .2, .3) equals (2.1, 2.2, 2.3) One should not use two lists of different length (the result would be null, which is explained below). List membership is supported using the is in operator: 3 is in (3, 4, 5) equals true 5 is in 5 equals true The result of is in is a list if the left argument is a list: (2, 3) are in (3, 4, 5) equals (false, true) (Note the use of the lexical variant are.) Lists can be aggregated using aggregation operators. They accept a list as input, they return a single value or another list, and their usage is the same as numeric functions. For example, count(1, 2, 3) equals 3 average(1, 2, 3) equals 2 minimum(1, 2, 3) equals 1 When an aggregation operator is applied to a single value, it treats the value like a list of length one: count(78) equals 1 Aggregation

operators can also be applied to empty lists:

count0 equals 0 To be used properly, aggregation operators usually require parentheses because of the high precedence of the aggregation operators. For example, the following is syntactically valid but usually undesired: count 1, 2, 3 equals (count l), 2, 3 Instead, use this: count(1,

2, 3)

The basic aggregation operators

are

count-number of elements exist-true if there is at least one element last-value at the end of a list first-value at the beginning of a list These aggregation operators

are used for numeric lists only:

average(avg) median sum

stddev-standard deviation variance-statistical variance These aggregation operators minimum(min)-smallest maximum(max)-largest

are used for numbers and strings:

value value

The Boolean aggregation operators any-true

are

if any element in the list is true

Writing Arden Syntax MLMs

all-true no-true

341

if all elements are true if none of the elements are true

For example, any(true, equals al&true, equals no(true, equals

false, true) true false, true) false false, true) false

When used on empty lists, the Boolean aggregation operators work as follows: any0 equals false allo equals true no() equals true

It is important to use aggregation with if-then statements. The if part of an if-then statement expects a single Boolean value; it does not know how to interpret multiple values. Therefore regardless of value of elements, a list is always counted as false. One should always use any, all, or no with lists so that they reduce the list to a single value: if (true, false, true) then a:= 3; /* NOT performed */ endif; if any(true, false, true) then a:= 3; /* IS performed */ endif;

The following aggregation operators

return list results:

minimum

from return smallest elements in C list > maximum from return < number > largest elements in C list > first from return first elements from last from return last elements from (In this example and others that follow, the phrase “” means any valid number, including a variable that has been assigned a number and an expression that results in a number. Similarly, “” means any list.) These aggregation operators return the number of elements requested, or only what is available if too many are requested. For example, first 2 from (1, 2,3) equals (1, 2) maximum 2 from (6, 4, 5) equals (6, 5) first 7 from (1, 2, 3) equals (1, 2,3) The following aggregation operators compare elements within a list. increase

< list > return difference between successive items decrease < list > additive inverse of increase % increase percent difference between successive items % decrease additive inverse of % increase

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For example, decrease(7, 3, 6) equals (4, -3) % increase(4, 5, 8) equals (25, 60) 5.6. Null

In the Arden Syntax, a rrull value symbolizes uncertainty or an error. A null value can result in several ways. The keyword null itself can be used. A null value can be returned from a database query. An error in execution will result in null, including the invalid use of an aggregation operator. All of the following expressions result in null. null 3/o log 0 true + 3 last0 sum(“asdf”,

2)

Note that these two are not the same: min 1 from () equals () /* a valid list */ min() equals null I* no way to pick one value *I Null can be an element in a list: (1, “asdf”, null, 4, null)

In general, any operation with null results in null. 3 + null equals null 3 >= null equals null 3 + (1, null) equals (4, null) (3 = null) equals null (null = null) equals null

To test for null, use is null, or its synonym is not present. (3, null) is null equals (false, true) (3, null) is present equals (true, false)

The Arden Syntax uses three-state logic, where null means “unknown”. tables for the three Boolean operators are as follows: OR

TRUE

FALSE

Null

TRUE FALSE NULL

true true true

true false null

true null null

AND

TRUE

FALSE

NULL

TRUE FALSE NULL

true false null

false false false

null false null

NOT

TRUE

FALSE

NULL

false

true

null

The truth

Writing Arden Syntax MLMs

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For example, (true or null) equals true (false or null) equals null (null and null) equals null

In an if-then statement, the Boolean expression must be true to execute the statements associated with then. Thus, null is treated the same as false, and the statements (if any) associated with else are executed instead. Non-Booleans are treated like null. if null then a:= 3.,

else a : = 0; /* this is chosen */ endif;

To separate the three logical states, check for null (existence) first, then check the truth value. if a is nuil then b : = “a is nuII”; eiseif a then b : = “a is true@; else b : = “a is false”; endif;

5.7. Time and duration An Arden Syntax time is a specific instant, which consists of a date and a time of day. It is known as a “timestamp” in some other languages. The format follows the IS0 8601 standard 141, where seconds and microseconds are optional. The general format is YYYY-MM-DDThh:mm:ss.mmmmmm where “YYYY” is year, “MM” is month, “DD” is day, “hh” is hour, “mm” is minutes, “ss” is seconds, and “mmmmmm” is microseconds. The T between the day and hour is required. For example, 1954-02-28Tl2:21 1993.Ol-02T14:30:34 1987-ll-12T02:00:21.123456 An Arden syntax duration is an unanchored duration operator to a number.

length of time, created by applying a

1 year 21 hours

The duration operators year month day hour minute second

are

years months days hours minutes seconds

There are several operators for combining time and duration. After adds a duration to a time, and before subtracts a duration from a time. They both correct for different days in a month.

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HKIPCSAK

1 month after 1990-Ol-OlT12:34:56 equals 1990-02-OlT12:34:56 1 month before 1~0-01-01T12:~:56 equals 1989-12-OlT12:34:56 1 month after 1990-Ol-31TOO:OO:O0 equals 1990-02-28TOO:OO:OO 30 days after 1990-Ol-31TOO:OO:O0 equals lo-03-02TOO:OO:~ One can also use arithmetic operators - < time > equals < duration > < duration > + < duration > equals < duration > < duration > equals < time > =Ctime > + < duration > eq~u~~< time > < duration > + < time > equals < time > < duration > * < number > equals < duration > < number > * equals < duration > < duration > / < number > equals < duration > I equals < number > For example, 1990-Ol-30TOO:OO:OO- 1990-01.01TOO:OO:00 equals 30 days 1 month + 0 seconds equals 2629746 seconds 1 month + 1 year equals 13 months Now is a special keyword that returns the current time. Multiple uses of now within a single MLM will return exactly the same time; it is as if time had stopped for the execution of the MLM. Ago is a special operator that is shorthand for times relative to now. 3 months ago equals 3 months before now Special temporal comparison operators < time > < time >

are available for comparing times:

is before is after is equal is within to is within preceding is within following is within surrounding is within the past is within same day as

For example, 1990-Ol-OITOO:OO:OOwas before 1990-Ol-02TOO:OO:OO equals true 19~-01-02T~:~:~ is within loom-OlT~:~:OO to l~-Ol-O3T~:~:~ equals true 1800-Ol-01T00:00:00 is within the past 3 days equals false Is after and is before are exclusive intervals, and the rest are inclusive intervals. 19~-Oi-0lTOO:OO:~ equals false

is after lo-Ol-OlT~:~:OO

Writing Arden Syntax MLMs

The regular comparison 19~-Ol-OlT~:~:~ equals true

operators

345

will also work:

< 1990-Ol-02T~:~:OO

5.8. Primary time

When data are retrieved from the patient database, each data value has associated with it a primary time. The following query assigns the patient’s potassium values to the variable K (details of the query syntax are explained later). K : = read {serum potassium};

Each value in K is really a pair: the actual value and the primary time that the value occurred. The primary time is the clinically most relevant time. For laboratory test results, it is the time that the test was performed on a patient. For physician orders, it is the time that the order was placed. Since primary times are not always available, one must often compromise. For example, if the time a test was performed on a patient is not available, then the time that the specimen arrives at the laboratory might substitute. If a patient had these potassium test results: 4.1 on 1991-01-01 4.2 on 1991-01-02 4.3 on 1991-01-03 then the above query would assign K these values: (4.1, 4.2, 4.3) One would access the primary times of the potassiums using the time function. The format is similar to numeric functions: time of K equals (1991-Ol-OlTOO:OO:OO, l~l-Ol-O2T~:~:~, 1991-Ol-03T~:~:OO)

When applied to something that has no intrinsic primary time, the time operator returns null : time of 3 equals null

The results of a query are always sorted in chronological order by primary time. Therefore, the first operator, which returns elements from the beginning of a list, will return the elements that are earliest in primary time. The last operator will return the elements that are the latest in primary time. Special comparison operators are available for primary time. They are analogous to the other comparison operators except that occur (or occur, occurs, occurred) replaces is. The effect is that instead of using the value of the left argument, the operator uses the time of the left argument. Thus, K occurred within the past 3 days is the same as (time of K) is within the past 3 days

For the potassium data listed above, the following are true: K occurred equat 1991-Ol-02TOO:OO:OO equals (false, true, false) K occurred before 1991-Ol-02T~:~:~ equals (true, false, false)

CBM24:5-c

G. HRIPCSAK

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The following comparison result of a database query). < qresult < qresult < qresult < qresult < qresult < qresult < qresult < qresult < qresult

> > > > > > > > >

occur occur occur occur occur occur occur occur occur

operators

work with occur (where “ < qresult >” is the

before after equal within to within preceding within < duration > following within surrounding within the past within same day as

Aggregation over primary time is also supported. To get the value in “ < qresult > ” whose primary time is closest to “ ,” use this: nearest


> from < qresult >

To find the slope coefficient of regression on values versus primary times, use this: slope < qresult >

To find the difference between successive primary times, use this: interval

< qresult >

When performing an operation on a variable, the primary times are maintained in the result. Using K from the above example, time of (2*K + 3) equals (1991-Ol-01T00:00:80, 1991-Ol-02TOO:OO:OO, 1991-Ol-03TOO:OO:OO)

The exception to this rule occurs when an operation is performed on two variables whose elements are derived from queries with different primary times. In this case the primary times are set to null. 5.9.

Where

One often needs to select items from a list. For example, select all potassiums less than 3; select all sodiums done in the past 3 days; or select all diagnoses related to neoplasm. This is accomplished with the where operator. Where expects a list as the left argument and a Boolean list as the right argument. It returns elements from the left list that correspond to true elements in the right list. All non-true elements are counted as false. (1,2,3) where (true, false, true) equals (1, 3) (10, 20, 30) where (true, null, 45) equals (,lO) /* length 1 list */ (1,2,3) where (1,2,3) > 1.5 equals (2,3) The keyword it (and its synonym they) can be used in the right list argument. It stands for the left argument. This is useful because the left argument is often referred to in the right argument. Using where and it, one can apply constraints to a list conveniently: (1,2,3) where (1,2,3) where (1,2,3) where equals (2,3) (1,2,3) where

it > 1.5 they > 1.5 (1,2,3) >1.5 I* all 3 are same *I it > 1 and it < 3

Writing Arden Syntax MLMs

347

equals (,2)

varl where it occurred within the past 3 days equals those values whose primary time > = now - 3 days 5.10.

Checking data types

Variables can be of any type, and lists can be mixed. Sometimes it is necessary to distinguish among the types. For example, one may want to select only the numeric values from a list. The type comparison operators are used to distinguish types. The is not construct can be used with each one. is present-same as is not null is null-same as is not present is boolean is number is time is duration is string is list For example, “asdf” is string equals true (1, true, null) is number equals (true, false, false) Whereas most of the type comparison operators return a list when their argument is a list, is list does not. It always returns a single value: true if the argument is a list, and false if it is not: (1, true, null) is list equals false

The operators

are often useful with the where operator:

(3 days, 0, null) where it is duration equals (, 3 days)

6. DATA

SLOT

6.1. Purpose The data slot maps an institution’s entities to MLM variables. There are several types of entities that need to be mapped: data in the patient database * cfinical events (store data, admit a patient) l other MLMs l providers (e.g. electronic mail address) l

For example, within an MLM, a patient’s serum potassium result may be called serumgotassium. The way the serum potassium is stored in the patient database will vary from institution to institution. Some institutions may use a name like “serum potassium” or “K”, and others may use a code like “23414”. It is the purpose of the data slot to isolate the parts of the MLM that vary from institution to institution (e.g. references to the patient database) from the parts that are more generic and sharable (e.g. the medical logic). 6.2. Read statement In addition to the assignment statements and if-then statements defined above, the data slot supports several special purpose statements. The read statement is used for

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queries to the patient database. It assigns database entities to MLM variables. It is only valid in the data slot. The components of a read statement are as follows: l l l l

variables to accept values aggregation function institution-specific mapping time constraints

For example, potassium : = read last ( {‘serum potassium’} where it occurred within the past 3 days );

The first part to notice is the institution-specific mapping, which is the part enclosed in { and }. It specifies what you want from the database. Its syntax varies among institutions, so it will need to be changed when MLMs are shared. The only requirement is that it must be enclosed in { and }, and that internal brackets ({ and }) must be matched. It should not contain aggregation or time constraints, since these are coded elsewhere. For example, to get the potassiums at one institution, this is used: {‘dam’ =“PDQRESl”; ; ‘any 32310 - intravascular

potassium’}

where PDQRESl specifies a database and 32310 is the class of all potassium tests. The use of different databases results in different syntaxes: SQL for relational databases; calls to data access subroutines; sets of parameters used in a query interface; or even just a comment, to be filled in later. All read statements must have an institution-specific mapping component. The time constraints specify a time interval that all retrieved data must lie in. The time constraints are optional; the default is to return all data regardless of primary time. The read statement accepts the temporal comparison operators that permit the where it occur form. (Note that “” can be the result of a previous query.) where where where where where

where where where where

it it it it it it it it it

occur occur occur occur occur occur occur occur occur

before < time > after equal within to within preceding within following within surrounding < time > within the past within same day as < time >

An aggregation operator can be applied to the result of a query. Conceptually, the aggregation operator is applied after the time constraints and the institution-specific mapping. Aggregation operators are optional; the default is to return the raw data as a list. These operators return a single value: last first count exist sum average (avg) minimum (mitt) maximum (max)

Writing Arden Syntax MLMs

These operators

349

return a list:

last from first from minimum < number > from maximum < number > from The following queries are valid. dx := read {‘admit diagnosis’}; dx := read last 3 from {‘admit diagnosis’); dx : = read (‘admit diagnosis’) where it occurred before the time of K; When both an aggregation required :

operator

and a time constraint

is used, parentheses

are

a : = read min ( {‘serum potassium’} where it occurred within the past 2 days); One can assign multiple variables in one read statement by using parentheses on the left side of the assignment operator. The corresponding elements in each variable will have the same primary times. These two are equivalent: (Na, K, Cl) : = read Iast 3 from {‘sodium’ , ‘potassium’, ‘chloride’}; let (Na, K, Cl) be read last 3 from (‘sodium’, ‘potassium’, ‘chloride’}; The result is that Na will be sodium, K potassium, and Cl chloride. The first element of Na, K, and Cl will have same primary time; the same will hold for the other elements. For example, if the patient had these test batteries: Na = 141, Cl = 101, HC03 = 31 on 1991-01-01 Na = 142, HC03 = 32 on 1991-01-02 Na = 143, Cl = 103, HC03 = 33 on 1991-01-03 then this query: (Na, Cl, HC03) : = read last 3 from (‘sodium’, ‘chloride’, ‘bicarbonate’}; would have these results: Na equals (141, 142, 143) Cl equals (101, NULL, 103) HC03 equals (31, 32, 33) time of Na equals time of Cl equals time of HC03 equals (1991-Ol-01T00:00:00, 1991-Ol-02TOO:OO:OO, 1991-Ot-03TOO:OO:OO) The second element of Cl is null because the corresponding test battery did not contain a chloride value. Thus, Arden Syntax queries fill in misisng values with null, but the primary times are maintained. One can then do correlated operations on all three variables since they are lists of same length. anion-gap : = Na - (Cl + HC03); This would result in: anion-gap equals (9, NULL, 7) time of anionsap equals time of Na

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350

One can also use the where operator across variables: Na where HC03

> 31 equals (142, 143)

In summary, the steps to creating a query are as follows: (1) “What data do you want?“- institution-specific mapping {‘discharge

diagnosis’}

(2) “Over what time interval?“-time

constraints

{‘discharge diagnosis’} where it occurred within the past 30 days (3) “How many do you want back?“-aggregation

operator

last 3 from ( {‘discharge diagnosis’} where it occurred within the past 30 days) (4) “What do you want to call it in the MLM?” discharge_DX : = read last 3 from ( {‘discharge diagnosis’} where it occurred within the past 30 days)

The following queries are examples from real databases. The first one gets the admitting service from IBM’s PCS Patient Management product via data access module GYDAPMP. service : = read last {‘dam’ = “GYDAPMP’, ‘constraints’ “HCASEX”; “HADMSVCL”};

In a HELP-based

= “i”;

database, the same query would look like this [lo]:

service : = read last {class WHICH IS 1.0.1.0.16.76.8.8);

The following query gets a list of up to 11 potassiums and calciums that occurred within the past year. (KS, CAs) : = read last 11 from ( {‘dam’ =“PDQRESl”; ; ‘any 32310 - intravascular potassium’; ‘any 32109 - intravascular calcium’} where they occurred within the past 1 year);

This query gets previous alerts from the same MLM. last-alert : = read last {‘dam’ =“PDQDECl”;

6.3.

‘mlmself’};

Other assignment statements

The data slot also supports assignments for special uariables. These variables are used only in particular contexts; they cannot be used in most expressions. The special variables and their assignments are listed here, but they are explained in later sections. var var var var

:= := := :=

event {‘result-of’, ‘serum potassium’}; mlm ‘hypokalemia_and_digoxin’; mlm ‘filename’ from institution “CPMC”; message {‘protocol’, ‘extubation’};

Writing Arden Syntax MLMs

351

var : = destination {‘[email protected],edu’}; var : = argument;

7. EVOKE

SLOT

The evoke slot defines what triggers an MLM. There are four kinds of triggers. occurrence of an event time delay after an event 0 periodically after an event 0 direct call from another MLM

l

l

An euent is any clinical occurrence for a patient: registration, admission, discharge, or transfer; the recording of clinical data (blood test, X-ray); a medical procedure (surgery); etc. An euent variable is defined in the data slot using the event assignment statement, which uses an institution-specific mapping like a read statement (with { and j). If the event variable is placed in the evoke slot, then the MLM will be triggered (evaluated) whenever the event occurs. For example {only the relevant lines from the data and evoke slots are shown), data: store-K : = event {‘result_of’, ‘serum potassium’};; evoke: store-K;;

This MLM will run whenever a serum potassium result is stored in the database. Note that the portion within { and} is institution-specific, so it would probably change when an MLM is shared. An MLM can be triggered by multiple events by using or or any as if it were a Boolean operation. data: admit : = event {‘management event’, ‘admit patient’); discharge : = event {‘management-event, ‘discharge patient’}; ;i evoke: admit or discharge;;

or its equivalent: evoke: any(admit,

discharge);;

It is possible to add logical criteria to an evoke statement using the evoke slot’s where operator. This was added to the Arden Syntax to support older systems, but it is recommended that it not be used. Put ail criteria in the logic slot instead. One can trigger after a time delay using the after operator: data: store-K := event (‘result_of’, ‘serum potassium‘};; evoke: 3 days after time of store-K;;

This MLM will run 3 days after a potassium result is stored. Periodic triggering is also supported: data: store-K : = event {‘result-of’, ‘serum potassium’}; K : = read last {‘serum potassium’};; evoke: every 1 day for 7 days starting 3 days after time of store-K until K > = 3;;

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This MLM will run eight times (includes start and end of seven day period) reaches three. Some MLMs are called directly from other MLMs using the call statement below). In this case the evoke slot will be empty.

unless

K

(defined

evoke: ;; Several

real evoke

slot examples

follow:

data: K-storage : = event (‘32506 -service event’, ‘32310 - intravascular potassium’};; evoke: K-storage;; or evoke: 1 day after time of K-storage;; or evoke: every 90 minutes for 1 day starting time of K-storage;; 8. LOGIC

SLOT

The logic slot uses data defined in data slot read statements. The slot generally tests some criteria or runs an algorithm, and concludes true or false. If it concludes true, then action slot is executed. Otherwise, the MLM terminates without further action. In addition to assignment statements and if-then statements, the logic slot supports the conclude statement. A conclude statement immediately ends execution of logic slot. If the conclusion is true, the action slot is run. If the conclusion is not true, the MLM is exited immediately. If these is no conclude statement, then the MLM terminates at the end of the logic slot without performing an action. conclude true; conclude false; conclude varl; For example, data : K : = rad last {‘dam’ = “PDQRESZ”; ; ‘32310 - intravascular potassium’}; old KS : = read last 10 from ( {Tdam’ = “PDQREST; ; ‘32310 - intravascular potassium’}; where they occurred before time of K);; logic : if K is not number then /* valid data? */ conclude false; elseif K < min(old_Ks where it is number) then conclude true; endif; I* else conclude false *I In the above example, the action slot is performed than the minimum of the previous ten. Another example follows: logic : I* baseline = second lowest creatinine *! baselinecreat : = max(min 2 from

only if the current

potassium

is less

Writing Arden Syntax MLMs

(old_creats where they are number)); I* if treat is significantly greater than baseline and rising, then perform (current_treat > l.l*baseline_creat + 0.3) and (current_treat > last old_treats) then conclude true; endif;; 9. ACTION

353

action */if

SLOT

The action slot carries out the MLM’s external behavior. One can store a message in the patient database, send a message via electronic mail, call another MLM (defined later), or perform any institution-specific action. Assignment statements are trot allowed. A limited if-then statement is allowed: the Boolean expression must be a single variable. This allows an MLM to choose from multiple actions, but it prevents an MLM author from putting medical criteria that ought to be in the logic slot into the action slot. The write statement supports message routing. How the message is routed is institution-specific. The write statement may store the resulting string in a database, send it over electronic mail, send it to a printer, etc. Often the default is to write the message to a database. For example, write “this is a simple message”;

The action slot has access to logic slot’s variables, allowing one to create customized messages : write “the result = @11varl;

One can tailor the message or the message’s destination using the message and destination variables defined in the data slot. They use institution-specific mappings enclosed in { and } (like the read statement), and they permit the use of coded messages. In the following example, mess1 contains a code rather than a string. The destination assignment causes the message to be sent via electronic mail. data: destl : = destination {‘[email protected]’}; mess1 := message {‘pneumonia’};; action: write mess1 at de&l;;

These action slots would also be valid: action: write action: write

I* coded message to default dest *I messl;; /* text message to email dest */ “a string” at destl;;

An example of a real action slot follows: write “The recent serum creatinine of’ 11current-et-eat 11n (performed on n IItime of current treat II “) is signifi~tly~i~her than the patient’s baseline of ’ 11baseline-et-eat 11“.“;

Note that IIis used to put variables into a text message. The text can cross a line, with the result that a single space is inserted in the output. For this write statement, the user might see this: The recent serum creatinine of 2.1 (performed higher than the patient’s baseline of 1.3.

on 4 July 1993 at 13:41) is significantly

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354

10. MLM

NESTING

The Arden Syntax has a provision for MLMs to call each other. This allows nesting like subroutine calls, which can reduce redundancy by sharing algorithms. It does break the independence assumptions of the Arden Syntax and can lead to more difficult maintenance and endless loops. The process is as follows. The calling MLM defines what MLMs to call, and then issues a call statement. In the process, it can pass an argument to the called MLM. The called MLM is triggered, accepts the argument (if present), and is executed. When the called MLM terminates, it can return a result, which the calling MLM then accepts. The calling MLM uses the call statement in the data or logic slot. Calling an MLM requires the coordination of several statements, including a special variable defined in data slot. For example, let calling MLM have these statements (only the relevant slots and statements are shown): filename: calling_mlm;; data: submlm : = mlm *called_mlm’;; logic: res : = call submlm with (3, 4);; Let the called MLM have these statements: filename: called_mlm;; data: arg : = argument;; logic: arg : = arg -t 1; conclude true;; action: return arg;; In this example, the calling_mlm passes (3,4) to called_mIm, and called_mlm returns (4, 5) to calling_mlm. The variable res will contain (4, 5) at the completion of called_mlm. One can call an MLM without passing an argument (no with), and one can terminate without returning a result (no return): a : = call submlm2; I* a equals null +f One can also call MLMs based on an event variable instead of an MLM name. This allows the triggering of multiple MLMs, and it can fool an MLM into thinking its regular triggering event has occurred. data: store_K : = event {‘result’, ‘~~iurn’~;; logic: a : = call store-K;; In this example, all MLMs normally triggered by potassium results will be triggered. The action slot has a special call statement. It never expects a result returned, and it allows an optional time delay before triggering the called MLM. In this case, the calling is done asynchronously. call submlm; call store-K delay 3 days; One can also link MLMs through their messages, rather than using the call statement. The storage of one MLM message in the patient database can trigger another MLM, just as a laboratory test result would. One can then treat patient database as a blackboard, where MLMs store intermediate results used by other MLMs. There are no explicit Arden Syntax statements for this; use read and write statements. 11. ARDEN 11.1. Check

SYNTAX

HINTS

for uaZid data

Real systems do not always behave as ptanned, so one cannot know what will come out of a real database: l

ridiculous value (potassium equals 50)

Writing Arden Syntax MLMs l l l

355

multiple values when one is expected error codes ~hemolyze~) old data (potassium from 10 years ago)

Therefore, one should account for data that do not fulfil expectations. The following example forces time constraints, picks only the last potassium value, and checks that the potassium is a number in a valid range: data: K : = read last ( {‘serum potassium’} where it occurred within the past 3 months);; logic : if K is not number then conclude false; elseif K > 10 or K < 1 then conclude false; endif; /* at this point, K is reasonable */ 11.2. Watch out for lists A potential problem is that read statements return lists by default, but if-then statements expect the Boolean expression to be a single item. The following example ihustrates the problem: data: bdate : = read {‘patient birthdate’f;; logic : /* DO NOT DO THIS */ if bdate is within past 3 days then conclude true; endif;; This will neuer conclude true, since bdate is a list even if only one birthdate is actually returned. A list of length one (even though it contains true) is not the same thing as a single true, and the if-then statement will not execute the statement associated with then. Instead, one must ensure that the variable is a single item, or one must use an aggregation operator in the if-then statement: bdate : = read last (‘patient birthdate’}; or if any (bdate is within past 3 days) then conclude true; endif;; 11.3. Working with codes Diagnoses, problems, and other medical data are often coded with a standard vocabulary, yet the Arden Syntax has no “code” data type. Some institutions treat codes like Arden Syntax numbers, and some treat codes like Arden Syntax strings. The codes are assigned to variables using read statements, and then classification is done using membership (is in). For example, one might read a coded patient diagnosis and want to know whether it is a pulmonary disease (e.g. pneumonia). The following excerpt accomplishes this. data: pulmonary : = read {‘codes’ =

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‘any 3290 - pulmonary disease’}; diags : = read {‘admit diagnosis’};; logic : if any (diags are in pulmonary) then conclude true; I* diag is pulmonary *I endif;; 11.4. ~upiicate aEerts The events that trigger MLMs are often redundant. The same result may be stored twice (preliminary, final, redundant tests may be ordered, or several providers may enter the same data. This causes a problem because one does not want to send redundant alerts. If health-care providers receive too many alerts that are not useful (false positive alerts), then they may ignore or miss the important ones. The code necessary to avoid duplicates is often complex. There is no single answer to the problem, since the answer depends on the context. The usual approach is to usually ask whether there has been an alert from the same MLM recently. One institution uses this look for alerts from the same MLM: r~undant alert : = read last (‘dam’ =“PDQDECl”; ‘mlmself’}; The old alert must then be analyzed to decide whether the new alert is redundant, whether this is actually a new problem.

or

11S. Phrasing of messages No system is perfect, so one must phrase messages to account for errors. It is often sufficient to show raw data, and leaving the interpretation to the provider. The real value of an alert is bringing this information to the provider’s attention; the provider is capable of drawing the correct conclusions once the data are pointed out. For example, after a number of discussions over what comprises a “positive” tuberculosis culture, the MLM filename and message were changed from ~sitive_TB_culture to the more neutral TB_cuIture_result, which showed the actual laboratory result.

11.6. MLM template Figure 3 contains a template of an MLM. It can be used as a quick reference creating new MLMs. The Appendix contains additional sample MLMs.

when

12. SUMMARY The Arden Syntax for Medical Logic Modules is a language for representing medical knowledge. It focuses on knowledge bases that consist of independent modules, and it has been used to generate alerts, interpretations, screen, and management messages. Each medical logic module (MLM) consists of slots grouped into three categories: maintenance, library, and knowledge. The slots define what event evokes the MLM, a set of logic criteria or algorithms that are evaluated, and an action to perform if the criteria prove true. One slot contains data mappings that translate from local MLM terms to institution-specific entities, such as database entries. Other slots specify knowledge base maintenance information. Most of the MLM slots follow a simple format. The more complex slots use a syntax that is similar to Pascal. MLMs can be written using a simple ASCII text editor. Minimal experience is required to read and understand most MLMs. Writing MLMs takes more practice, but with the tutorial and sample MLMs, a novice user can become a successful MLM author. Sample MLMs are available electronically over anonymous ftp on server cucis.~is.co~umbia.~u in directory pub/mim. Acknow~e~ge~e~~-Tee

help of T. Allan Pryor, James J. Cimino. and Paul D. Clayton is greatly appreciated.

Writing Arden Syntax MLMs

357

maintenance:

**

title:

filename: : 1.00;

veraion

;;

institution: author : specialist:

(youraffiliation) (your name)

;;

:;

(always blank to start) (t&y’s date, like 1993-10-31) ii testing;: (skzrt with t&in&

date : validation:

library

;

lsomebrief descripiitml nane for the MLM)

1 * (a w&e kdwaysstait’with 1.00)

: purpose :

(why urouldyocr use the MLM) ;; fhow G&Wthe MLM workj

explanation:

;;

kwrdsor phrases that describe the ML?&

:

keywords

;

knowledge: type: data:

;

:i

data-driven;; (always data-driven) (define the event used in the evoke slot* and read data for the logic slot)

;; event should trigger this MLM) ;; (what logic : (create the k&c necessary to carry out the task and canclird~ true when you want to write a message1

evoke :

i; action

:

(writi a message, combining narrative text and calculated values)

;; end :

Fig. 3. Template MLM.

REFERENCES 1. G. Hripcsak, P. D. Clayton, T. A. Pryor, P. Haug, O.B. Wigertz and J. van der Lei, The Arden Syntax for Medical Logic Modules, Proceedings of ihe Fourteenth Annual Symposium on Computer Appficut~o~ in Medical Cure Washington, DC, 4-7 November 1990, R. A. Miller, Ed., pp. 200-204. IEEE Computer Society Press, New York (1990). 2. American Society for Testing and Materials, E 1460 standard specification for defining and sharing modular health knowledge bases (Arden Syntax for medical logic modules), 1992 Annual Book of ASTM Standards, Vol. 14.01, pp. 539-587. American Society for Testing and Materials, Philadelphia (1992). 3. G, Hripcsak, P. Ludemann, T. A. Pryor, 0. B. Wigertz and P. D. Clayton, Rationale for the Arden Syntax. Comput. Biomed. Res. 27, 291-324 (1994). 4. International Organization for Standardization, Data Elements and Interchange Formats-lnformarion Interchange-Representutions of Dares and Times (IS0 8601: 1988E). International Organization for Standardization, Switzerland (1988). 5. D. A. B. Lindberg and B. L. Humphreys, Toward a unified medical language, ~edicu~~nformat~~ Europe ‘87, Proceedings of the Seventh Internationu~ Congress, Rome, Italy, pp. 23-31 (1987). 6. Internatjonal Committee of Medical Journal Editors, Uniform requirements for manuscripts submitted to biomedical journals, New Eagf. J. Med. 324, 424-428 (1991). 7. T. A. Pryor, R. M. Gardner, P. D. Clayton and H. R. Warner, The HELP system, ~nformut~on Systems For Patient Cure, B. I. Blum, Ed., pp. 109-128. Springer, New York (1984). 8. C. J. McDonald, L. Blevins. W. M. Tierney and D. K. Martin, The Regenstrief medical records, MD Comput. 5(S). 34-47 (1988). 9. American Society for Testing

and Materials, E 1238 standard specification for transferring clinical laboratory data messages between independent computer systems, 1992 Annual Book of ASTM Standards, Vol. 14.01, pp.322-385. American Society for Testing and Materials, Philadelphia (1992). 10. T. A. Pryor and G. Hripcsak, Sharing MLMs: an experiment between Columbia-Presbyterian and LDS Hospital, Proceedings of the Seventeenth Annual Symposium on Computer Applications in Medical Care. Washington, DC, 30 October-3 November 1993 C. Safran, Ed., pp. 399-403. McGraw-Hill, New York (1994).

G. HRIPCSAK

358

APPENDIX:

THREE

ADDITIONAL

SAMPLE

MLMs

maintenance: Provide comaon calculations for the chemistry profile T;; chemistrygrofile_calculations;; veraion: 1.14;; institution: Columbia-Presbyterian bIedica1 Center;; author: George Hripcsak, M.D. (hripcsa8cucis.cis.columbia.edu);; specialist: ;; date: 1993-05-05;; validation: testing;; title:

filename:

library: purpose: Provide comnon calculation8 for the chemistry "chemistry profile T" teat.::

laboratory’8

explanation: The following formulae are used: anion gap (mSq/L) = Na - (Cl t HC03) serum osmolality (mOsm/kg H20) = 2(Na) t glucose/l8 t BlJN/2.8 BUN-creatinine ratio = BUN / creatinine corrected calcium = Ca t 0.8*(4.0-albumin) if albumin < 4.0 Ca - 0.8*(albumin-5.0) if albumin > 5.0 if albumin 4-5 Ca ;; keywords: chemistry profile: anion gap; osmolality; corrected calcium: BUN-creatinine ratio;; citations:

;;

knowledge: type:

data-driven:;

data: chemT_atorage := event ('32506','32693'); (sodium,chloride,bicarbonate,BDN,creatinine,glucose, calcium,tprotein,albumin) := read last ('evok&ng', 'dam'="PDQPESl"; ; '32112'; '32714'; '32115'; '32116'; '32718'; '32717'; '32119'; '32723'; '32124'): interp_dest := destination ('interpretation');

;; evoke :

chemT_storage;;

logic: /* do not bother ia data are not back yet */ if creatinine is not number and sodium ia not number then conclude false; endif: anion-gap

:= 8odium - (chloride t bicarbonate): oemxolality := 2*8odium + glucose/18 t BDN/2.8; BUN_creatinine := BUN/creatinine; if albumin < 4.0 then corrected_ca := calcium t 0.8*(4.0-albumin); elseif albumin > 5.0 then corrected_ca := calcium - 0.8*(albumin-5.0): el8e corrected_ca := calcium; endif;

Writing ArdenSyntax MLMs /* display

lnion_qap2

359

procaaaing */ := int(Q.!l+wtion_gap);

oelaolelity2:= fnt[O.S+oezaolelity); BUN_crrrtinine2 := int(O.S+BUH_creetinine);

correctad ce2 := O.l*int(0.5+10*correct*d_ce) if nion~ap2 < 0 then 8g_sp := “-: llsrff anion_gep2 < 10 then

l

8g_sp := n n; elm ag_ap := '*"; endif;

if Bm_creetininei!< 10 then bc_ap := " '1; 0180 bc_ap := 'I": endif; if corracted_ce%< 10 then if corrected_ca2= int(corrected_cel)then cc_ap : = ” ‘I; llea ec_ap := 1''I: endif;

else if corrected._ce? = fnt(corrected_eatfthen cc_ap := * *a: else cc_ap := ““; endif; endif; conclude

;;

true;

action: write

"2he following were calculated

%‘\n

"\n

8nion gap

from chem. profile on "1 I time of cre8tiniae II ranae’~ I I

= "II enion_ge@ II ag_ap II” -s/L

-17 )"I1 ( 1 I "I1 oemol8lity2 II 1' mOsm/kg 1120 ( 285 - 310 )"[I “\a BUN/creatinino BUN_creatinineP iI bc_8p II = "II " (5 - 25 f”lf "\n corrected calcium = " 11 correctec_cal 11 cc_ap I( ( 8.5 - 10.5)"~~ ” mg/cu "\n

serum oemlality

"\n\nThe following formulae were wed:" II *'\n anion gap = Na - (cl + Izo3)"1l "in serum osmolatity = 2(Na) + glucose118 + BUS@/?.8"ll “\n corrected calci&u - Ce + 0.8~(4.0-albuadn) “II “if albundn f8 les8 than 4.O”ll "\n ca - 0.8*(albumin-9.0) “I I “if

"\n et interp_dest:

otherwise corrected

albumin ie greetor than S.O"ll celcium = tot31 calcium"

end:

maintenance:

title: Screen for hypokelemiawith digorin therepy (trigumred digoxin level storage);;

by

360

G. HRIPCSAK

digoxin_and_hypokalexia;; f ilenama: veraion: 1.04;; institution: Columbia-Presbyterian l&d&al author: George Hripcsak, M.D.;; specialist: ;; date: 1993-03-2s;; validation: testing;;

Center:;

library: purpose: the health care provider digoxin therapy.;:

Warn

of

hypokalexda in the setting

of

explanation: Whenever a aerurndigoxin level is stored, it is checked to see if it ia non--ILero. If so, the last valid serum potassium (within the past 3 montha) is retrieved. If the potassium is less than 3.3, then an alert is generated. It warns that hypokaleuda may potentiate digoxin arrhythmias. Repeat alerts are not generated if there has not been a new digoxin level since the last alert.;:

:

keyworda

hypokalemia; digoxin; arrhythmia;:

citationa:

;;

knowledge: type:

data-driven;:

data:

/* evoke on atorage of a serum digoxin level */ atorage_of_cUgoxin := WENT {'32506','1730'); /* read the digoxin that evoked the MU4 */ digoxin := READ LAST ('evoking', 'dam'="PDQPXSl"; ; ‘1730’); /* get the rawgotaaaiw {

last 3 potaasiuma l/ := READ

LAST

3 FROM (

'dam'="PDQRESl"; ;

'1301','1608','1609','1610','1656','1698','32713')

where they occurred within the past 3 months); /* get the last related alert */ last-alert := RXAD LAST ( ( ‘dam’X”GyDmEP” ; “HYPOKALEMIA ANl_DIGOXIN", "DIGOXIN_AND_XTPOXALXMIA":

“MLM FI LENAt& ) where it occurred within the past 3 months); /* email for research log */ email dest := destination TIemailW, 'narm'="[email protected]");

;; evoke : /* evoke

on storage

of

a serum

digoxin level

storage_of_digoxin;;

logic:

/* exit if the digoxin level is invalid */ if digoxin ia not number then conclude false;

l/

Writing Arden Syntax MLMs

36

endif;

I* ait if the digoxin level i8 0 */ if digoxin <= 0 then conclude false: endif; /* get the last valid potassium */ potassium := lastfrargotassiums where they

are

number);

/* exit if no hypokalexia is found *I if potassium < 3.3 then ; /* continue */ else conclude false: e?Idif;

/* exit if an alert has occurred since 2 weeks before */ /* the last digoxin level (ie, avoid redundant alerts) */ if last_alert occurred after 2 week8 before time of tigoxin then conclude false; endif: /* send an alert *I conclude true:

;; action: write "The patient's sexux digoxin level f" IIdigoxinl I n ng/ml on * I jtime of digoxinl I I')indicates that the patient is taking digoxin. The patient'8 most recent potassium level (” I IpotassiumlI ” mEq/l on " 1ltime of potassiuml I I*)is low, and the hypokalemia may potentiate the development of digoxin-related arrhythmias."; /* send email for research */ write "k=" I I potassium 11 '1on m 11 time of potassium II '1,digorin=" I I digoxin II Ivon '1 II time of digoxin at email_dest;

urgency :

SO; ;

end:

maintenance: title: Fractional excretion of sodium;: filename: fractional-na;: version: 1.07;; institution: Columbia-Presbyterian Medical Center;; author: George Hripcsak, M.D. ([email protected]);; specialist: ;; date: 1993-03-25;; validation: testing;;

library:

CEM24:5-O

362

G. HRIPCSAK purpo8e: Calculate the fractional excretion creatinine ia atored.;:

of sodium whenever a urine

explanation: Thi8 MLM ia triggered whenever a apot urine creatinine ia atored. The ML!4 then waita for up to one day for a 8pot urine aodium, 8erum creatinine, and serum aodium to come back from the lab. The urine valuea muat have been collected within an hour of each other, and the serum value8 mu8t have been collected within a day of the urine valuea. The fractional excretion of sodium is calculated from theae numbera, and presented when the urine creatinine is reviewed. An explanatory paragraph is included with the interpretation. ;; keyword8:

fractional

excretion; aodium; arotemia: oliguria; creatinine;;

citations: 1. Steiner RW. Interpreting the fractional excretion of sodium. Am J Med 1984;17:699-102.;; knowledge:

type:

data-driven;;

data: let urine_creat_atorage be event ('32506','1763');

/* get evoking 8pot urine creatinine */ let (updatetime, urine_treat) & read last ('evoking', qdam1=VDQRES2", 'di8play_header'="TU"; ; '1763'): /* event_db_key (acce88ion number) is to find previourr interprets */ let event-&-key be read last ('pcodes'="now event_db_key "1; let urine_na_list be read ('dam'="PDQRESl"; ; '1534') where it occurred within 1 hour aurrounding time

of

let (8erum_na_list, serum_creat_list) be read ('dam'="PDQRESl"; ; '1612','1613','1657','32712'; '1598','1599','1651','32718') where they occurred within 1 day 8urrounding time /* get key of data that has already been let interp_accnoa be read ('dun'="GYDADEP";

where it occurred

of

urine_treat;

urine_creat;

interpreted */

"FRACTIONAL NA"; "E_DB_KEY") within updatetime to now;

/* code aa an interpretation rather than alert (default) */ let interp_deat be destination ('interpretation'); /* email for research log */ let email_deat be de8tination ('email', 'name'='WbfractRcuci8.ci8.columbia.edu"I;

;; evoke :

/* keep trying to calculate fractional Na until all re8ult8 are back */ every 90 minutes for 1 day atarting tima of urine_creat_atorage /* 8top if U treat is not ready, MIM will be triggered again anyway */ until urine_treat i8 not number /* al8o 8top if thi8 ver8ion of te8t ha8 already been interpreted l/ or any (interp_accnoa = event_db_key]::

Writing Arden Syntax MLMs logic

363

:

/* get the appropriate values from lfata */ let urine_na be ne8rmt (time of urine_creat) from (urine_na_list'rhexe it ia number); let aerum_na be nemeat (tinm of urine_crest) from (aerum_na_list where it is number); let aerum_treat be nearest (time of urine_creat) from (serum_creat_liat where it is number); /* calculate fractional excretion of sodium */ let fractional na be 100 * (urL*_na I urine_treat) / (serrum_na / aeruxr_creat); /* send interpretation if frational Wa is valid if fractional_na is number then /* send message */ conclude true: else /* no meaaage *I conclude false; endif;

*/

;; action: write "The fractional excretion of aodium f6 m ii int(O.5+fractiona~_na*lOO)/1OOlI"# baaed upon a urine creatinine of "I /urine_creat1I" (“I I time of urine_creatI I*'), urine aodium of “1 Iurine_nalf" (“1 Ithue of urine_naI I'*), aerum creatinine of “I Iaerum_creatI1” (“I Itime of serum_creat~ I*'),and serum aodium of "1 Iaerunx_naI I** (“lltime of aerum_naIl"). In the patient with oliguria and lrotemia (elevated BUN), acute tubular necroeia may be diatinguiahed from reduced perfuaion of normal kidneya by measuring the fractional excretion of aodium. The fraction uecretion of sodium ia reduced (l.O$) in the patient with acute renal failure due to acute tubular necro6ia." at interp_deat; i* aend email for research *I "f ractna - TV 1 Ifractional_naI I ” I ~int(O.5+fr~ct~onal_na*lOO)/lOO~ I l;nu creatirmn I (urine_creatJ I ” on 1' I ltime of urine_creatj I " , u na=" I Iurine_naI I '1on '1 I ltime of urine_naI 1 "\na na-'1 IIaem_naI I " , a treat=" I Iserum_creatII '1on '1 IItime of serum nail "\neventtime=" 1leventtimel I 11 , now=" llnowll II updatetime=" I Iupdatetfmel I "inev db key=** 1levent_db_key at m;;il~dest;

write

AbouttheAuthor-GEORGE HRIPCSAK, M.D.,is an assistant professor in the Center for Medical Informatics at the Columbia-Presbyterian Medical Center. Dr Hripcsak wrote the first draft document that later became the ASTM’s Arden Syntax for Medical Logic Modules. He is now chairman of the ASTM’s Arden Syntax Task Group. Dr Hripcsak is in charge of automateddecision support systems at Columbia-Presbyterian Medical Center, where the Arden Syntax is used for clinical care, research, and quality management.