Validation of a simple measure of psoriasis severity based on a longitudinal study of Chinese patients.

Abstract

The product of Physician Global Assessment (PGA) and Body Surface Area (BSA) (PGA × BSA) has been proposed as a simple and sensitive instrument for measuring psoriasis severity. To assess the simple measure, PGA × BSA, with respect to criterion validity, reproducibility, responsiveness, and interpretability among Chinese patients with psoriasis. Assessments of psoriasis severity were performed by two dermatologists independently for the baseline survey and by one dermatologist during follow-up. Criterion validity and reproducibility were assessed using Spearman correlation coefficients (r). Responsiveness was assessed by comparing the percentage changes in PGA × BSA (PGA × ΔBSA [%]) between patients grouped by disease improvement. The receiver operating characteristic (ROC) curve was used to determine the threshold of PGA × ΔBSA for disease improvement, anchored by 50% and 75% reduction in Psoriasis Area Severity Index (PASI). A total of 276 patients participated in the baseline survey, of whom 93 were followed. PGA × BSA highly correlated with PASI (r = 0.94), Simplified PASI (SPASI, r = 0.93), and Psoriasis Log-based Area and Severity Index (PLASI, r = 0.90) measured at baseline, indicating good criterion validity. The between-evaluator consistency of PGA × BSA was r = 0.95, indicating high reproducibility. PGA × ΔBSA highly correlated with both ΔPASI (r = 0.86) and ΔPLASI (r = 0.85), suggesting good responsiveness. The threshold of ΔPGA × BSA for disease improvement was 57% and 73%, as indicated by 50% and 75% reduction in PASI, respectively. PGA × BSA demonstrates good biometric properties and may be used to measure the severity of psoriasis among Chinese patients.