Interpretation of laboratory test results is much more complicated than simply comparing the test result against a so-called normal range, labeling the test values normal or abnormal according to the normal range limits, and then fitting the result into patterns that indicate certain diseases. Certain basic considerations underlie interpretation of any test result and often are crucial when one decides whether a diagnosis can be made with reasonable certainty or whether a laboratory value should alter therapy.
All laboratory tests have certain attributes. Sensitivity refers to the ability of the test to detect patients with some specific disease (i.e., how often false negative results are encountered). A test sensitivity of 90% for disease Z indicates that in 10% of patients with disease Z, thetest will not detect the disease. Specificity describes how well test abnormality is restricted to those persons who have the disease in question (i.e., how often false positive results are produced). A specificity of 90% for disease Z indicates that 10% of test results suggestive of disease Z will, in fact, not be due to disease Z.