Background Recent studies using setting-specific health insurance or clinical datasets have demonstrated the importance of considering sexual activity when calculating Chlamydia incidence and screening rates, particularly for adolescents. Using data from a provincial adolescent health survey we assessed the impact of adjusting for sexual activity on population-based Chlamydia incidence and screening rates among adolescents in British Columbia (BC), Canada.

Methods We estimated the proportion of adolescent males (15–18 years) and females (14–18 years) who had ever had sexual intercourse (i.e., sexually active) using data from a cluster-stratified survey of public school students (Grades 7–12) completed by ∼30,000 BC students in 2003 and 2008. Using provincial Chlamydia surveillance and testing data we compared adolescent Chlamydia screening and incidence rates in BC by age and gender, using total and sexually active populations as denominators.

Results During these time periods, an estimated 32% and 33% of males 15–18 years and 28% and 31% of females 14–18 years were sexually active in 2003 and 2008 respectively. Regardless of denominator used, screening and incidence rates increased with age, and were higher among females compared to males. Sexually active incidence and screening rates were consistently higher with a more pronounced impact at younger ages. For example, in 2008 screening rates among 14 year old females were 26.2% vs 2.5% in sexually active and total populations respectively, while the corresponding rates among 18 year old females were 60.2% vs 28.9% (2.1 times higher).

Conclusions Using data representing the entire population of BC adolescents we demonstrated that without adjustment for sexual behaviour, adolescent Chlamydia incidence and screening rates are substantially under-estimated, particularly at younger ages. Adjusting for sexual behaviour using population survey data is essential for accurately monitoring the population impact of prevention and screening programmes among adolescents.