title: Using chlamydia positivity to estimate prevalence: evidence from the Chlamydia Screening Pilot in England.
abstract: Studies have suggested that positivity can be used to estimate the prevalence of Chlamydia trachomatis in large-scale chlamydia screening programmes. A recent pilot of opportunistic screening in England estimated that the prevalence among 16-24-year-old women in Portsmouth and Wirral was 9.8% and 11.2%, respectively. This study assessed the continued validity of positivity as an approximate for prevalence. We re-analysed data from the Chlamydia Screening Pilot to estimate positivity,calculated as total positive tests divided by total tests, and compared these estimates with the previously reported prevalence, measured as the number of women testing positive divided by the total number of women screened. Overall positivity was 9.4% in Portsmouth and 11.0% in the Wirral; these estimates were not statistically different from prevalence, regardless of health-care setting, age group or symptoms. We conclude that positivity can be used as a proxy for prevalence.
authors.affiliation: Health Protection Agency, Communicable Disease Surveillance Centre, HIV and STI Department, 61 Colindale Avenue, London NW9 5EQ, UK. scott.lamontagne@hpa.org.uk
authors.forename: D S
authors.initials: DS
authors.lastname: LaMontagne
authors.forename: K A
authors.initials: KA
authors.lastname: Fenton
authors.forename: J M
authors.initials: JM
authors.lastname: Pimenta
authors.forename: M
authors.initials: M
authors.lastname: Catchpole
authors.forename: P A
authors.initials: PA
authors.lastname: Rogers
authors.forename: S
authors.initials: S
authors.lastname: Randall
authors.forename: W G
authors.initials: WG
authors.lastname: Hewitt
authors.forename: H
authors.initials: H
authors.lastname: Mallinson
authors.forename: G S
authors.initials: GS
authors.lastname: Underhill
authors.forename: L
authors.initials: L
authors.lastname: McLean
authors.forename: T
authors.initials: T
authors.lastname: Gleave
authors.forename: V
authors.initials: V
authors.lastname: Harindra
authors.forename: A K
authors.initials: AK
authors.lastname: Ghosh
authors.forename: J M
authors.initials: JM
authors.lastname: Tobin
mesh_terms: D000293:Adolescent; D000328:Adult; D002690:Chlamydia Infections / Q000175:diagnosis* / Q000453:epidemiology*; D003627:Data Interpretation, Statistical; D005260:Female; D006801:Humans; D008403:Mass Screening; D010865:Pilot Projects; D015995:Prevalence; D015203:Reproducibility of Results; D006113:United Kingdom