BACKGROUND: A reliable, developmentally appropriate and standardized method for assessing postconcussive symptoms (PCS) is essential to accurately determine recovery postconcussion and to effectively manage return to normal activities. The aim of this study was to develop an evidence-based, psychometrically validated approach to determining clinically useful cutoff scores by using a commonly administered PCS measure. METHODS: The current study was a prospective, longitudinal observational study conducted between July 2013 and November of 2015 at a statewide tertiary pediatric hospital. Participants were 120 children (5-18 years of age) presenting to the emergency department with a concussion within 48 hours of injury. PCS were assessed by using the Postconcussion Symptom Inventory (PCSI), acutely, 1 to 4 days postinjury and 2 weeks postinjury. Using comprehensive clinical assessment as gold standard, we assessed the clinical cutoff discrimination ability of PCSI at 2 weeks postinjury by using published approaches, and then varying each approach to optimize their discrimination ability. RESULTS: Existing and potential clinical cutoff scores were explored in predicting delayed recovery. Receiver operating characteristic curve results returned acceptable discrimination and sensitivity when PCSI items increased in severity from preinjury by 1 or more. Compared with a published cutoff score being 3+ items with increased severity, the current study suggests a more stringent cutoff requirement of 2+ is better able to accurately classify symptomatic children. CONCLUSIONS: This study provides the first validated index (2+ items, 1+ severity) of concussion recovery for children and youth. Further studies in more varied samples are needed to establish the effectiveness of this method.