Commentary: Chlamydia trachomatis screening: what are we trying to do?

Chlamydia trachomatis is a sexually transmitted infection that can cause pelvic inflammatory disease (PID) which can lead to infertility and ectopic pregnancy. Infection is often asymptomatic, detectable with a urine test, and cured by single dose therapy. These characteristics are similar to another infection Neisseria gonorrhoeae. In the US, a N. gonorrhoeae control programme began in the mid-1970s, and between 1975 and 1996, rates of N. gonorrhoeae fell from 467.7 to 121.8 per 100 000 population. The prevalence of C. trachomatis infection among persons who were tested (test positivity) fell when C. trachomatis screening was introduced in Sweden, British Columbia, and the Northwestern United States. A study found screening was associated with lower rates of PID, and several systematic reviews concluded that sexually active women under age 25 years should be screened for C. trachomatis every year. Now Low et al., in another review of the evidence conclude that ‘there is an absence of evidence supporting opportunistic chlamydia screening . . . the most commonly recommended approach.’ What is going on? Rates of reported C. trachomatis infection have since increased in Sweden, and British Columbia. The prevalence among persons tested has also increased in the Northwestern United States, and did not fall when C. trachomatis screening was introduced in nine other regions. Trends in PID are difficult to evaluate because the diagnosis is non-specific and there are other causes of PID, but rates appeared to decrease among inpatients (by 68%) and outpatients (by 47%) between 1985 and 2001 in the United States. Most of this decrease occurred by 1995, before screening recommendations were published. Thus, in the present context, it is appropriate to reexamine the evidence, and remind ourselves what we are trying to do. PID could be prevented by either preventing C. trachomatis infection in the first place, or by curing infections before they progress to PID. This distinction is important. If screening is intended to interrupt progression, then, presumably, treating recently acquired infections would prevent more PID than treating longstanding infections. Screening routinely once per year would mean the average asymptomatic infection would be about 6 months old. Screening high-risk women more frequently could decrease the average duration of infection and increase the likelihood of preventing PID. For example, women who are diagnosed with a C. trachomatis infection could be asked to return in 3 months for rescreening. If lowering the incidence of C. trachomatis is important, then screening and treatment should be done in a way that lowers the likelihood of new infection. If everyone is screened annually, with screening spread evenly across 250 working days in a year, every day 0.4% of all infections in a community would be cured. That small drop in prevalence leaves plenty of opportunity for re-infection of persons who were treated or new infection in previously uninfected persons. If everyone were screened and treated on the same day, the risk of re-infection or new infection would be considerably less. Alternatively, screening could focus on likely sexual networks, or screening programmes could emphasize treatment of past partners and screening of future partners. Low et al. found two studies that evaluated screening to cure infection before it progressed to PID. Scholes et al. screened 645 women, treated 44 for C. trachomatis infection, and after 1 year of follow-up, the group had 12 fewer cases of PID than expected based on the comparison group of unscreened women. This study did not address the prevention of incident infections. Curing 44 infections among the 36 547 women in the health maintenance organization (with partners that were not screened and were not necessarily part of the same health plan) is unlikely to influence the incidence. Nevertheless, these findings suggest that treating four infected women will prevent one case of symptomatic PID. Ostergaard et al. used a different definition of PID, but had similar success; testing 867 female students, detecting 43 infections and preventing nine reported * Corresponding author. Division of STD Prevention, Centers for Disease Control and Prevention, Atlanta, GA 30333, USA. E-mail: tap1@cdc.gov Division of STD Prevention, Centers for Disease Control and Prevention, Atlanta, GA 30333, USA. Published by Oxford University Press on behalf of the International Epidemiological Association 2009