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Combining Two Developmental Screening Tests to Improve Predictive Accuracy. OBJECTIVE: Single developmental screening tests are insufficient for supporting clinical decisions, showing less than 50% of those referred are actually delayed. Combining two tests can address this problem. We used likelihood ratios (LR) to evaluate the predictive value (PV) of combining the Ages and Stages Questionnaires (ASQ) and Parents Evaluation of Developmental Status (PEDS) screening tests. METHODS: We used data from Limbos and Joyce that included information on the ASQ, PEDS, and an age-appropriate diagnostic assessment of delay on 328 children (ages 12-60 months) drawn from primary care. LRs were calculated for each of the available ASQ/PEDS combinations then rank ordered. PV was examined using different LRs to define referral and nonreferral thresholds. RESULTS: Setting the referral threshold at LR 2.0 for combined scores decreased the number of false positives from 66 to 28, increased false negatives from 6 to 9 with positive predictive value (PPV) improving significantly from 30% to 47% (P < .05). Sensitivity dropped from 82% to 74% and specificity improved from 72% to 90%. When the LR referral threshold increased, PV improved but an inconclusive group emerged that required more information for decision-making. CONCLUSIONS: Combining the ASQ and PEDS significantly improved predictive accuracy of a referral in a single threshold analysis. Predictive value improved further as requirements for referral increased and an inconclusive group needing further study was recognized. Systematic research is needed to identify what readily available information will assist decision making in this group.