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

Abstract

Abstract 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.