Regional differences between Japan and the Netherlands in atopic dermatitis patient‐reported outcome measures: which interpretation?

Abstract

enable the physician to evaluate easily the severity of patients’ symptoms and the effects of drugs and/or nonpharmacological interventions. The special interest group Scoring Itch in Clinical Trials of the International Forum for the Study of Itch has identified ItchyQoL as the first pruritus-specific instrument for data collection on QoL. ItchyQoL measures the QoL of patients with chronic pruritus. It was developed in the USA with patient input and its content validity was ensured with subsequent patient interviews. In its initial validation in the USA, and in a cross-European validation in Austria, France, Germany, Italy, Poland, Russia, Spain, Switzerland and Turkey, ItchyQoL showed excellent reliability, validity and responsiveness. It consists of 22 items with five response options from 1 (never) to 5 (all the time). The original US American version reports three domains of the ItchyQoL: symptoms (items 1–6), functioning (items 7–13) and emotions (items 14–22). Structural validity is an important measurement property that describes the degree to which the scores of a patient-related outcome measure are an adequate reflection of the dimensionality of the construct to be measured. Item response theory (IRT), Rasch analyses or factor analyses as part of classical test theory (CTT) are the preferred statistical methods to assess structural validity. In this issue of the BJD, Gabes et al. assessed the structural validity of ItchyQoL using these methods. With the help of 551 patients from nine European countries, who completed the 22 items of the ItchyQoL, they found that IRT analysis supported the revision of response options from five to three. This revision was supported by structural validity using CTT. The overall fit to the Rasch model was confirmed. Unidimensionality was supported by the ItchyQoL overall scale as well as by the single subscales. Consequently, Gabes et al. suggest a revision of the response categories of the ItchyQoL from a five-point to a three-point scale. On application of this revision, ItchyQoL showed excellent structural validity according to CTT, IRT and Rasch criteria. The calculation of an overall ItchyQoL sum score is thereby reproducible and validated. This simplification would allow easier use of ItchyQoL in clinical trials as well as in daily practice.