Problems in interpreting laboratory· tests

Problems in interpreting laboratory·tests What do unexpected results mean?

Malcolm L. Brigden, MD

John C . Heathcote, MD

PREVIEW Serial laboratory test results on a seemingly healthy patient show unexpected discrepancies, or an elderly patient's test results are outside the " normal range." Why? Drs Brigden and Heathcote explore the possible answers to this question-answers that can spare patients unnecessary further testing or therapy.

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hen performing serial laboratory tests on a patient, physicians are often faced with the problem of interpreting unanticipated abnormal results. Though a ch an ge in clinical status, laboratory error, or variations in test conditions or procedures may be responsible, a common cause is biologic variation within the individual. Elderly patients in particular experience various physiologic changes that cause reference ranges for a variety of tests to differ from those of the rest of the population. Physicians need to be aware of the magnitude of these changes so that any inappropriate follow-up investigation or th erapy is not undertaken.

Illustrative case histories The following five case histories / illustrate the confusion and inap-

propriate follow-up that can occur in the wake of unexpected results on serial tests.

Case 1 A 40-year-old woman was found to have a hemoglobin level of 10.8 g/dL. No unusual features were noted on blood film examin ation. A repeat h emoglobin determination at the same laboratory 3 days later indicated a level of 12.0 g/dL. The patient's ph ysician was certain that specimens must have been switched, as there had been n o change in therapy or clinical status in the interval between tests. Case 2 While at a shopping mall in a n eighboring state, a 65-year-old man had a cholesterol evaluation performed by fingerstick sample. He was told that the result, a reading of 245 mg/dL, placed him

at risk for a possible heart attack. Repeat testing by his own physician at his hometown laboratory yielded a value of 197 mg/dL. The patient, having been informed after the first test that he would require treatment, was concerned and confused by the different results.

Case 3 A 16-year-old boy with a sore throat was found to have a leukocyte count of 12.5 X 103/pL with an absolute neutrophil count of 8, 100/}JL When the test was repeated a day later, the total leukocyte count was 8.5 X 103/}JL with an absolute neutrophil count of 5,900/}JL The patient's physician angrily demanded to know wh at type of quality control was being performed on the laboratory's analyzer, because he believed such discordant results must represent analytical error. Case 4 A frail, 77-year-old, 50-kg (110-lb) woman who was being treated with aminoglycoside for a neutropenia associated with cancer chemotherapy had a serum creatinine level of 1.13 mg/dL. The physician ordered gentamycin in continued

VOL 107 I NO 7 I JUNE 2000 I POSTGRADUATE MEDICINE I PROBLEMS IN INTERPRETING LABORATORY TESTS

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