Measuring Fatigue in Multiple Sclerosis: There may be Trouble Ahead.

Most existing fatigue PROs do not meet COSMIN content validity requirements. Although two PROs scored well on aggregate (NFI-MS and FSIQ-RMS), our subsequent evaluation of the item sets that generated their scores implied that both PROs have weaker content validity than COSMIN suggests. This indicates that COSMIN criteria require further development, and raises significant concerns about how we have measured one of the most common and burdensome MS symptoms. A detailed head-to-head psychometric evaluation is needed to determine the impact of different PRO development qualities and the implications of the problems implied by our analyses, on measurement performance.