Short Form-36 psychometric properties in idiopathic inflammatory myopathies: reliability, validity and responsiveness

OBJECTIVE: To evaluate the psychometric properties of Short Form-36 (SF-36) as a health-related quality of life (HRQoL) measure in idiopathic inflammatory myopathies (IIMs). METHODS: Patients fulfilling the EULAR/ACR classification criteria or with myositis-specific antibodies were enrolled from two clinical trials (Tocilizumab in Myositis and Abatacept in Myositis) and one prospective observational study. Data collection at 0, 3 and 6 months included all myositis core set measures (CSMs) including manual muscle testing-8 (MMT-8), and SF-36 summary scores and all eight subdomains including physical component summary score (PCS) and physical function. Test-retest reliability (Pearson correlation), validity (association with other valid measures) and responsiveness using clinically important difference (CID) and effect size were calculated. RESULTS: The study included 105 IIM patients (44% DM, 30% anti-synthetase syndrome, 18% PM, 8% necrotizing myopathy), with mean age: 52.3 years, 64% females and 85% White. HRQoL scores were significantly lower in IIM compared with the general population. All SF-36 domain scores were significantly lower in patients with active disease and muscle weakness. All SF-36 components demonstrated good test-retest reliability at 1 month (P < 0.001), with significant baseline and longitudinal associations with most myositis CSMs. Changes in MMT-8 strongly correlated with changes in PCS and physical domains. PCS illustrated significant and concordant change with 2016 ACR/EULAR myositis response criteria and physician/patient assessments of change, showing a large effect size. We present preliminary CIDs based on a cohort with minimal clinical change. CONCLUSION: SF-36 demonstrates good reliability, validity and responsiveness in IIM and should be utilized for assessing HRQoL in clinical practice and myositis trials.