title: Timing of progression from Chlamydia trachomatis infection to pelvic inflammatory disease: a mathematical modelling study.
abstract: BACKGROUND
  Pelvic inflammatory disease (PID) results from the ascending spread of microorganisms from the vagina and endocervix to the upper genital tract. PID can lead to infertility, ectopic pregnancy and chronic pelvic pain. The timing of development of PID after the sexually transmitted bacterial infection Chlamydia trachomatis (chlamydia) might affect the impact of screening interventions, but is currently unknown. This study investigates three hypothetical processes for the timing of progression: at the start, at the end, or throughout the duration of chlamydia infection.
  
  
  METHODS
  We develop a compartmental model that describes the trial structure of a published randomised controlled trial (RCT) and allows each of the three processes to be examined using the same model structure. The RCT estimated the effect of a single chlamydia screening test on the cumulative incidence of PID up to one year later. The fraction of chlamydia infected women who progress to PID is obtained for each hypothetical process by the maximum likelihood method using the results of the RCT.
  
  
  RESULTS
  The predicted cumulative incidence of PID cases from all causes after one year depends on the fraction of chlamydia infected women that progresses to PID and on the type of progression. Progression at a constant rate from a chlamydia infection to PID or at the end of the infection was compatible with the findings of the RCT. The corresponding estimated fraction of chlamydia infected women that develops PID is 10% (95% confidence interval 7-13%) in both processes.
  
  
  CONCLUSIONS
  The findings of this study suggest that clinical PID can occur throughout the course of a chlamydia infection, which will leave a window of opportunity for screening to prevent PID.
authors.affiliation: Institute of Social and Preventive Medicine, University of Bern, Bern CH-3012, Switzerland. herzog.sereina@gmail.com
authors.forename: Sereina A
authors.initials: SA
authors.lastname: Herzog
authors.forename: Christian L
authors.initials: CL
authors.lastname: Althaus
authors.forename: Janneke Cm
authors.initials: JC
authors.lastname: Heijne
authors.forename: Pippa
authors.initials: P
authors.lastname: Oakeshott
authors.forename: Sally
authors.initials: S
authors.lastname: Kerry
authors.forename: Phillip
authors.initials: P
authors.lastname: Hay
authors.forename: Nicola
authors.initials: N
authors.lastname: Low
mesh_terms: D000293:Adolescent; D002692:Chlamydia trachomatis / Q000472:pathogenicity*; D005260:Female; D006801:Humans; D008219:Lymphogranuloma Venereum / Q000150:complications* / Q000473:pathology*; D008962:Models, Theoretical; D000292:Pelvic Inflammatory Disease / Q000473:pathology*; D016032:Randomized Controlled Trials as Topic; D055815:Young Adult