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“Sensitivity” and “Specificity” Reconsidered: The Meaning of These Terms in Analytical and Diagnostic Settings
- Alfred J. Saah, MD, MPH; and
- Donald R. Hoover, PhD, MPH
- From the Johns Hopkins University School of Public Health, Baltimore, Maryland. Grant Support: In part by U.S. Public Health Service cooperative agreement U01-AI-35042 from the National Institute of Allergy and Infectious Diseases. Requests for Reprints: Alfred J. Saah, MD, MPH, Johns Hopkins University, School of Public Health, 615 North Wolfe Street, Room E6006, Baltimore, MD 21205. Current Author Addresses: Dr. Saah: Johns Hopkins University, School of Public Health, 615 North Wolfe Street, Room E6006, Baltimore, MD 21205.
Imprecise usage of the terms “sensitivity” and “specificity” produces confusion in the diagnostic use of sophisticated laboratory test results.“Analytical sensitivity” represents the smallest amount of substance in a sample that can accurately be measured by an assay. “Analytical specificity” refers to the ability of an assay to measure one particular organism or substance, rather than others, in a sample. An assay's analytical sensitivity and analytical specificity are distinct from that assay's clinical diagnostic sensitivity and diagnostic specificity. “Diagnostic sensitivity” is the percentage of persons who have a given disorder who are identified by the assay as positive for the disorder. High analytical sensitivity does not guarantee acceptable diagnostic sensitivity. “Diagnostic specificity” is the percentage of persons who do not have a given condition who are identified by the assay as negative for the condition. False-positive reactions occur because of sample contamination and diminish the diagnostic specificity of the assay. The terms “sensitivity” and “specificity” should be used with the requisite adjectives because the “diagnostic” and the “analytical” meanings of these terms are very different.
The terms “sensitive” and “specific” are used in many different contexts. In fact, in a discussion of molecular assays [1], the term “ultrasensitive” was introduced to describe an assay that offered “more sensitivity” than its predecessor. Most health care providers have at least an intuitive notion of these concepts, but the vernacular of test sensitivity and specificity has become more complex and seemingly more contradictory as the use of molecular assays has become more widespread [2]. Thus, difficulties arise when the clinical performance of an assay does not seem to meet expectations. For instance, as many as 2% to 15% of persons who are documented as being seropositive for human immunodeficiency virus type 1 (HIV-1) infection have negative results on the exquisitely sensitive polymerase …
