A comparison of statistical techniques to evaluate the performance of the glucometer elite® blood glucose meter

Clinical Biochemistry, Vol. 29, No. 6, pp. 521-527, 1996 Copyright © 1996 The Canadian Society of Clinical Chemists Printed in the USA. All rights reserved 0009-9120/96 $15.00 + .00 ELSEVIER

PH S0009-9120(96)00092-6

A Comparison of Statistical Techniques to Evaluate the Performance of the Glucometer Elite ® Blood Glucose Meter BERN HARRISON, ROSALIE MARKES, PATTI BRADLEY, and IBRAHIM A. ISMAIL Bayer Corporation, Diagnostics Division, 1884 Miles Ave., Elkhart, IN 46514, U.S.A. Objectives: The objective of the study was to measure the performance of the Glucometer Elite®, a technique-independent device for self-monitoring of capillary blood glucose. Several potential sources of error were incorporated into the study, including variability between lots, the age of a lot, variability between subjects, and variability between strips. Design and Methods: The Glucometer Elite® was tested using capillary samples from 86 individuals. Duplicate readings with each of 8 sensor lots were done for each participant. Glucometer Elite® readings were compared to YSI capillary plasma equivalent values, and the data was evaluated using a variety of both technical and clinical analysis methods. Results: All evaluation methods showed excellent agreement between the Glucometer Elite® and the YSI plasma equivalent values. Conclusion: The Glucometer Elite® system provides accurate and clinically valuable informatior~ to the diabetic home tester.

KEY WORDS: Glucometer Elite®; self-monitoring; electrochemical; technique independent.

Introduction ince the findings of the Diabetes Control and

s C o m p l i c a t i o n s T r i a l w e r e a n n o u n c e d , selfmonitoring of blood glucose has become increasingly important in the care of individuals with diabetes mellitus (1-2). The blood glucose monitoring system chosen by the patient :must not only be accurate and precise, b u t also easy to use. This paper describes the performance of a capillary blood glucose monitoring system, the Glucometer Elite ® Diabetes Care System, which consist~s of the Glucometer Elite ® Meter, Test Strip, and Code Strip. The meter is small (86 × 54 x 13 mm, 1.75 ounces) and has a biosensor that utilizes an electrochemical test reagent employing the following reactions:

Glucose + glucose oxidase (oxidized) -~ gluconolactone + glucose oxidase (reduced) Fe(CN)6 -3 + glucose oxidase (reduced) -~ glucose oxidase (oxidized) + Fe(CN)6 -4 Fe(CN)6 ~ o.5v > Fe(CN)6-3 + eA test strip is inserted into the meter and the end of the strip is touched to a drop of whole blood. Blood is drawn by capillary action into the sample chamber, which holds approximately 3 ~LL of capillary whole blood. In the chamber, ferricyanide is reduced by glucose in the presence of glucose oxidase. After a 1-min incubation period, the reduced potassium ferrocyanide is oxidized at the cathode in the presence of a constant voltage to give an electrical current proportional to the amount of glucose in the sample. The glucose concentration appears on a large digital display window. The system requires no wiping or blotting of blood, and the test is initiated automatically when the sample chamber fills. The performance of this system was evaluated using fingerstick capillary whole blood specimens. Although the performance of the Glucometer Elite ® has been previously described (3-5), this study involved 8 different lots to examine the often neglected effect of lot-to-lot variability. Also, this study used capillary rather than venous blood on the meter to test the system more closely to the way it is intended to be used in the hands of the user. Capillary blood was also used on the reference method to avoid the problem of accounting for differences between capillary and venous blood glucose (6). Additionally, the analysis of data was conducted with a variety of statistical techniques currently in use, including both technical and clinical approaches.

Methods Correspondence: Dr. Ibrahim A. Ismail. Bayer Corporation, Diagnostics Division, 1884 Miles Ave., Elkhart, IN 46514 U.S.A. Manuscript received February 20, 1996; revised and accepted May 16, 1996. C L I N I C A L B I O C H E M I S T R Y , V O L U M E 29, D E C E M B E R 1996

Eighty-six adults (42 diabetic and 44 nondiabetic) were recruited from the local community and employees within the company to participate in this study. The study was conducted in our laboratory 521

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YSI Capillary Plasma Equivalent Glucose (mmol/L) Figure 1 - - Linear Regression Analysis. The top panel shows the least-squares linear regression of Glucometer Elite ® capillary readings against YSI capillary plasma equivalent glucose values. The 95% confidence interval is indicated by dotted lines, but the large number of data points makes this interval barely distinguishable from the regression line. The bottom panel shows the percent bias of Glucometer Elite ® readings. Dotted lines in this panel indicate the TNO limits: _+15% if/> 6.5 mmol/L, _+1 mmol/L if < 6.5 mmol/L.

over a 2-day period. Informed consent was given by each participant. Our own personnel performed finger punctures and made duplicate readings on each of 8 lots of Glucometer Elite ® Test Strips (Bayer, Elkhart, IN), aspirating the blood sample directly from the finger. Capillary blood samples were also collected in a heparinized Microtainer ® (Becton Dickinson, Rutherford, NJ) brand tube for duplicate 522

whole blood glucose measurements on a laboratory comparison method, the YSI 2300 STAT ® (Yellow Springs Instr. Co., Yellow Springs, OH) Glucose analyzer. Due to the difficulty in some cases of obtaining adequate fingerstick blood for direct measurement on plasma, YSI whole blood glucose values were converted to plasma equivalent values using the equation: plasma glucose = whole blood glucose/ CLINICAL BIOCHEMISTRY,VOLUME 29, DECEMBER 1996

EVALUATION OF GLUCOMETER ELITE TABLE 1

Statistics for ¥SI Whole Blood Glucose, Hematocrit, and Calculated Plasma Glucose Measurements for Capillary Specimens in Diabetic and Normal Populations :Diabetics (n = 42)

Nondiabetics (n = 44)

All donors (n = 86)

YSI WB

Hct

YSI P1

YSI WB

Hct

YSI P1

YSI WB

Hct

YSI P1

10.6 3.8 25.0

40.8% 32.5% 50.5%

11.8 4.2 28.0

4.8 3.9 6.4

40.7% 32.5% 48.0%

5.3 4.2 7.1

5.5 3.8 25.0

40.8% 32.5% 50.5%

6.1 4.2 28.0

Median Min Max

(1 - (0.24* hematocrit)) (7). The hematocrit of each sample was measured, on a Compur M 1100 Minicentrifuge (Compur ]Electronic, Munich) using a separate specimen. STATISTICAL ANALYSIS

Glucometer Elite ® capillary glucose results obtained from whole blood specimens were plotted and compared to YSI plasma equivalent glucose values. Least-squares linear regression equations were det e r m i n e d for each of the 8 lots as well as the total data set. The systematic error estimate was calculated at several key concentrations: 3.3 mmol/L (60 mg/dL), 7.8 mmol/L (].40 mg/dL), 11.1 mmol/L (200 mg/dL), and 22.2 mmol/L (400 mg/dL). These levels were chosen using the following criteria: 3.3 mmol/L represents a commonly used cutoff for hypoglycemia, 7.8 mmol/L represents the cutoff for hyperglycemia in a fasting individual, 11.1 mmol/L represents an unequivocal elevation of plasma glucose (after ingestion of a glucose bolus) t h a t is considered to be d iag n o s tic of d i a b e t e s m e l l i t u s , a n d 22.2 mmol/L was included to assess the upper end of the clinically relevant glucose range (8). It should be noted t h a t this analysis did not consider the correlation coefficient. Thi:3 overused statistic is a measure of association, not agreement, and is irrelevant to the objectives of this study (9). The percent of readings falling within _+ 15% of reference was determined, as recommended by the American Diabetes A~sociation (ADA) (10). Also det e r min ed was the percent of readings falling within

limits recommended by TNO (Netherlands Governm e n t Research Organization): _+ 15% of reference if glucose >t 6.5 mmol/L, +_ 1 mmol/L if glucose < 6.5 mmol/L (11). The TNO limits approach the d a ta from a more clinical perspective, recognizing t h a t a larger relative deviation from reference can be tolerated at low glucose without altering the clinical interpretation. The Clarke E r r o r Grid Analysis (12), a method for determining clinical performance, was also applied to the data. This analysis divides the scatter plot into five error zones (A, B, C, D, and E) with the assumption t h a t the clinical goal is to keep blood glucose levels between 3.9 and 10 mmol/L (70 and 180 mg/dL). Meter readings within Zone A deviate from the reference by no more t h a n 20% (or both m e t e r and reference are less t h a n 3.9 mmol/L) and l ead to clinically a c c u r a t e decisions. R e a d i n g s within Zone B deviate from the reference by more t h a n 20%, but would lead to appropriate or no treatments. Readings within Zone C produce an intervention t h a t overcorrects the reference blood glucose levels. Such overcorrect i ons m i g h t cause a c tu a l blood glucose to fall below 3.9 mmol/L or rise above 10 mmol/L. Zone D represents "dangerous failure to detect and treat" errors, as patient-generated values within this zone are within the t arget zone of 3.9 to 10 mmol/L, and actual values are outside the ta rg e t range. Readings with Zone E are opposite to the actual values (hypoglycemic result from a hyperglycemic individual and vice versa), and t r e a t m e n t decisions would be opposite to those t h a t would be appropriate.

TABLE 2

Linear Regression Equation and Estimated Glucometer Elite ® Absolute and Percent Bias From YSI Plasma Glucose Mean for 8 Sensor Lots at 4 Key Glucose Concentrations Glucometer Elite ® Sensor lot 2J04A 2L01B 4A01FA 3M04CA 4A05EA 4A1C1B 3M1D3B 3M1G3B All 8 Lots

Estimated bias (% bias) at 4 YSI plasma glucose levels

vs

YSI Plasma Glucose y y y y y y y y y

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1.00(x) + 0.2 0.99(x) + 0.1 0.99(x) + 0.2 0.94(x) + 0.1 0.93(x) + 0.3 1.00(x) - 0.2 1.02(x) + 0.1 1.02(x) + 0.0 0.98(x) + 0.1

3.3 mmol/L

7.8 mmol/L

11.1 mmolfL

22.2 mmol/L

0.2 (7.1%) 0.1 (1.8%) 0.1 (4.2%) -0.1 (-2.6%) 0.1 (2.9%) -0.2 (-5.3%) 0.1 (3.6%) 0.1 (3.0%) 0.1 (1.8%)

0.2 (2.9%) 0.0 (0.0%) 0.1 (1.0%) -0.4 (-4.7%) -0.2 (-3.0%) -0.2 (-2.5%) 0.2 (2.6%) 0.2 (2.6%) -0.0 (-0.1%)

0.2 (2.0%) -0.0 (-0.4%) 0.0 (0.3%) -0.6 (-5.2%) -0.5 (-4.3%) -0.2 (-1.9%) 0.3 (2.3%) 0.3 (2.5%) -0.1 (-0.6%)

0.2 (0.9%) -0.2 (-0.9%) -0.1 (-0.5%) -1.3 (-5.7%) -1.3 (-5.8%) -0.3 (-1.2%) 0.5 (2.1%) 0.5 (2.4%) -0.2 (-1.1%)

CLINICAL BIOCHEMISTRY, VOLUME 29, DECEMBER 1996

523

HARRISON

TABLE 3 Percentage of Readings F a l l i n g

Within _+15% and TNO Limits Lot

-+ 15%

TNO*

2J04A 2L01B 4A01FA 3M04CA 4A05EA 4A1C1B 3M1D3B 3M1G3B

97.1% 94.2% 99.4% 95.9% 97.7% 97.7% 95.3% 93.0%

97.7% 97.1% 99.4% 99.4% 97.7% 99.4% 97.1% 94.8%

A l l 8 Lots

96.3%

97.8%

* TNO limits: _+15% of reference if /> 6.5 mmol/L, _+1 mmol/L if < 6.5 mmol/L. The data was also evaluated by the Koschinsky Analysis (13). This method for analyzing the total error of a glucose meter was developed by Theodor Koschinsky of the Diabetes Research Institute in Dusseldorf, G e r m a n y . An a s s u m p t i o n of leastsquares linear regression that is often overlooked is that the standard deviation of residuals (Sy x) is constant throughout the range of the independent variable (9). Koschinsky recognized that this assumption is not satisfied by data generated with blood glucose meters. (See Figure 1 in Results for proof that the variability of Elite ® readings does, indeed, change over the glucose range.) A much better assumption is that the S. x is proportional to the lab reference, so Koschins~y's method involves leastsquares linear regression of log-transformed values. The log transformation has the effect of making the Syx constant. Unlike the regression of untransf o r m e d d a t a , t h e r e f o r e , t h e Sy.x of t h e logtransformed data provides a single estimate of random error that describes the entire glucose range. The other component of total error, the systematic error, is calculated using the log-log least-squares linear regression equation at the limits of the clinically relevant range, 1.7 and 19.4 mmol/L (30 and 350 mg/dL). If the response is linear, the systematic error across the relevant glucose range will fall between the systematic error values determined at the limits. These calculations of the random and systematic error of the log values are then converted back into clinical units by taking their anti-log. This conversion causes the Sy x to become a Standard Deviation Factor (SDF) and the systematic error to become an Accuracy Factor (AF). These factors allow the error to be interpreted in terms of proportional deviation relative to the reference, rather than in terms of absolute deviation. Total error, a combined measure of systematic and random error, is calculated at the glucose level showing the greatest systematic error (either 1.7 or 19.4 mg/dL). Usually, total error is determined as the sum of the bias and 2 standard deviations but, because the Koschinsky Analysis involves proportional rather than absolute estimates of error, it is calculated as the product of 524

E T AL.

the AF and the square of the SDF. This product is called the Total Deviation Factor (TDF). TDF values provide a technical measure of performance. In addition, the Koschinsky Analysis uses TDF values to define 3 Acceptance Classes based on clinical performance criteria: Good, Acceptable, and Unacceptable. Results SUBJECT DEMOGRAPHICS AND GLUCOSE DESCRIPTIVE STATISTICS

Of the 86 volunteers who participated in the study, 42 were diabetic and 44 were nondiabetic, 56 were women and 30 were men. Though ages were not recorded, they covered a range from approximately 30 to 80 years, with no single age group dominating. The medians and ranges of YSI whole blood glucose, hematocrit, and calculated plasma glucose for capillary specimens in the entire population, as well as in the diabetic and nondiabetic subpopulations, are shown in Table 1. LINEAR REGRESSION OF GLUCOMETER ELITE ® RESULTS

Least squares regression and percent deviation plots of Glucometer Elite ® capillary readings, in comparison to YSI capillary plasma equivalent results, are shown in Figure 1 for all 8 test strip lots combined. The regression equations for each individual test strip lot are shown in Table 2. To translate these regression equations into more meaningful terms, Table 2 also shows the estimated average Glucometer Elite ® reading and percent bias at several key concentrations: 3.3 mmol/L, representing a commonly used cutofffor hypoglycemia; 7.8 mmol/L, representing the cutoff for hyperglycemia in fasting individuals; 11.1 mmol/L, representing an unequivocal elevation of plasma glucose diagnostic of diabetes mellitus in nonfasting individuals; and 22.2 mmol/L, representing the high end of the measured clinical glucose range (8). PERCENT WITHIN LIMITS

The percentages of readings falling within _+ 15% (10) and TNO limits (_+ 15% if I> 6.5 mmol/L, _+ 1 mmol/L if < 6.5 mmol/L) (11), are shown in Table 3. TNO limits are also indicated in the bottom panel of Figure 1. All lots performed very well, achieving an average of 96% within + 15%, and 98% within TNO limits. Examination of the plot in Figure 1 reveals a sample t h a t w a r r a n t s f u r t h e r explanation. All sixteen Elite ® readings generated with the sample with the highest glucose concentration (28.0 mmol/L) were lower than the reference, appearing to indicate a trend toward negative bias at very high glucose levels. However, the mean bias for this sample was only -4.5%, or 4.3% lower than the average bias for all samples. CLINICAL BIOCHEMISTRY, VOLUME 29, DECEMBER 1996

EVALUATION OF GLUCOMETER ELITE

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YSI PlasmaEquivalentGlucose(mmol/L) Figure 2 -- Clarke Error Grid Analysis. Of the 1376 readings generated on the Glucometer Elite ®(86 donors, 8 sensor lots, duplicate readings), 1362 readings (99.0%) fell in Zone A and 15 readings (1.0%) fell in Zone B. No readings fell in Error Zones C, D, or E.

ERROR GRID ANALYSIS

The Clarke Error Grid Analysis (12) was applied to all readings from all: 8 lots and is shown in Figure 2. As would be expected from the regression and limit results already shown, clinical performance was excellent. Ninety-nine percent of the Glucometer Elite ® results fell in Zone A, 1% were in Zone B, and no readings were obtained in the critical "C", "D", or "E" Zones. None of the 1376 readings obtained from the 86 donors in the study would have led to an incorrect t r e a t m e n t according to this analysis. KOSCHINSKY ANALYSIS

The log regression (113) of the combined Glucometer Elite ® readings from all 8 test strip lots is shown in Figure 3, and the TDF values obtained for each individual lot, compared to YSI plasma glucose values, are listed in Table 4. The TDF is reported for both ends of the clinically relevant glucose range CLINICAL BIOCHEMISTRY, VOLUME 29, DECEMBER 1996

(1.7 and 19.4 mmol/L) in both directions from the regression line (up and down), with the maximal value for each lot indicated in bold. Each lot was given an acceptance class (AC) based on whether the maximal TDF fell within certain limits. An AC of Good was given to a lot with a maximal TDF falling between 1.20 -1 and 1.20. A lot with a maximal TDF beyond the limits of 1.20 -1 to 1.20, but within 1.85 -1 to 1.55 at the 1.7 mmol/L level or within 1.55 -1 to 1.85 at the 19.4 mmol/L level, was given an AC of Acceptable. The Koschinsky Analysis gave 7 of the 8 Glucometer Elite ® test strip lots an Acceptance Class of Good. The one lot given an Acceptable rating just barely fell below the boundary of the Good zone (1.21-1 at 1.7 mmol/L).

DISCUSSION The performance of the Glucometer Elite ® system for self-monitoring of blood glucose was found to be excellent by using a variety of both technical and clinical evaluation methods currently in use, includ525

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Figure 3 - - Koschinsky Analysis. The top panel shows the relationship between Glucometer Elite ® and YSI plasma equivalent glucose measurements on a log-log scale. The bottom panel shows the ratio of Glucometer Elite ® readings to YSI plasma equivalent glucose values, also on a log-log scale. Such a ratio is known as a Deviation Factor, or DF, in the Koschinsky Analysis. The labeled values on the y axis of the DF plot indicates the key ratios that form the boundaries of the Acceptance Classes (AC). Solid lines indicate the limits of the "Good" AC and the limits of the "Acceptable" AC. ing e s t i m a t i o n of s y s t e m a t i c e r r o r a t k e y glucose c o n c e n t r a t i o n s (8), p e r c e n t a g e of r e a d i n g s w i t h i n _+ 15% (10) a n d T N O limits (11), C l a r k e e r r o r grid analysis (12), a n d K o s c h i n s k y analysis (13). T h e 8 t e s t strip lots e v a l u a t e d in the s t u d y included 2 lots t h a t were n e a r the end of t h e i r shelf-life, a n d t h e s e were found to p e r f o r m a t t h e s a m e level as the 6 n e w e r lots. Due to its r e d u c e d d e p e n d e n c e on u s e r

technique, p a t i e n t s u s i n g this s y s t e m are expected to achieve a level of a c c u r a c y a n d precision similar to t h a t d e m o n s t r a t e d herein. In addition to providing a c c u r a t e glucose results, the G l u c o m e t e r Elite ® s y s t e m was u s e r - f r i e n d l y as a r e s u l t of its small size, small s a m p l e v o l u m e (3 ~L), capillary action sampling, a n d a u t o m a t i c t u r n - o n a n d turn-off.

526

CLINICAL BIOCHEMISTRY, VOLUME 29, DECEMBER 1996

EVALUATIONOF GLUCOMETER ELITE TABLE 4 Koschinsky Analysis

Lot

TDFu 1.7"

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TDFu 19.4

TDFd 19.4

ACt

2J04A 2L01B 4A01FA 3M04CA 4A05EA 4A1C1B 3M1D3B ~M1G3B ~dl 8

1.17 1.15 1.14 1.09 1.14 1.05 1.14 1.15 1.14

1.09 -1 1.16 -1 1.13 -1 1.19 -1 1.13 -1 1.21-1 1.12 -1 1.16 -1

1.15 1.15 1.14 1.08 1.08 1.12 1.17 1.19 1.15

1.11-1 1.16 -1 1.12 -1 1.20 -1

Good Good Good Good Good Acceptable Good Good Good

1.16 -1

1.19 -1

1.14-1 1.10 -1 1.11-1 1.15 -1

* TDF~/d 1.7/19.4, Total Deviation Factor at 1.7 and 19.4 mmol/L up (u) and down (d) from the regression function log(y) = log(a) + b. log(x), or y = a . x b. t AC, Acceptance Class; Good, maximal TDF within 1.20 -1 to 1.20; Acceptable, maximal TDF within 1.85 -1 to 1.55 if at the 1.7 mmol/L level er within 1.55 -1 to 1.85 if at the 19.4 mmol/L level. Acknowledgements

We thank Dr. Dave Michaels and Dr. Donald Parker for assistance in the preparation of this manuscript. References

1. The Diabetes Control and Complications Trial Research Group: The effect of intensive treatment of diabetes on the development and progression of longterm complications in insulin-dependent diabetes mellitus. N Engl J Med 1993; 329: 977-86. 2. ADA Consensus Statement: Self-monitoring of blood glucose. Diabetes Ca~e 1994; 17: 81-6. 3. White JR, Campbell RK, Freerksen A, Gould B. A comprehensive evalnation of a blood glucose selfmonitoring system f~r diabetes care. Curr Ther Res 1994; 55: 1127-35. 4. MacKinnon DT, Henderson AR. A laboratory Assessment of the Miles Glucometer Elite Blood Glucose Meter. Clin Biochem 1994; 27: 501-5. 5. Innanen VT, Barqueira-de Campos F. Point-of-care glucose testing: Cost savings and ease of use with the Ames Glucometer Elite. Clin Chem 1995; 41: 1537-8. 6. Melnik J, Potter JL. Variance in capillary and venous

CLINICAL BIOCHEMISTRY,VOLUME 29, DECEMBER 1996

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8.

9. 10. 11.

12.

13.

glucose levels during a glucose tolerance test. A m J Med Tech 1982; 48: 543-5. Fogh-Anderson N, Wimberly PD, Thode J and Siggaard-Anderson. Direct reading glucose electrodes detect the molality of glucose in plasma and whole blood. Clin Chim Acta 1990; 189: 33-8. ADA Position Statement. Office guide to diagnosis and classification of diabetes mellitus and other categories of glucose intolerance. Diabetes Care 1993; 16(Supplement 2): 4. Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet 1986; i: 307-10. ADA Consensus Development Panel. Consensus statement on self-monitoring of blood glucose. Diabetes Care 1987; 10: 95-9. Quality Guidelines: Portable blood glucose monitors for self-monitoring. Leiden, The Netherlands: TNO Medical Technology Service, February 1991. Clarke WL, Cox D, Gonder-Frederick LA, Carter W, Pohl SL. Evaluating clinical accuracy of systems for self-monitoring of blood glucose. Diabetes Care 1987; 10: 622-7. Koschinsky T, Karsten D, Gries FA. New approach to technical and clinical evaluation of devices for selfmonitoring of blood glucose. Diabetes Care 1988; 11: 619-29.

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