Reliability of the migraine disability assessment score in a population- based sample of headache sufferers

Background. The Migraine Disability Assessment (MIDAS) score is used to quantify headache-related disability. In a previous study, we showed that the MIDAS score was highly reliable in population-based samples of migraine headache sufferers in two countries. Objectives. To examine the test-retest reliability and internal consistency of the five items comprising the MIDAS score and the overall MIDAS score in a population-based sample of both migraine and nonmigraine headache sufferers. Methods. Using a clinically validated telephone interview, a population-based sample of migraine and nonmigraine headache sufferers was identified in Baltimore, Maryland, USA. A total of 97 migraine cases and 80 nonmigraine subjects completed the MIDAS questionnaire on two occasions an average of 3 weeks apart. The MIDAS score is derived from five questions about missed time from work (or school) and household work (one question each about missed days and days with at least 50% reduced productivity) and missed days of nonwork activities. Results. Among all headache sufferers the test-retest Spearman's correlations of individual MIDAS questions ranged from 0.67 to 0.73. The Spearman's correlation for the MIDAS score (i.e., sum of lost days and reduced effectiveness days in each domain) was 0.84. Cronbach's alpha, a measure of internal consistency, was 0.83. Mean and median item values and the overall MIDAS scores differed between migraine and nonmigraine cases. Even after adjusting for differences in headache frequency, the mean MIDAS scores differed substantially (i.e., 10.3 points) between migraine cases and nonmigraine cases. Conclusions. The reliability and internal consistency of the MIDAS score are high, as tested in a population-based sample of headache sufferers. MIDAS scores are substantially higher in migraine cases than in nonmigraine cases, supporting the validity of the measure.