Chlamydia positivity versus prevalence -: What's the difference?

Background: Data on chlamydia screening collected as part of Regional Infertility Prevention Projects often do not include personal identifiers, therefore repeat tests for patients during a year cannot be identified. Consequently, positivity is calculated and used to monitor chlamydia prevalence. Goals: To assess how well positivity can estimate prevalence in family planning and sexually transmitted disease (STD) clinic settings. Study Design: Analyzed data from chlamydia screening programs in three geographic areas of the United States that used unique patient identifiers. Results: The relationship between positivity and prevalence is related to both the percentage of tests that are repeat tests and the percentage of repeat tests that are positive. On average, the percentage of positive repeat tests was the same as or higher than prevalence in family planning clinics; thus, positivity was the same as or higher than prevalence, In STD clinics, the percentage of positive repeat tests was consistently lower than prevalence; thus, positivity underestimated prevalence, However, the absolute difference between positivity and prevalence was less than 0.5% in family planning and STD clinics. Conclusions: Positivity can be used to monitor chlamydia prevalence in women screened in family planning and STD clinic settings.