Development and Validation of a Patient-Reported Outcome Measure to Assess Disease Control in Chronic Prurigo

Abstract

Chronic prurigo (CPG), including prurigo nodularis, is a difficult disease to treat and considerably affects patients' quality of life. Helping patients obtain control of CPG is a major treatment goal. To develop and validate the Prurigo Control Test (PCT), a tool for assessing disease control in CPG, and to identify a cutoff value for controlled disease to aid treatment decisions. This qualitative study followed the current recommendations for patient-reported outcome measure development in the generation and validation of the PCT. The final PCT was obtained after item generation, followed by reduction and selection, and was then tested for internal consistency and test-retest reliability, convergent validity, known-group validity, screening accuracy, and banding. The item-generation phase resulted in an unselected list of 69 potential PCT items. Impact analysis, interitem correlation, and review for content (face) validity resulted in final set of 5 PCT items. The validation study was performed among patients across 2 expert centers in Germany. Data were analyzed from February 2017 to November 2019. A 5-item PCT with a recall period of 2 weeks was developed. A cutoff value of 10 points or higher was determined as suitable for identifying patients with well-controlled vs poorly controlled CPG. Of the 95 patients included in the validation study, the median (range) age was 63 (19-87) years, 50 patients (53%) were women, and the median (range) disease duration was 72 (9-774) months. The validation study yielded good internal consistency reliability (Cronbach α, 0.86) and a high degree of convergent validity. The PCT demonstrated good known-group validity and could discriminate between patients who differed in prurigo control. Test-retest reliability was high, and the intraclass correlation coefficient was 0.94, indicating excellent reproducibility. This qualitative study showed that the PCT is able to assess disease control in patients with CPG. Its retrospective approach, brevity, and simple scoring likely make the PCT suitable for clinical practice and trials.