Abstract
The C19-YRS was the first condition-specific for long COVID/post-COVID syndrome. Although the original C19-YRS evolved to the modified version (C19-YRSm) based on psychometric evidence, clinical content relevance, as well as feedback from patients and healthcare professionals, it has not been validated through Rasch analysis. The study aim was to psychometrically assess and validate the C19-YRSm using newly collected data from a large-scale, multicenter study (LOCOMOTION). In total, 1278 patients (67% Female; mean age = 48.6, SD 12.7) digitally completed the C19-YRSm. The psychometric properties of the C19-YRSm Symptom Severity (SS) and Functional Disability (FD) subscales were assessed using a Rasch Measurement Theory framework, assessing for individual item model fit, targeting, internal consistency reliability, unidimensionality, local dependency (LD), response category functioning and differential item functioning (DIF) by age group, sex and ethnicity. Rasch analysis revealed robust psychometric properties of both subscales, with each demonstrating unidimensionality, appropriate response category structuring, no floor or ceiling effects, and minimal LD and DIF. Both subscales also displayed good targeting and reliability (SS: Person Separation Index (PSI) = 0.81, Cronbach's α = 0.82; FD: PSI = 0.76, Cronbach's α = 0.81). Although some minor anomalies are apparent, the modifications to the original C19-YRS have strengthened its measurement characteristics and its clinical and conceptual relevance. Trial Registration: NCT05057260, ISRCTN15022307.