Development and initial content validation of the Vienna Post-Exertional Malaise Questionnaire: an iterative mixed-methods study

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Abstract
Objectives Post-exertional malaise (PEM) is increasingly understood not as a singular symptom but as a multidimensional exertion-response pattern involving diverse triggers, variable onset, and prolonged recovery. This study aimed to develop and content-validate the Vienna Post-Exertional Malaise (VPEM) Questionnaire, a multidimensional patient-reported outcome measure for PEM in myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS), designed to assess the consequences of PEM, including functional decline, prolonged recovery, new crash-associated symptoms, and pacing-related adaptations. Methods An iterative mixed-methods instrument-development study was conducted, combining quantitative ratings and qualitative feedback across sequential evaluation cycles involving individuals with lived experience and an interdisciplinary expert panel. An initial 13-item draft, derived from PEM, fatigue, and functional-capacity instruments, as well as consensus literature, was evaluated by individuals with lived experience of ME/CFS or Long COVID using Likert ratings, open-ended feedback, and semi-structured interviews. Quantitative data were summarised descriptively; qualitative data were analysed using deductive–inductive structuring content analysis. Revisions were agreed upon by a core development team and documented in revision matrices. The revised instrument version was evaluated by an interdisciplinary expert panel using domain-level ratings and item-level ratings of relevance and clarity. Results Twelve individuals with ME/CFS and one individual with Long COVID who also fulfilled ME/CFS criteria evaluated the questionnaire. Ratings showed high approval for questionnaire length (92.3%) and terminology (76.9%), while ratings for completeness and clarity were more heterogeneous. Qualitative analysis revealed conceptual ambiguities concerning the distinction between PEM and non-specific fatigue, the absence of trigger domains such as sensory, orthostatic and positively valenced emotional triggers, and inadequate representation of intra-individual variability. Following revision, the expert panel (n = 11) rated PEM coverage as either complete or largely complete (100%), and the multidimensional structure as either suitable or very suitable (90.9%). Item-level ratings revealed high content validity in terms of relevance (S-CVI/Ave = 0.96) and acceptable clarity (S-CVI/Ave = 0.82). The complete revised questionnaire was subsequently pretested online with five participants. Conclusions The VPEM Questionnaire operationalises PEM as a multidimensional pattern of post-exertional deterioration. Findings support initial content and face validity and provide the basis for psychometric validation.

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