Estimated effect of reduced asymptomatic testing on missed

Background: Widespread testing for asymptomatic chlamydia is under debate. In the Netherlands, from 2025, Methods: We included consultations of women and heterosexual men (≥16 years) tested for CT at the CSH Amsterdam, the Netherlands, 2015-2023. We assessed consultations where no test would have been performed under the new guideline (neither symptoms nor notified), including CT diagnosed during those consultations (=undetected CT). Determinants of undetected CT were assessed using multivariable logistic regression with cluster-robust standard errors. Analyses were stratified by age (<25/≥25 years) and gender (women/men). Findings: In total 241,005 consultations among 124,527 individuals were analysed. Under the new guideline, CT testing would not have been performed in 167,476 (69.5%, 95% confidence interval [CI]; 69.3-69.7%) consultations; mostly among women <25 years (n = 100,756/167,476, 60.2%). In total 17,771/32,360 (54.9%, 95% CI; 54.4-55.5%) CT cases would have remained undetected and untreated: 12,092 among women <25 years, 1498 among women ≥25 years, 3428 among men <25 years, and 753 among men ≥25 years. Determinants of undetected CT were younger age, no/primary/secondary education, and recent bacterial STI. Interpretation: The new guideline limiting CT testing might lead to a more than twofold reduction in diagnoses and antibiotic prescriptions in women and heterosexual men. Surveillance of STIs and associated complications is essential to monitor unintended consequences of the guideline change. Funding: None.