Background Young patients with repeat infections of Chlamydia trachomatis(Ct) are a key population for prevention as they indicate ongoing risk for spread and complications in women. We estimated the hidden key population, i.e. missed repeat testers and repeat positives, to effectively focus screening strategies.

Methods Data covered all youngsters (16–29 years,n = 42,894) in Eastern South-Limburg, the Netherlands (2006–2010) including all their genital Ct tests by any care provider. Using logistic regression, determinants (age, sex, socio-economic status (SES)) for not having a repeat test (in positives) and for having a positive repeat test (in repeated testers) were evaluated. Using Geographic-Information-Systems and spatial statistics (SaTScan purely spatial Poisson model, Bivariate Local Moran`s I), spatial clusters and correlations with SES of repeat (positive) tests were evaluated.

Results Overall 10,044 (23.4%) youngsters were tested of whom 944(9.4%) were positive. Of positives, 423(44.8%) had no repeat test (more often older, ORperyear 0.96 95% CI 0.92–1.00, and male, OR2.26 95% CI 1.69–3.02). Of repeat testers, 111(21.3%) were repeat positive. Spatial clusters were found in four municipalities(3 low SES) and low SES correlated with repeat positive tests. We estimate that 230 repeat positives (0.5% of total youngsters) are missed in care. These include 90 (21.3% of 423) who were lost in care for repeat testing follow-up and 140 repeat positives in the 32,850 youngsters who were never tested before (assuming 2.0% positivity and 21.3% repeat positivity). Overall, an estimated 67.4%(230/(230+111)) of all repeat positive patients is thereby missed in current care.

Conclusion Two-thirds of repeat positive patients are hidden to current care, some (–40%) because they missed a repeat test and others (–60%) because they are never tested. As they comprise a central but small part of the total young population, control strategies targeting this key population should be highly acuminated. Geo-spatial analysis, which pointed to low SES high prevalence areas informs more effective Ct control.