Understanding relationships between chlamydial infection, symptoms, and testing behavior: an analysis of data from Natsal-3.

Background: Genital chlamydia is the most commonly diagnosed sexually transmitted infection worldwide and can have serious long-term sequelae. Numerous countries invest substantially in testing but evidence for programs' effectiveness is inconclusive. It is important to understand the effects of testing programs in different groups of people. Methods: We analyzed data on sexual behavior and chlamydia tests from 16-to 24-year olds in Britain's third National Survey of Sexual Attitudes and Lifestyles, considering test setting, reason, and result. We conducted descriptive analysis accounting for survey design and nonresponse, and Bayesian analysis using a mathematical model. Results: Most men testing due to symptoms tested in sexual health settings (63%; 95% confidence interval 43%-84%) but most women testing due to symptoms were tested by general practitioners (59%; 43%-76%). Within behavioral groups, positivity of chlamydia screens (tests not prompted by symptoms or partner notification) was similar to population prevalence. Screening rates were higher in women and in those reporting more partners: median (95% credible interval) rates per year in men were 0.30 (0.25-0.36) (0 new partners), 0.45 (0.37-0.54) (1 new partner), and 0.60 (0.50-0.73) (>= 2 new partners). In women, they were 0.61 (0.53-0.69) (0 new partners), 0.89 (0.75-1.04) (1 new partner), and 1.2 (1.0-1.4) (>= 2 new partners). Conclusions: Proportion of testing occurring in sexual health is not a proxy for proportion prompted by symptoms. Test positivity depends on a combination of force of infection and screening rate and does not simply reflect prevalence or behavioral risk. The analysis highlights the value of recording testing reason and behavioral characteristics to inform cost-effective control.