Clinical utility of a quick and easy-to-use international tool for assessing and identifying impaired physical fitness in people with severe mental illness - The PsychiActive project.

BACKGROUND: Severe mental illness (SMI) is a leading cause of mortality, disability, and frailty, largely driven by physical multimorbidity. Physical fitness is a strong predictor of health outcomes, yet routine assessment is rarely implemented in SMI due to limited resources. Self-reported tools may offer a feasible alternative, although their validity and clinical utility remain unclear. This study evaluated the validity, reliability, and clinical utility of the International Fitness Scale (IFIS) in SMI. METHODS: In a multicenter cross-sectional study, 234 adults with SMI (18-65 years, 62 females) completed the IFIS and a battery of objective physical fitness tests. Convergent validity was assessed using ANOVA and ANCOVA. Test-retest reliability over two weeks was assessed using weighted kappa and percentage agreement. Clinical utility was evaluated using area under the curve, sensitivity, specificity, and positive predictive value. RESULTS: The IFIS effectively discriminated objective physical fitness across all domains and response categories in adults with severe mental illness (p < 0.005), with lower self-reported fitness consistently associated with poorer objective performance. Test-retest reliability was fair for the five-response scale (κ = 0.22-0.32) and improved after category reduction (κ = 0.26-0.50). The highest positive predictive value was observed for muscular strength (75%) and cardiorespiratory fitness (46%). The IFIS showed moderate discriminatory accuracy for cardiorespiratory fitness and flexibility (AUC = 0.64 for both domains). Mean completion time was 1.8 ± 1.2 min. CONCLUSIONS: The IFIS is a valid, reliable, and time-efficient tool for identifying impaired physical fitness in adults with SMI.