Validation of the Pediatric Index of Mortality 3 in a pediatric intensive care unit in Thailand: A prospective, single-center observational study

Purpose The Pediatric Index of Mortality 3 (PIM3) is a widely used severity-of-illness score. This study aimed to validate PIM3 performance in patients aged < 18 years admitted to a Thai pediatric intensive care unit (PICU). Methods This prospective, single-center, observational study included patients aged < 18 years admitted to the PICU between April 1, 2024, and March 31, 2025. PIM3 performance was assessed using the area under the receiver operating characteristic curve for discrimination, the Hosmer–Lemeshow goodness-of-fit test for calibration, and the standardized mortality ratio. Results A total of 617 patients were enrolled, with an observed mortality of 2.43%, lower than the PIM3-predicted mortality of 3.63%. The area under the receiver operating characteristic curve was 0.917 (95% CI 0.842–0.993). The Hosmer–Lemeshow goodness-of-fit test showed no statistically significant difference between observed and predicted mortality (χ² = 7.493; P  = 0.484). The standardized mortality ratio was 0.67 (95% CI 0.39–1.08). Conclusion The PIM3 demonstrated good discrimination and satisfactory calibration in this Thai PICU population aged < 18 years, although it tends to overestimate mortality risk in this setting.