Determination of Perceived Levels of Physical Work Demand Thresholds, and Reliability and Responsiveness of the Modified-Spinal Function Sort Questionnaire in a Multidisciplinary Occupational Rehabilitation Setting.

PURPOSE To determine the levels of perceived work demand capacity corresponding to the Modified Spinal Function Sort (M-SFS) score and precise reliability validity and responsiveness. METHODS This prospective validation study included patients with chronic musculoskeletal impairments who underwent multidisciplinary occupational rehabilitation. After determining the percentiles of the work demand thresholds corresponding to the spinal function sort (SFS), the percentiles were transposed to the M-SFS. Reliability was assessed using the intraclass correlation coefficient and limits of agreement. Correlations with other questionnaires and a lifting task were measured to assess validity. Responsiveness was determined using anchor- and distribution-based approaches. RESULTS 288 patients were included. The following thresholds were obtained for the M-SFS: 0-43 points, minimal; 44-50, very light; 51-58, light; 59-64, light to medium; 65-70, medium; 71-76, heavy; and 77-80, very heavy. Reliability was confirmed. The correlation between the M-SFS and SFS scores was good at 0.89 (95% CI, 0.86-0.91) and moderate according to the PILE-test result of 0.60 (95% CI, 0.50-0.67). We could not calculate a valid anchor-based minimal clinically important difference. The standard error of measurement was 3.9 points, and the smallest detectable change was 10.8 points. CONCLUSIONS On the basis of the comparison of the M-SFS and SFS scores, the M-SFS score can be interpreted in relation to the levels of work demand. This study confirms the good reliability and validity of the M-SFS questionnaire in assessing perceived physical capacity. Further studies are needed to determine its responsiveness.