The Patient Specific Functional Scale - Brazil as an instrument for the functional assessment of patients with chronic non-specific low back pain: Construct validity (hypothesis testing and structural validity) and test-retest reliability

The objective was to assess the validity and reliability of the Patient-Specific Functional Scale-Brazil (PSFS-B) in patients with non-specific chronic low back pain. A methodological study was conducted, and 101 volunteers with chronic non-specific low back pain were interviewed and asked to complete the PSFS-B questionnaire and the Oswestry Disability Index-Brazil 2.0 reference questionnaire. The Oswestry Disability Index-Brazil 2.0 is a standardized tool used to assess functional disability of the lumbar spine. A new assessment (follow-up) was conducted one week later using the same instruments. The COSMIN checklist was utilized as a guide. Two questionnaires were employed to assess patients: the Patient-Specific Functional Scale-Brazil (PSFS-Br) and the Oswestry Disability Index-Brazil 2.0 (ODI-Br). Data were analyzed to assess construct validity (hypothesis testing and structural validity), internal consistency, and test-retest reliability. The statistical methods employed included Pearson's correlation coefficient, confirmatory factorial analysis, Cronbach's alpha, and intraclass correlation coefficient. The results of the Patient-Specific Functional Scale-Brazil demonstrated a moderate negative correlation with the ODI-Br in hypothesis testing (r = −0.691 and r = −0.754) and in structural validity (Comparative Fit Index (0.986), Tucker-Lewis Index (0.958), Root Mean Square Error of Approximation (0.194), and Standardized Root Mean Square Residual (0.134). Furthermore, the instrument demonstrated excellent internal consistency (α = 0.951) and test-retest reliability (intraclass correlation coefficient = 0.978), and was additionally validated by the Bland-Altman plot (0.125). In conclusion, the Patient-Specific Functional Scale-Brazil is a valid and reliable tool for the evaluation and follow-up of patients with non-specific chronic low back pain.