Methods
Data were extracted from computerised health plan databases for women aged 15–44 years enrolled in Group Health Cooperative (GH), a mixed model managed care organisation serving Washington State and Western Idaho. Data on chlamydia testing and test results were collected from inpatient, ambulatory care and utilisation databases for women enrolled in the GH integrated group practice (where enrolees receive the majority of their care through GH) between 1 January 1997 and 31 December 2007. For each enrolee, annual person-years were calculated based on the time she was enrolled in GH.
Chlamydial infection was defined by a positive chlamydia test. Re-infection was defined as a positive chlamydia test that occurred at least 30 days after a previously documented infection. Annual re-infection rates per 100 000 person-years were calculated using three denominators of annual person-years contributed by GH women enrolees aged 15–44 years: (1) all women enrolled; (2) women with a prior documented chlamydial infection during the study period; and (3) women with a prior documented chlamydial infection during the study period who were retested within the subsequent 14 months. Because current guidelines in the USA recommend rescreening approximately 3 months after treatment or at the first healthcare visit in the 12 months following treatment, we used 14 months to allow for time after the initial diagnosis date for treatment and follow-up. For denominator choices #1 and #2, re-infections could occur at any time in the study interval at least 30 days following an initial infection. Year was defined by the year the re-infection was identified, and women could have multiple re-infections. For denominator choice #3, re-infections were restricted to those within 14 months following a previous infection in the study interval. Only the first re-infection was included and year was defined by year of the initial infection.
The re-infection date was defined as the date of the positive result. Because chlamydia is often asymptomatic, this may not be the date when the re-infection actually occurred. Therefore, when evaluating the third denominator choice, we conducted a sensitivity analysis reassigning the date of re-infection to the midpoint between the initial and re-infection test dates (denominator #3a). Analyses were performed in SAS V.9.2 (SAS Institute Inc., Cary, North Carolina, USA). All study protocols received review and approval by the Group Health Institutional Review Board.