OBJECTIVE: To develop and validate a predictive score to avoid unnecessary screening and prophylactic antibiotic use in abortion clinics by identifying a group of women who are at very low risk for Chlamydia trachomatis (CT) infection. METHODS: This population-based retrospective study includes 1000 women who underwent surgical abortion between January and September 2010. The main outcome measure was the rate of CT infection among women seeking an induced abortion according to sociodemographic and clinical data. The score was developed by using two-thirds of the data set as the derivation sample to identify the strongest predictors of CT. A receiver operating characteristic curve established cutoffs and applied the score to the remaining one-third (validation sample). RESULTS: The rate of CT infection was 6.7%. Three criteria were independently associated with CT: gestation more than 10 weeks (adjusted odds ratio [aOR], 1.96; 95% confidence interval [95% CI], 1.06-3.64), not using contraception (aOR, 2.70; 95% CI, 1.41-5.16), and having 0 or 1 child (aOR, 3.46; 95% CI, 1.34-8.93). The CT score was based on these 3 criteria. The low-risk group was derived from values of the score (probability of CT, 1.3% [95% CI, 0-3.0]). Application of these criteria to the validation data set confirmed the diagnostic accuracy of the score (probability of CT, 0%). Sensitivity was 100% and specificity was 26.9% for the score in the validation data set. When applied to the validation data set, the score avoided 25.4% of CT tests and screened 100% of CT-infected women before surgical abortion. CONCLUSIONS: This easy-to-calculate score may prove useful for avoiding CT test in 25% of patients seeking surgical abortion.