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Pitfalls in Laboratory Tests*
Pitfalls in Laboratory Tests#
28.
Lack of knowledge of normal laboratory values, particularly those of infants, and as a result making incorrect diagnoses. Examples. Alkaline phosphatase can be as high as 20, possibly 25, Bodansky units in premature infants, and as high as 15 units in early childhood. The protein-bound iodine, transaminase and bilirubin concentrations are normally very high by adult standards in the neonatal period. The platelet count can be as low as 50,000 in the first month of life. Electrocardiographic variations are common, particularly right axis deviation in infancy. The serum gamma globulin values can normally be as low as 0.2 to 0.4 gm. per 100 mi. in infants one to six months of age. The hemoglobin values at birth should be above 15 gm. per 100 mi., but not over 23 gm.
29.
Overuse of laboratory studies. This pitfall can be minimized if one always asks whether therapy or diagnosis would be changed on the basis of the laboratory findings, whether normal or abnormal.
'" Pitfalls numbers 28 and 37 were concurred in by both seminar and corresponding participants, and numbers 29-32, 34-36, 38-42, 46, 47 and 49 by the seminar participants.
17
18
PITFALLS IN LABORATORY TESTS
30. Believing that the laboratory investigation of a patient's clinical problem is usually "academic" as opposed to good clinical judgment. Good clinical judgment is developed only by taking the opportunity to relate the patient's manifestations to total clinical course (history, physical examination and laboratory tests) as objectively as possible. It is not "laboratory" versus "clinical," but the proper fusion of both that makes a clinician.
31. Excessive dependence on "negative" laboratory data. For example, believing a. That meningitis is ruled out if the spinal fluid is normal. b. That a negative Coombs test result rules out erythroblastosis. c. That a negative (normal) urine examination finding indicates absence of renal disease. d. That a negative tuberculin test result excludes tuberculosis.
32. Using inactive laboratory reagents or testing material. Examples are Coombs's reagent, metabisulfite, outdated tuberculin, and bilirubin standard which has been exposed to daylight.
33. Failing to perform the best available test for excluding phenylpyruvic oligophrenia in the neonatal period, and not repeating the ferric chloride test between the first and second months of life.
PITFALLS IN LABORATORY TESTS
34.
19
Attempting to make a diagnosis of the bacteriologic cause of pharyngitis from the smear of the pharyngeal exudate. The type of bacteria (gram-negative, gram-positive, cocci or rods) found in pharyngeal smears is worthless for diagnosing the cause. Only thrush, Vincent's stomatitis and diphtheria can be diagnosed (at times) by this method, but not beta hemolytic streptococcal pharyngitis.
35.
Complete dependence on the total white blood cell or differential count to differentiate, establish or confirm the diagnosis of a bacterial or viral respiratory infection. Many reports have shown that viral infections are often associated with a markedly elevated white cell count, not infrequently with elements of the myeloid series predominating.
36. Failing to ascertain the gamma globulin concentration of plasma in an infant with a single serious infection or in children with (relatively) serious, repeated infections. And thereby m1ssmg hypogammaglobulinemia or agammaglobulinemia. 37.
Routinely obtaining coagulation studies prior to circumcision or tonsillectomy and adenoidectomy. The family history and a history of bleeding in the patient are important to obtain. If these indicate abnormality, comprehensive coagulation studies may be indicated, rather than a crude, relatively unreliable screening test. If the history is normal, determination of the bleeding and coagulation times is virtually worthless.
20
PITFALLS IN LABORATORY TESTS
38. Failing to appreciate that platelet counts are notoriously inaccurate. A properly prepared blood smear which shows some platelets in each field almost always indicates that there is an adequate number of platelets, whereas the absence of platelets from the blood smear indicates an abnormal, low count.
39. Failing to obtain at least one hemoglobin value in all children some time during late infancy (between six months and one year of age). Infants with hemoglobin values less than even 7 gm. per 100 ml. may appear normal on physical examination.
40. Using the glucose oxidase method for testing routine urine samples for reducing activity. The conventional screening tests, Clinitest or Benedict's test, for ascertaining the presence of reducing substances in urine are important for detecting the presence not only of glucose, but also of other chemicals, including galactose, other carbohydrates, and amino acids. A screening test which indicates the presence of all reducing substance should be used for all routine urine testing, and then, if there is evidence that a reducing substance is present, a glucose oxidase test is valuable at this point to exclude glucose as the reducing substance. Other tests should be performed immediately on the same urine to determine the nature of the reducing substance. Note: The Combostix uses a glucose oxidase method; it is therefore not the recommended method for testing routine urine samples.
PITFALLS IN LABORATORY TESTS
21
41.
Overemphasizing the significance of a few amebic or giardia} cysts in the stool.
42.
The use of inappropriate tests to ascertain the presence of blood in stools, and the misinterpretation of the significance of positive guaiac tests. The guaiac test, if strongly positive, usually indicates the presence of blood in the stool. Gastrointestinal bleeding may be the result, however, as well as the cause of iron deficiency anemia. The recent report of Dr. L. Diamond indicates that moderately severe iron deficiency is associated with an enteropathy which in turn is associated with bleeding. Adequate iron therapy for a few weeks USlJ::\lly corrects both conditions (the enteropathy and the iron deficiency anemia).
43.
Using the presence of eosinophils in the stool as evidence of intestinal allergy.
44.
Failing to realize that nasal eosinophilia in a child three months of age or less is physiologic, and not an indication of allergy.
45. Failing to obtain ( an annual) routine urinalysis. There were mixed feelings as to the value of annual urinalysis. Some preferred performing urinalysis more frequently in female patients, but less frequently in asymptomatic male patients, possibly every two years (see pitfall number 47).
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PITFALLS IN LABORATORY TESTS
46.
Examining improperly collected urine, or not obtaining re· peated urinalyses in children with acute or chronic fever of unknown etiology.
47.
Believing that an annual routine urinalysis is either indicated or the most appropriate screening method for excluding the presence of urologic infection. Some doubted that asymptomatic children (particularly male children) require an annual urine examination. Some felt that it would be more valuable to obtain a truly comprehensive urine evaluation less frequently. At this time a midstream sample of urine should be obtained after overnight fasting and thirsting. The urine should be sent promptly to the laboratory for quantitative bacteriuria, specific gravity, and the usual chemical and microscopic tests. Such a comprehensive examination, performed every two or three years, might be more meaningful than a casual urine examination performed annually.
48.
Testing for coproporphyrins on nonfresh urine. The urine should be put in a dark bottle, kept refrigerated, and examined as soon as possible. Coproporphyrins are extremely sensitive to light and bacterial decomposition.
49.
Failing to obtain chest x-rays in newborn infants with respiratory distress. Although most (95 to 99 per cent) newborns with respiratory distress have hyaline membrane disease, a few have a treatable cause. Some of the treatable causes of respiratory distress that are missed on the basis of a physical examination alone are atelectasis, diaphragmatic hernia, tension cysts, tumors and others. It is not possible to diagnose most of the treatable conditions other than by x-ray examination.
28.
Lack of knowledge of normal laboratory values, particularly those of infants, and as a result making incorrect diagnoses. Examples. Alkaline phosphatase can be as high as 20, possibly 25, Bodansky units in premature infants, and as high as 15 units in early childhood. The protein-bound iodine, transaminase and bilirubin concentrations are normally very high by adult standards in the neonatal period. The platelet count can be as low as 50,000 in the first month of life. Electrocardiographic variations are common, particularly right axis deviation in infancy. The serum gamma globulin values can normally be as low as 0.2 to 0.4 gm. per 100 mi. in infants one to six months of age. The hemoglobin values at birth should be above 15 gm. per 100 mi., but not over 23 gm.
29.
Overuse of laboratory studies. This pitfall can be minimized if one always asks whether therapy or diagnosis would be changed on the basis of the laboratory findings, whether normal or abnormal.
'" Pitfalls numbers 28 and 37 were concurred in by both seminar and corresponding participants, and numbers 29-32, 34-36, 38-42, 46, 47 and 49 by the seminar participants.
17
18
PITFALLS IN LABORATORY TESTS
30. Believing that the laboratory investigation of a patient's clinical problem is usually "academic" as opposed to good clinical judgment. Good clinical judgment is developed only by taking the opportunity to relate the patient's manifestations to total clinical course (history, physical examination and laboratory tests) as objectively as possible. It is not "laboratory" versus "clinical," but the proper fusion of both that makes a clinician.
31. Excessive dependence on "negative" laboratory data. For example, believing a. That meningitis is ruled out if the spinal fluid is normal. b. That a negative Coombs test result rules out erythroblastosis. c. That a negative (normal) urine examination finding indicates absence of renal disease. d. That a negative tuberculin test result excludes tuberculosis.
32. Using inactive laboratory reagents or testing material. Examples are Coombs's reagent, metabisulfite, outdated tuberculin, and bilirubin standard which has been exposed to daylight.
33. Failing to perform the best available test for excluding phenylpyruvic oligophrenia in the neonatal period, and not repeating the ferric chloride test between the first and second months of life.
PITFALLS IN LABORATORY TESTS
34.
19
Attempting to make a diagnosis of the bacteriologic cause of pharyngitis from the smear of the pharyngeal exudate. The type of bacteria (gram-negative, gram-positive, cocci or rods) found in pharyngeal smears is worthless for diagnosing the cause. Only thrush, Vincent's stomatitis and diphtheria can be diagnosed (at times) by this method, but not beta hemolytic streptococcal pharyngitis.
35.
Complete dependence on the total white blood cell or differential count to differentiate, establish or confirm the diagnosis of a bacterial or viral respiratory infection. Many reports have shown that viral infections are often associated with a markedly elevated white cell count, not infrequently with elements of the myeloid series predominating.
36. Failing to ascertain the gamma globulin concentration of plasma in an infant with a single serious infection or in children with (relatively) serious, repeated infections. And thereby m1ssmg hypogammaglobulinemia or agammaglobulinemia. 37.
Routinely obtaining coagulation studies prior to circumcision or tonsillectomy and adenoidectomy. The family history and a history of bleeding in the patient are important to obtain. If these indicate abnormality, comprehensive coagulation studies may be indicated, rather than a crude, relatively unreliable screening test. If the history is normal, determination of the bleeding and coagulation times is virtually worthless.
20
PITFALLS IN LABORATORY TESTS
38. Failing to appreciate that platelet counts are notoriously inaccurate. A properly prepared blood smear which shows some platelets in each field almost always indicates that there is an adequate number of platelets, whereas the absence of platelets from the blood smear indicates an abnormal, low count.
39. Failing to obtain at least one hemoglobin value in all children some time during late infancy (between six months and one year of age). Infants with hemoglobin values less than even 7 gm. per 100 ml. may appear normal on physical examination.
40. Using the glucose oxidase method for testing routine urine samples for reducing activity. The conventional screening tests, Clinitest or Benedict's test, for ascertaining the presence of reducing substances in urine are important for detecting the presence not only of glucose, but also of other chemicals, including galactose, other carbohydrates, and amino acids. A screening test which indicates the presence of all reducing substance should be used for all routine urine testing, and then, if there is evidence that a reducing substance is present, a glucose oxidase test is valuable at this point to exclude glucose as the reducing substance. Other tests should be performed immediately on the same urine to determine the nature of the reducing substance. Note: The Combostix uses a glucose oxidase method; it is therefore not the recommended method for testing routine urine samples.
PITFALLS IN LABORATORY TESTS
21
41.
Overemphasizing the significance of a few amebic or giardia} cysts in the stool.
42.
The use of inappropriate tests to ascertain the presence of blood in stools, and the misinterpretation of the significance of positive guaiac tests. The guaiac test, if strongly positive, usually indicates the presence of blood in the stool. Gastrointestinal bleeding may be the result, however, as well as the cause of iron deficiency anemia. The recent report of Dr. L. Diamond indicates that moderately severe iron deficiency is associated with an enteropathy which in turn is associated with bleeding. Adequate iron therapy for a few weeks USlJ::\lly corrects both conditions (the enteropathy and the iron deficiency anemia).
43.
Using the presence of eosinophils in the stool as evidence of intestinal allergy.
44.
Failing to realize that nasal eosinophilia in a child three months of age or less is physiologic, and not an indication of allergy.
45. Failing to obtain ( an annual) routine urinalysis. There were mixed feelings as to the value of annual urinalysis. Some preferred performing urinalysis more frequently in female patients, but less frequently in asymptomatic male patients, possibly every two years (see pitfall number 47).
22
PITFALLS IN LABORATORY TESTS
46.
Examining improperly collected urine, or not obtaining re· peated urinalyses in children with acute or chronic fever of unknown etiology.
47.
Believing that an annual routine urinalysis is either indicated or the most appropriate screening method for excluding the presence of urologic infection. Some doubted that asymptomatic children (particularly male children) require an annual urine examination. Some felt that it would be more valuable to obtain a truly comprehensive urine evaluation less frequently. At this time a midstream sample of urine should be obtained after overnight fasting and thirsting. The urine should be sent promptly to the laboratory for quantitative bacteriuria, specific gravity, and the usual chemical and microscopic tests. Such a comprehensive examination, performed every two or three years, might be more meaningful than a casual urine examination performed annually.
48.
Testing for coproporphyrins on nonfresh urine. The urine should be put in a dark bottle, kept refrigerated, and examined as soon as possible. Coproporphyrins are extremely sensitive to light and bacterial decomposition.
49.
Failing to obtain chest x-rays in newborn infants with respiratory distress. Although most (95 to 99 per cent) newborns with respiratory distress have hyaline membrane disease, a few have a treatable cause. Some of the treatable causes of respiratory distress that are missed on the basis of a physical examination alone are atelectasis, diaphragmatic hernia, tension cysts, tumors and others. It is not possible to diagnose most of the treatable conditions other than by x-ray examination.