External validation of the Prehospital Return of Spontaneous Circulation (P-ROSC) score for predicting prehospital return of spontaneous circulation among patients with out-of-hospital cardiac arrest

• We validated the P-ROSC score using a nationwide Japanese OHCA registry. • The P-ROSC score showed acceptable discriminative ability and moderate calibration for predicting prehospital ROSC. • Its predictive performance varied across regions, supporting its use with attention to regional heterogeneity. The Prehospital Return of Spontaneous Circulation (P-ROSC) score was developed using the Pan-Asian Resuscitation Outcomes Study registry to predict prehospital ROSC. Although external validation studies have been conducted in other Asian countries, their results have been inconsistent. This study aimed to externally validate the P-ROSC score using a nationwide Japanese registry and assess its performance across geographic regions. We analysed out-of-hospital cardiac arrest (OHCA) cases of medical origin registered in the All-Japan Utstein Registry between 2015 and 2020. The primary outcome was prehospital ROSC, defined as ROSC at the scene or during transport. Model discrimination was evaluated using the area under the receiver operating characteristic curve (AUC), and calibration was assessed using calibration plots. Subgroup analyses were conducted across eight regions of Japan. A total of 493,695 OHCA cases were included, with a median age of 81 years, and 56.5% were men. Overall, 46,760 patients (9.5%) achieved prehospital ROSC. The AUC of the P-ROSC score based on the predefined model was 0.794 (95% confidence interval [CI]: 0.792–0.796). The calibration plot showed moderate agreement between predicted and observed outcomes. The region-specific AUCs varied across Japan, with the highest AUC in Tohoku (0.822, 95% CI: 0.816–0.828) and the lowest in Hokkaido (0.772, 95% CI: 0.764–0.780). The P-ROSC score demonstrated fair discrimination and moderate calibration for predicting prehospital ROSC in Japanese patients with OHCA. Its application should consider regional variations in patient characteristics and emergency medical service structures that may influence transportation and generalisability.