Reliability, Minimum Detectable Change and Construct Validity of the Functional Rating Index in Italian Patients with Chronic Non-Specific Low Back Pain

Background and Objectives: To assess the reliability and construct validity of the Functional Rating Index (FRI) in Italian-speaking individuals with chronic non-specific low back pain (CLBP), in order to improve assessment and clinical management in this population. Materials and Methods: This cross-sectional study consecutively enrolled 75 individuals with CLBP (52 females; mean age 48.71 ± 19.18 years; mean pain duration 298.64 ± 427.52 weeks). Internal consistency and test–retest reliability were evaluated using Cronbach’s α and the intraclass correlation coefficient [ICC2,1], respectively, while measurement error was estimated through the minimum detectable change (MDC). Construct validity was examined by testing a priori hypotheses through correlations (Pearson’s r) between the FRI and disability measures (Roland–Morris Disability Questionnaire, RMQ; Oswestry Disability Index, ODI), pain intensity (Numerical Rating Scale, NRS), and quality of life (Short-Form Health Survey, SF-36). Results: Cronbach’s α was 0.88, and test–retest reliability showed an ICC2,1 of 0.86 (95%CI: 0.82–0.93). The MDC was 18.05, corresponding to approximately 20% of the total score. The Italian FRI demonstrated strong correlations with the RMQ (r = 0.70) and ODI (r = 0.77), and a moderate correlation with the NRS (r = 0.60). The physical and social domains of the SF-36 showed stronger negative correlations with the FRI than the mental and emotional domains. Conclusions: The Italian version of the FRI is a reliable and valid instrument for individuals with CLBP and is recommended for both clinical practice and research applications.