Fundamental to patient care is understanding their diagnosis before proceeding to treatment. Diagnostic tests are used to “ruling in” or “ruling out ” specific diagnoses from a list of differentials. The probabilities of a diagnostic test correctly testing positive when a disease is actually present or correctly testing negative in the absence of disease are dependent on the test’s sensitivity and specificity and the pre-test probability. A review and clinical application of these concepts are presented in order to help the clinician assess a test’s risk of a false positive or a false negative in a world of uncertainty.



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