Evaluating the Mental Fatigue Scale for long COVID: A content validity study

This study aimed to evaluate the content validity of the Mental Fatigue Scale (MFS) in individuals with long COVID. Content validity was assessed using COSMIN methodology through cognitive interviews with individuals with long COVID (n = 10) and focus groups with healthcare professionals (n = 8). The interviews were conducted at a Danish long COVID clinic and a community-based rehabilitation centre. MFS was generally perceived as relevant, comprehensive, and comprehensible for assessing mental fatigue in individuals with long COVID. Most participants found the items appropriate; however, some described experiences, such as word-finding difficulties and racing thoughts, not fully captured by the scale. Participants also compared symptoms to their pre COVID condition rather than to the previous month, suggesting that the timeframe may require adjustment. Several noted that the MFS reflects symptoms at the body-function level and offers limited insight into how mental fatigue affects participation in work, social, and family life. The MFS appears useful for assessing mental fatigue in individuals with long COVID but may benefit from supplementary items relevant to this population or from being combined with tools that capture broader participation-related and contextual factors. Future studies should adapt and evaluate the scale in this population.