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

Background 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. Objectives To assess the simple measure, PGA × BSA, with respect to criterion validity, reproducibility, responsiveness, and interpretability among Chinese patients with psoriasis. Materials & methods 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). Results 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. Conclusion PGA × BSA demonstrates good biometric properties and may be used to measure the severity of psoriasis among Chinese patients.