Observer disagreement in rating neurologic impairment in multiple sclerosis: facts and consequences.

The interobserver variability of the expanded disability status scale (EDSS) was studied in 59 patients with multiple sclerosis (MS). Interrater agreement was measured by the kappa coefficient. Agreement was low in patients with mild disability (EDSS less than 5); it was higher in patients with EDSS equal to or greater than 5. The difference between ratings of 2 independent examiners was equal to at least 1 point in 34% of the MS cases. This variability must be taken into consideration in designing clinical trials in MS.