Construct Validity and Validity to Change of the Patient-Specific Functional Scale in Patients with Shoulder and Low Back Pain: A Clinimetric Study

Background: Patient-specific and condition-specific measures are widely used in clinical practice and research to measure disability or change over time. While condition-specific outcome measures comprise a range of restrictions generally relevant for all patients, the Patient-Specific Functional Scale measures restrictions chosen by the individual patient. Objectives: Based on the hypothesis that patient-specific and conditionspecific scales deliver comparable results when used on group level. The aim of this study was to test for floor and ceiling effects, to evaluate construct validity and validity to change of the Patient-Specific Functional Scale when compared to condition-specific outcome measures. For this purpose, two datasets from patients with shoulder pain and low back pain were analyzed. Methods: Patient-Specific Functional Scale scores were compared to the Shoulder Pain and Disability Index and the Roland Morris Disability Questionnaire at 4 time-points using stem-and-leaf-plots and correlations using Pearson’s r. Hypothesis-driven correlation levels for data interpretation were predefined, with r ≥0.75=high, r ≥0.5=moderate, r ≥0.25=low. Results: Patient-Specific Functional Scale floor effects were comparable to condition-specific outcome measures in both samples. At none of the timepoints did the Patient-Specific Functional Scale correlate with the conditionspecific outcome measures in the expected manner. Conclusion: Hypotheses regarding expected ranges of correlation between the Patient-Specific Functional Scale and the condition-specific outcome measures for construct validity and validity to change were not met. While the use of the Patient-Specific Functional Scale in a clinical context has its advantages, the measure is not recommended for group-level evaluations.