Hyperhidrosis quality of life index (HidroQoL): further validation by applying classical test theory and item response theory using data from a phase III clinical trial

Abstract Background The Hyperhidrosis Quality of Life Index (HidroQoL ©) is a well-developed and validated patient-reported outcome measure assessing the quality-of-life impacts in hyperhidrosis with 18 items. Our aim was to extend the already existing validity evidence for the HidroQoL, especially in relation to structural validity. Especially Rasch analysis has not been applied to the final 18-item HidroQoL before. Methods Data from a phase III clinical trial were used. Confirmatory factor analysis was conducted to confirm the two a priori HidroQoL scales within classical test theory. Furthermore, the assumptions of the Rasch model (model fit, monotonicity, unidimensionality, local independence) and Differential Item Functioning (DIF) were assessed using item response theory. Results The sample included 529 patients with severe primary axillary hyperhidrosis. The two-factor structure could be confirmed by the confirmatory factor analysis (SRMR = 0.058). The item characteristic curves showed mainly optimally functioning response categories, indicating monotonicity. The overall fit to the Rasch model was adequate and unidimensionality for the HidroQoL overall scale could be confirmed, since the first factor had an eigenvalue of 2.244 and accounted for 18.7%. Local independence was below assumed thresholds (residual correlations ≤ 0.26). DIF analysis, controlling for age or gender, was critical for four and three items, respectively. However, this DIF could be explained. Conclusion Using classical test theory and item response theory/Rasch analyses, this study provided further evidence for the structural validity of the HidroQoL. This study confirmed several specific (measurement) properties of the HidroQoL questionnaire in patients with physician-confirmed severe primary axillary hyperhidrosis: the HidroQoL is a unidimensional scale allowing the summation of scores to generate a single score, and simultaneously it has a dual structure, also allowing the calculation of separate domain scores for daily activities and psychosocial impacts. With this study, we provided new evidence of the structural validity of the HidroQoL in the context of a clinical trial. Trial registration The study was registered (ClinicalTrials.gov identifier: NCT03658616, 05 September 2018, https://clinicaltrials.gov/ct2/show/NCT03658616?term=NCT03658616&draw=2&rank=1 ).