Background: The prevalence of Chlamydia trachomatis genital infection in the United States population is unknown. Using a new urine test for C. trachomatis, we conducted a pilot survey as part of the National Health and Nutrition Examination Survey III (NHANES III).

Goal: To determine whether the prevalence of chlamydial infection in a convenience sample of NHANES participants was high enough to justify testing for C. trachomatis in a national survey.

Study design: NHANES III, conducted from 1988 to 1994, was based on a stratified multistage probability sample of the United States population. Non-Hispanic blacks and Mexican-Americans were oversampled. Using the ligase chain reaction assay for C. trachomatis, we tested urine from participants 12 to 39 years of age from 10 of the 89 sites of NHANES III. The prevalence of infection was calculated by racial or ethnic group.

Results: We tested 1,144 study participants, of whom 65% were female, 30% were non-Hispanic blacks, and 30% were Mexican-American. Prevalence was higher for non-Hispanic blacks (7%) than for Mexican-Americans (3%) and non-Hispanic whites (2%). Prevalence was higher for women than men in non-Hispanic blacks (7% vs. 6%), Mexican-Americans (5% vs. 2%), and non-Hispanic whites (2% vs. 1%). In 15- to 19-year-old women, prevalence was 13% in non-Hispanic blacks, 11% in Mexican-Americans, and 5% in non-Hispanic whites.

Conclusion: The prevalence of C. trachomatis genital infection was high enough to suggest that a reliable national prevalence estimate could be obtained in a national probability sample survey.