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 rho, 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 (kappa 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 (rho = 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 (kappa = 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.