Assessment of disability in idiopathic inflammatory myopathy: a call for linearity

Objectives To evaluate the clinimetric properties of the Academic Medical Centre Disability Score (ALDS) in patients with idiopathic inflammatory myopathy (IIM). Methods We used prospectively collected data of IIM patients who completed a phase-2 study with first-line IVIG monotherapy. The ALDS is a patient-reported questionnaire which contains 25 items relevant for disability in myositis. ALDS and all core set measures (CSMs) for myositis [including HAQ-Disability Index (HAQ-DI)] were evaluated at baseline and 9 weeks follow-up. In addition, the 2016 ACR/EULAR myositis response criteria outcome called Total Improvement Score (TIS) was evaluated at 9 weeks. We examined floor/ceiling effects, reliability and construct validity of the ALDS. To examine known-group validity, ALDS change scores over time were compared with TIS and physician impression of clinical response. Results Nineteen patients with IIM [median age 59 years, 12 (63%) female] were enrolled. At baseline, ALDS showed a median score of 65.4 (IQR 58.2-73.5), good Cronbach's alpha (alpha = 0.84) and a small ceiling effect (11%). Construct validity was confirmed by moderate to strong correlations between ALDS and HAQ-DI [r(s) = -0.57 (baseline); -0.86 (follow-up)]. ALDS change score correlated with TIS (r(s) = 0.70), discriminated between responders and non-responders (TIS >= 40; P = 0.001), between groups based on physician impression of clinical response (P = 0.03), and detected deterioration. Conclusion The ALDS showed promising clinimetric properties and detected relevant changes in disability in patients with myositis. These results warrant further investigations.