Objective To test the interrater reliability, internal consistency and aspects of validity of the myositis damage index (MDI) in the assessment of damage in adult patients with idiopathic inflammatory myopathy (IIM).

Methods 95 patients were assessed in six centres as part of this cross-sectional international study. Two parts of a MDI were used to assess disease damage, the MDI and the myositis damage score (MYODAM). The myositis disease activity assessment tool (MDAAT) was used to assess disease activity. Interrater reliability was assessed using intraclass correlation coefficient (ICC). Spearman's rank correlation coefficient was used to measure the convergent validity of cross-sectional scores between the two parts of the damage tool and to determine the correlation between the respective components of the damage and activity tools.

Results In general, the damage index appears to have good interrater reliability for most of the systems with an ICC greater than 0.65. Convergent validity between the two parts of the damage tool showed good correlation for the individual organ systems (r>0.8). There were weak correlations between some parts of the MDI and corresponding components of the MDAAT.

Conclusion The MDI is a comprehensive tool to assess damage in patients with myositis. With physician education and emphasis to record items that have been diagnosed since the myositis diagnosis, the MDI will provide a valuable tool to assess damage in future clinical trials and longitudinal studies.