Reliability and validity of the Shoulder Pain and Disability Index in a sample of patients with frozen shoulder

Background: The Shoulder and Pain Disability Index (SPADI) is a widely used outcome measure. The aim of this study is to explore the reliability and validity of SPADI in a sample of patients with idiopathic frozen shoulder. Methods: The SPADI was administered to 124 patients with idiopathic frozen shoulder. A sub-group of 29 patients were retested after 7 days. SPADI scores were correlated with other outcome measures (i.e., Disabilities of the Arm, Shoulder and Hand Questionnaire - DASH; Numerical Pain Rating Scale-NPRS; and 36-item Short Form Health Survey-SF-36) to examine construct validity. Structural validity was assessed by a Two-Factors Confirmatory Factor Analysis (CFA). Internal consistency, test-retest reliability, and measurement error were also analyzed. Results: The construct validity was satisfactory as seven out of eight of the expected correlations formulated (≥ 75%) for the subscales were satisfied. The CFA showed good values of all indicators for both Pain and Disability subscales (Comparative Fit Index = 0.999; Tucker-Lewis Index = 0.997; Root Mean Square Error of Approximation = 0.030). Internal consistency was good for pain (α = 0.859) and disability (α = 0.895) subscales. High test-retest reliability (Intraclass correlation coefficient [ICC]) was found for pain (ICC = 0.989 [95% Confidence Interval (CI = 0.975-0.995]) and disability (ICC = 0.990 [95% CI = 0.988-0.998]). Standard Error of Measurement values of 2.27 and 2.32 and Minimal Detectable Change values of 6.27 and 6.25 were calculated for pain and disability subscales, respectively. Conclusion: The SPADI demonstrated satisfactory reliability and validity properties in a sample of patients with idiopathic frozen shoulder.