title: Chlamydia positivity versus prevalence. What's the difference?
abstract: 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.
  
  
  OBJECTIVE
  To assess how well positivity can estimate prevalence in family planning and sexually transmitted disease (STD) clinic settings.
  
  
  METHODS
  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.
authors.affiliation: Division of Sexually Transmitted Diseases Prevention, Centers for Disease Control and Prevention, Atlanta, Georgia 30333, USA.
authors.forename: L W
authors.initials: LW
authors.lastname: Dicker
authors.forename: D J
authors.initials: DJ
authors.lastname: Mosure
authors.forename: W C
authors.initials: WC
authors.lastname: Levine
mesh_terms: D002690:Chlamydia Infections / Q000175:diagnosis / Q000453:epidemiology*; D002692:Chlamydia trachomatis; D005193:Family Planning Services; D005260:Female; D006801:Humans; D015995:Prevalence