Pityriasis rubra pilaris (PRP) is a rare inflammatory cutaneous condition characterized by widespread erythematous scaly plaques and palmoplantar keratoderma. PRP skin involvement can cause functional impairments and emotional distress leading to decreased quality of life. There is no clinical tool explicitly developed for PRP. The Individual Pityriasis Rubra Pilaris Area and Severity Index (iPRPASI) was developed as a patient-reported outcomes measure to capture the more nuanced features of PRP, such as the high prevalence of palmoplantar keratoderma. An assessment of the validity, reliability, and responsiveness of the iPRPASI is conducted in this study.