Relevance of Sensitivity and Specificity for Diagnosis

The underlying reason for ordering a diagnostic laboratory test is based on the knowledge of the underlying pathophysiology of the disease and the relationship of the test to the disease process. Most laboratory tests are ordered as part of a diagnostic process—the patient presents with a symptom that prompts the clinician to order a test. A laboratory test with poor sensitivity and/or specificity can lead to an inaccurate diagnosis due to the risk of missing a condition where it exists or producing a positive test result when the patient does not have a disease. The possibility of an inaccurate test result is almost always balanced by other factors in the diagnostic process such as history, symptoms, physical examination findings, and other test results. Furthermore, the sensitivity and specificity of a test can vary in different clinical scenarios because of changing biologic variables, with age being a very important one. For example, although the “mono spot” test is very sensitive in adults and older children, its sensitivity is less than 50% in young children. In evaluating whether to order a diagnostic test, there are many important considerations including:

• How does the test you are ordering compare with other laboratory tests?
• What will the lab test do to help you care for the patient?
• Would you still order the test if the patient did not have insurance?
• Can you interpret the test?
• How will the results of the test affect your clinical decision making? Which is the best test to eliminate the differentials and determine the diagnosis?
• Does testing improve the patient’s health outcomes?
• How much does the test cost? Is the test cost effective?
• Have the test parameters (sensitivity, specificity, accuracy, precision, and predictive values) been evaluated?