The Myositis Activities Profile -- initial validation for assessment of polymyositis/dermatomyositis in the USA

• To evaluate some measurement properties of the Myositis Activities Profile (MAP) in adult patients with polymyositis (PM) and dermatomyositis (DM) in the United States.Methods.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.Results.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 rs = 0.69 and rs = 0.68, respectively, with lower correlations to the MMT (rs = −0.35), and the FI-2 (rs = −0.29 to −0.47) and disease activity measures (rs = 0.36–0.41). Cronbach’s alpha coefficients for the 4 subscales varied between 0.85 and 0.95. Weighted kappa coefficients (Kw) 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).Conclusion.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.

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