Background: The Psychosis Polyrisk Score (PPS) was developed to characterise exposure to environmental and developmental risk factors for psychosis. In previous studies, the PPS showed promise particularly in discriminating between individuals at clinical high risk for psychosis (CHR-P) and community controls (CC).

Objectives: This study aimed to (1) perform an external validation of the PPS model in a large independent sample and (2) refine the model using expanded predictor data.

Methods: Participants were recruited through the AMP SCZ program: n=1642 CHR-P, n=519 CC. We performed a formal external validation of the PPS original model. The model was then refined incorporating expanded predictor data (perceived loneliness, neighbourhood quality) and increasing the granularity of existing predictors (ethnicity, childhood trauma, parental severe mental illness). Next, we evaluated the refined model through repeated nested cross-validation and compared it to the original model retrained in the new dataset. Discrimination (Harrell's C-index) and calibration (intercept and slope) were used to assess model performance.

Findings: The externally validated PPS original model showed good discrimination (C=0.777, 95% CI 0.756 to 0.797) between CHR-P and CC and good calibration (intercept=0; slope=1.003) after recalibration. The refined PPS model exhibited higher internally cross-validated discrimination than the retrained original model (C=0.876, 95% CI 0.860 to 0.891 vs C=0.796, 95% CI 0.775 to 0.816), as well as good calibration performance (intercept=-0.008; slope=1.031).

Conclusions: The PPS showed good generalisability to a large, independent sample, and performance improved after model refinement.

Clinical implications: The PPS represents a potential scalable tool within stepped assessment frameworks for identifying individuals who warrant further CHR-P assessment.