The effectiveness of selective screening for control of Chlamydia trachomatis is unknown. In 1986, a statewide screening program began in family planning clinics in Wisconsin after the prevalence of infection among women was found to be 10.7% in four nonurban clinics and 13.7% in an urban Milwaukee clinic. In 1990, endocervical specimens were obtained from 1,757 women attending these same clinics; 5.2% of women in the non-urban clinics and 6.9% in the Milwaukee clinic tested positive for C. trachomatis. Prevalence of infection had decreased similarly (by 53% overall) in both high- and low-risk groups in all five clinics. Although reported condom use increased from 16% to 31%, most other demographic and behavioral risk factors for infection did not significantly change; in contrast, the prevalence of clinical signs of infection decreased. The percentage of infections identified by selective screening criteria decreased from 77% to 55%. Selective screening and attendant activities, as well as an increase in condom use, were associated with a decrease in prevalence of C. trachomatis infection in this population.

In Reply.— We appreciate the comments of Drs Addiss and Davis and Mr Vaughn. We contend that if the clinician is interested in selective screening, then he or she needs to be able to estimate a patient's individual probability of infection to decide whether to test. Although our model's performance is less than ideal, advantages over previous attempts at risk stratification include better discrimination and the ability to stratify patients into multiple risk groups, and reliability verified by prospective validation. We support the work of these authors in developing a selective screening program. We agree that one must know the local prevalence of disease to use the model to estimate probability and that the thresholds of risk that should guide decision making and screening policy remain unknown. Eventually the development of credible thresholds and generalizable diagnostic models may lead to both more effective and less costly screening programs.

To determine prevalence and risk factors for endocervical Chlamydia trachomatis infection in an urban midwestern population and to evaluate two non-culture direct tests for C. trachomatis, we studied 849 women attending two family planning clinics and a community health clinic in Milwaukee, Wisconsin. Adequate endocervical specimens were obtained from 751 women for chlamydial isolation in tissue culture and antigen tests using direct fluorescence (DFA) and enzyme immunoassay (EIA); 93 (12.4%) patients had cultures positive for C. trachomatis. Compared to culture, the DFA test had a 77.4% sensitivity, 96.8% specificity, and a predictive value positive (PVP) of 77%. For the EIA, these values were 83.9%, 97.0%, and 80%, respectively. No single historical, clinical, or laboratory variable, including the previously described cervicitis index and specific cytologic findings on Pap smear, had sufficient predictive value to be used as the only criterion for selective screening in this population. Criteria for selective screening were proposed that would result in screening 43% of patients and would identify 71% of infections. PVP of both non-culture tests was 89% in persons identified by these criteria to be at increased risk of C. trachomatis infection.

Chlamydia trachomatis infections constitute the most prevalent bacterial sexually transmitted disease (STD) in Wisconsin. In 1987, chlamydia became Wisconsin's most frequently reported STD. Several significant clinical syndromes have been associated with chlamydia infection. Prevention and control of STD needs to become a basic part of primary care within the private medical setting. Determination of the need to test a patient for chlamydia infection cannot be based solely on the presence of signs or symptoms. Physicians in private practice need to identify high-risk patients and selectively screen high-risk men and women, rapidly initiate treatment of chlamydia-infected patients and their sexual partners, and work closely with public health personnel as part of disease intervention.

To determine the prevalence of, and identify risk factors for, Chlamydia trachomatis infection, we studied 380 women attending four Wisconsin family planning clinics in October 1985. The patients completed self-administered sexual history questionnaires, were examined by nurse clinicians and had specimens taken for direct fluorescent antibody (DFA) testing for C. trachomatis. Of 335 women with adequate specimens, 10.7 percent had positive DFA tests. Selective screening criteria were developed based on the following risk factors for C. trachomatis: Age less than 20 years and recent exposure to either a new sexual partner or a partner with more than one partner; a partner with symptoms of urethritis; a diagnosis of cervicitis; and inflammatory changes on Pap smear. Thirty-six percent of patients met one or more of these screening criteria, and the criteria had a sensitivity of 72 percent.