Demographic and behavioural profile of adults infected with chlamydia: a case-control study.

OBJECTIVES: To determine which demographic and behavioural parameters are independently associated with chlamydial infection in adults. METHODS: Subjects were recruited prospectively from male and female attendees at a large clinic for sexually transmitted infections (STI). All subjects were tested for chlamydia and gonorrhoea and asked to complete a questionnaire addressing demography, sexual and non-sexual (including drug taking) behaviour, and history of STI. Cases were those attending with a new clinical episode and found to be infected with chlamydia, but who did not have gonorrhoea. A control group was selected randomly from those found to be negative on screening for both infections. RESULTS: 986 cases and 1212 controls were recruited over one calendar year. The following were found to be independent risk factors for chlamydial infection on multivariate analysis (odds ratios with 95% confidence intervals in parentheses): being unmarried (1.8; 1.1-3.1); black Caribbean ethnicity (2; 1.5-2.7). Increasing age, fewer partners, and higher reported use of condoms were associated with a lower risk of infection. CONCLUSION: Black Caribbeans are at increased risk from chlamydia after controlling for sexual behaviour and socioeconomic status. Future research should seek an explanation elsewhere-for example, in terms of differences in sexual mixing or effectiveness of healthcare interventions.