Minimal clinical important difference (MCID) of the Thai chronic urticaria quality of life questionnaire (CU-Q2oL)

Background: Chronic urticaria (CU) has negative impacts on patients’ daily lives. The Chronic Urticaria Quality of Life Questionnaire (CUQ2oL) evaluates quality of life impairment attitudes among chronic urticaria patients. Although the CU-Q2oL has been validated in several languages, the minimal clinical important difference (MCID) of the CU-Q2oL has never been determined. Objective: This study aimed to investigate the validity, reliability, responsiveness to change, and MCID of the Thai CU-Q2oL. Methods: The Thai CU-Q2oL was translated with permission from the authors of the original Italian version. The Thai CU-Q2oL, the validated Thai Dermatology Life Quality Index (DLQI), and the Urticaria Activity Score were assessed for 166 patients to evaluate validity and internal consistency. The three questionnaires were then administered to 124 patients to determine the test-retest reliability, responsiveness, and MCID of the Thai CU-Q2oL. Results: The Thai CU-Q2oL contained only three domains, whereas the Italian version revealed six domains. Nevertheless, the total variance of the Thai CU-Q2oL (60.5%) was very close to that of the Italian version (60.0%). The validity of the Thai CU-Q2oL was shown by strong correlations between CU-Q2oL and DLQI scores. The Thai CU-Q2oL also had high internal consistency and test-retest reliability. Distribution-based, receiver operating characteristic analysis, and anchorbased approaches yielded MCID values of 3.9– 8.0, 15, and 21.1, respectively. Conclusions: The Thai CU-Q2oL is a valid and reliable instrument. We propose that a difference in the Thai CU-Q2oL score of 15 (MCID) is the smallest change patients perceive as a meaningful improvement.