Disease activity score for children with juvenile dermatomyositis: reliability and validity evidence

OBJECTIVE: To assess the reliability and validity of the Disease Activity Score (DAS), an instrument used to evaluate children with juvenile dermatomyositis (JDM). METHODS: Psychometric study of internal consistency, reliability, rater agreement, and the relationship with measures of muscle strength and disability was conducted. RESULTS: The DAS ratings are internally consistent (reliability = 0.89) and describe a wide range of disease activity. The pediatric rheumatologists in this study agree on the presence of most of the disease indicators. Their disagreements tend to cancel each other, resulting in highly correlated (r = 0.79) overall measures across raters. Estimates of muscle weakness using the DAS and ratings of muscle strength obtained independently from therapists are highly related (r = -0.77), but estimates of disease activity and disability are weakly related (r = 0.20). CONCLUSION: The DAS exhibits evidence of good reliability and validity. The combination of skin and muscle strength assessments makes this easily administered instrument a useful addition in the evaluation of children with JDM.