The Myositis Activities Profile — Initial Validation for Assessment of Polymyositis/Dermatomyositis in the USA

Abstract

To evaluate some measurement properties of the Myositis Activities Profile (MAP) in adult patients with polymyositis (PM) and dermatomyositis (DM) in the United States. To assess content validity, patients with PM/DM rated difficulty and importance of items of the MAP using a visual analog scale (VAS), range 0-10. For construct validity, consecutive patients with PM/DM performed the 6-item core set for disease activity including the manual muscle test (MMT) and the Health Assessment Questionnaire (HAQ), the Functional Index-2 (FI-2; muscle endurance), and the MAP plus disease effect on well-being on a VAS. Item fit within subscales was analyzed by Cronbach's alpha. Patients with stable disease activity filled out the MAP again 1 week later. The median combined difficulty and importance, 0-10, of the 31 items was 5.00 (range 2.10-5.95). One item was added, giving a 32-item MAP. Correlations between the median of subscales/single items of the MAP and the HAQ and disease effect on well-being were r(s) = 0.69 and r(s) = 0.68, respectively, with lower correlations to the MMT (r(s) = -0.35), and the FI-2 (r(s) = -0.29 to -0.47) and disease activity measures (r(s) = 0.36-0.41). Cronbach's alpha coefficients for the 4 subscales varied between 0.85 and 0.95. Weighted kappa coefficients (K(w)) ranged between 0.77 and 0.93 for the 4 subscales and between 0.74 and 0.83 for the 4 single items without systematic variations (p > 0.05). This initial validation of the MAP indicates promising measurement properties for assessing limitations in activities of daily living and participation in patients with PM/DM in the United States.