Strengths and limitations
hyperhidrosis
With a sample size of Additionally, significant DIF regarding age and gender was found. This can possibly affect the validity of the HidroQoL, since the response to the items showing DIF is governed by something other than the underlying construct health-related QoL. One common solution is to remove the items showing DIF from the questionnaire in order to preserve its validity. Nevertheless, the HidroQoL is a well-established and much used questionnaire in the clinical assessment of hyperhidrosis. Removal of items always needs to be balanced against maintaining a questionnaire in its original format enabling standardized assessment and comparability. For this reason, we refrained from removing these items right now. If future research also reports DIF in the same items, removal should be considered again since they can detrimentally affect the validity of the HidroQoL.In this study, we could confirm the unidimensionality of the HidroQoL, as well as an underlying two-factor structure. This might be confusing, since both findings do not seem to align with each other. The HidroQoL as a whole scale is unidimensional (meaning that the HidroQoL has one underlying construct: health-related QoL) allowing the calculation of a sum score (confirmed by Rasch analysis and CFA). At the same time, it has two subscales (daily life activities and psychosocial domain) that are capturing different aspects of health-related QoL (confirmed by CFA). Both approaches aim for a different construct of hyperhidrosis impacts, and are not exclusive. They are based on different levels (two-factor solution: lower level, i.e. daily life activities or psychosocial impact versus unidimensionality: higher level, i.e. health-related QoL). The single factor solution (based on the Rasch analysis and CFA) seemed to be a more robust factor extraction approach than the two-factor solution (based solely on CFA), since in the first approach, the factor solution could be confirmed by CFA and PCA. Additionally, this study and the development study reported a correlation of the two subdomains of 0.651 and 0.645, respectively [In summary, our study extends existing evidence on the measurement properties of the HidroQoL regarding structural validity, based on data from a large clinical trial in people with confirmed primary axillary hyperhidrosis. According to the COSMIN methodology, PROMs can be placed in the highest recommendation category A if evidence of sufficient content validity and at least low-quality evidence of sufficient internal consistency is provided. In addition, sufficient internal consistency requires at least low evidence of sufficient structural validity [