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

Abstract

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.