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The Michigan Fatigability Index (MIFI) is a patient reported outcome (PRO) measure that was developed in people with multiple sclerosis (MS) or fibromyalgia (FM). The MIFI consists of three items banks and corresponding 6-item short forms that assess physical fatigability, mental fatigability, or emotional fatigability. This report details examination of convergent and known-groups validity (i.e., construct validity) of the MIFI short forms for MS and FM.

A US web-based cross-sectional survey was distributed to n = 2650 participants (FM n = 1242; MS n = 1408) who completed the MIFI measure and measures of fatigue intensity, severity, impact, fatigability, physical and cognitive functioning, anxiety, and depressive symptoms to establish convergent validity. Tests of known groups validity were based on disease severity scores for MS and FM and PROMIS measures of global physical and mental health scores, which were dichotomized into upper and lower tertile health groups.

Convergent validity of the MIFI short forms was supported by moderate to high correlations (r = > 0.60) with measures of similar constructs. Each MIFI fatigability short form was able to distinguish between lower and upper tertiles on global physical and mental health with large effects (η²>0.3), and between lower and upper disease severity subgroups with small to large effects, supporting known groups validity.

Results support the construct validity of the MIFI physical fatigability, mental fatigability, and emotional fatigability short forms for MS and FM. The MIFI measures, which are brief and developed specifically for use in clinical populations with high fatigue burden, should be considered for use in clinical and research applications.