Development and initial content validation of the Vienna post-exertional malaise questionnaire: an iterative mixed-methods study

Objectives Post-exertional malaise (PEM) is a multidimensional exertion-response pattern involving diverse triggers, variable onset, and prolonged recovery. This study developed and content-validated the Vienna Post-Exertional Malaise (VPEM) Questionnaire, a patient-reported outcome measure assessing functional decline, recovery, crash-associated symptoms, and pacing adaptations in ME/CFS. Methods An iterative mixed-methods instrument-development study combined quantitative ratings and qualitative feedback across sequential evaluation cycles. An initial 13-item draft, informed by existing PEM, fatigue, and functional-capacity instruments and consensus literature, was evaluated by individuals with ME/CFS or Post-COVID-19 syndrome (PCS) using Likert ratings, open-ended feedback, and interviews. Quantitative data were summarised descriptively, and qualitative data were analysed using deductive–inductive structuring content analysis. Revisions were documented in revision matrices. The revised instrument was evaluated by an interdisciplinary expert panel using domain-level and item-level ratings of relevance and clarity. Results Twelve individuals with ME/CFS and one individual with PCS fulfilling ME/CFS criteria evaluated the questionnaire. Ratings showed high approval for length (92.3%) and terminology (76.9%), while completeness and clarity were more heterogeneous. Qualitative findings identified ambiguities in distinguishing PEM from non-specific fatigue, missing trigger domains, and insufficient representation of intra-individual variability. After revision, the expert panel ( n = 11) rated PEM coverage as complete or largely complete (100%) and the multidimensional structure as suitable or very suitable (90.9%). Item-level ratings showed high relevance (S-CVI/Ave = 0.96), whereas clarity ratings were more variable (S-CVI/Ave = 0.82). Conclusions The VPEM Questionnaire operationalises PEM as multidimensional post-exertional deterioration. Findings support initial content and face validity, warranting further psychometric validation.