Measurement properties and interpretability of the Patient-Reported Impact of Dermatological Diseases (PRIDD) measure.

Background: Patient-reported outcome measures (PROMs) are crucial in assessing the impact of dermatological conditions on people's lives, but the existing dermatology-specific PROMs are not recommended for use, according to COSMIN. We developed the Patient-Reported Impact of Dermatological Diseases (PRIDD) measure in partnership with patients. It has strong evidence of content validity, structural validity, internal consistency, acceptability and feasibility.

Objectives: To test the remaining measurement properties of the PRIDD and establish the interpretability of scores against the COSMIN criteria, using classic and modern psychometric methods.

Methods: A global longitudinal study consisting of two online surveys administered 2-4 weeks apart was carried out. Adults (≥ 18 years of age) living with a dermatological condition were recruited via the International Alliance of Dermatology Patient Organizations' (GlobalSkin) membership network. Participants completed PRIDD, a demographics questionnaire and other related measures, including the Dermatology Life Quality Index. We tested the criterion validity, construct validity and responsiveness (Spearman's ρ, independent-samples t-tests and Anova); test-retest reliability [interclass correlation coefficient (ICC)]; measurement error [smallest detectable change or limits of agreement (LoA), distribution-based minimally important change (MIC)]; floor and ceiling effects (number of minimum and maximum scores and person-item location distribution maps), score bandings (κ coefficient of agreement) and the anchor-based MIC of the PRIDD.

Results: In total, 504 people with 35 dermatological conditions from 38 countries participated. Criterion validity (ρ = 0.79), construct validity (76% hypotheses met), test-retest validity (ICC = 0.93) and measurement error (LoA = 1.3 < MIC = 4.14) were sufficient. Floor and ceiling effects were in the acceptable range (< 15%). Score bandings were determined (κ = 0.47); however, the anchor-based MIC could not be calculated owing to an insufficient anchor.

Conclusions: PRIDD is a valid and reliable tool to evaluate the impact of dermatological disease on people's lives in research and clinical practice. It is the first dermatology-specific PROM to meet the COSMIN criteria. These results support the value of developing and validating PROMs with a patient-centred approach and using classic and modern psychometric methods. Further testing of responsiveness and MIC, cross-cultural translation, linguistic validation and global data collection are planned.