Reliability,validity and responsiveness to change of the Patient Report of Extent of Psoriasis Involvement (PREPI) for measuring body surface area affected by psoriasis

Background The development of a simple, reliable, valid and responsive method for measuring the extent of skin involvement in psoriasis is important for use in epidemiological studies. Objectives We sought to investigate the psychometric characteristics of the Patient Report of Extent of Psoriasis Involvement (PREPI), a single‐question method for measuring body surface area affected by psoriasis. Methods This was a cross‐sectional study of 140 patients with psoriasis, with an exploratory prospective longitudinal cohort component. Reliability was measured via a test‐retest approach and criterion validity was investigated by comparing the PREPI with an assessment of body surface area of involvement by a dermatologist. We additionally compared Skindex‐29 scores with the PREPI. To demonstrate responsiveness and establish a minimally important difference in the PREPI, we created receiver operating characteristic curves for the PREPI instrument. Results The test‐retest reliability of the PREPI was nearly perfect [intraclass correlation coefficient (ICC) = 0·99, 95% confidence interval (CI) 0·97–0·99], and there was substantial agreement between patient and physician assessments (ICC = 0·82, 95% CI 0·75–0·87). The PREPI showed significant correlations with all Skindex‐29 domains. We found the PREPI to be responsive to change and identified changes in the PREPI score that have good discrimination between patients with and without a minimally important clinical difference. Conclusions Our study suggests that the PREPI is a reliable, valid and responsive measure of body surface area affected by psoriasis that may be useful for future epidemiological research.