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REPLY
Sensitivity, Specificity, Prevalence, and Disease Stage
Walter E. Stamm, MD
15 February 1994 | Volume 120 Issue 4 | Page 345
IN RESPONSE:
I appreciate Dr. Redd's comment and agree that, in a statistical sense, neither sensitivity nor specificity should vary with disease prevalence. However, with chlamydial infection (and others as well), biologic factors may influence the sensitivity and specificity of the test in populations having different prevalences [1, 2]. For example, in patients infected with chlamydia who have recently acquired infection (usually seen in populations with a high prevalence), the number of organisms (and amount of antigen or DNA) present is greater than in populations characterized by low prevalence. In the latter populations, more infections are long-lived, are subacute, and characterized by fewer chlamydial organisms. Hence, test performance will change because infections will be more difficult to detect in a population composed largely of patients harboring fewer organisms.
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Author and Article Information
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Harborview Medical Center; Seattle, WA 98104
1. Barnes RC, Katz BP, Rolfs RT, Batteiger B, Caine V, Jones RB. Quantitative culture of endocervical Chlamydia trachomatis. J Clin Microbiol. 1990; 28:774-80.
2. Schachter J, Stamm WE, Chernesky MA, Hook EW, Jones RB, Judson FN, et al. Nonculture tests for genital tract chlamydial infections. What does the package insert mean, and will it mean the same thing tomorrow? Sex Trans Dis. 1992; 29:243-4.
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