Abstract
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.