I appreciated the thoughtful and critical analysis by the authors of the AFP Journal Club on the impact of procalcitonin measurement in treating children one to 36 months of age presenting with fever without a source. However, in the discussion of the impact of disease prevalence on a diagnostic test's performance characteristics, the authors may have confused two concepts in clinical epidemiology: sensitivity/specificity and predictive value.1 One author stated that the sensitivity and specificity of a test may change depending on the prevalence of disease in the population in which the test is used. Yet, according to the textbook Clinical Epidemiology: The Essentials, "...the sensitivity and specificity of a test are said to be independent of the prevalence of the diseased individuals in the sample in which the test is being evaluated."2 In theory, the sensitivity and specificity of procalcitonin measurement are independent of the prevalence of the target disease.
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