INTRODUCTION AND OBJECTIVE: Preoperative comorbidity influences postoperative outcomes. Comorbidity-driven surgical risk assessment is essential for informed counseling, risk stratification, and clinical research. Existing mortality-focused indices have mixed success at risk-adjustment in children, where mortality is relatively rare compared to adults. Postoperative complications may serve as a better marker of surgical risk. We sought to develop a new multispecialty risk index predicting 30-day postoperative complications in children.METHODS: Children who underwent surgeries from 1/2014 to 9/2015 were identified using MarketScanÒ Research databases. Retrospective review identified preexisting comorbidities and 30-day postoperative complications using ICD-9/10-CM codes. The risk index was derived separately for ambulatory and inpatient surgical patients using logistic regression with backward selection. We compared the performance of the novel index in discriminating postoperative complications vis-a-vis 3 existing comorbidity indices using bootstrapping and area under the receiver operating characteristic (ROC) curve (AUC).RESULTS: We identified 293,212 patients (190,629 ambulatory, 22,633 inpatient). The inpatient cohort was older (median 10 vs. 6 years), with higher rates of general surgery procedures (43.5% vs. 17.2%) and pre-existing medical comorbidities, compared to ambulatory patients. The novel index had the best performance for discriminating postoperative complications for inpatients (AUC 0.76, 95% CI 0.75-0.77) relative to the Charlson Comorbidity Index (CCI, 0.56, 0.56 -0.57), Van Walraven Index (VWI, 0.60, 0.60 -0.61), and Rhee Score (RS, 0.69, 0.68 -0.70). In the ambulatory cohort, the novel index outperformed existing indices, though none was ideal (novel score 0.68, 0.67 -0.68; CCI 0.53, 0.52 -0.53; VWI 0.53, 0.52 -0.53; RS 0.50, 0.49 -0.50).CONCLUSIONS: In both inpatient and ambulatory settings, our novel pediatric comorbidity index demonstrated better performance at predicting postoperative complications than 3 widely used comorbidity indices. This index can be used for risk adjustment for surgical outcomes in pediatric health services research and may be adaptable to clinical settings to facilitate identification of high risk surgical patients.