PaSi

PASI, short for Psoriasis Area and Severity Index, is a standardized tool used to quantify the severity and extent of psoriasis. It combines assessments of how much skin is involved with how severe the lesions are, providing a single numerical score that can be used to track disease activity and treatment response.

The body is divided into four regions for scoring: head, upper limbs, trunk, and lower limbs. In each region, three clinical signs are evaluated: erythema (redness), induration or thickness, and scaling. Each sign is scored from 0 to 3, with higher numbers indicating greater severity. The area of skin involvement in the region is scored from 0 to 6, reflecting the percentage of skin affected. The region’s score is calculated by combining the sum of the three sign scores with the area score and applying a region-specific weighting factor. The four regional scores are then summed to obtain the total PASI score, which commonly ranges from 0 to 72.

PASI is widely used in clinical trials and practice to assess baseline disease activity and monitor response to therapies. It is often reported as achieving certain thresholds, such as PASI-75 (a 75% reduction from baseline), PASI-90, or PASI-100, to indicate clinically meaningful improvement.

Limitations of PASI include inter-rater variability and limited sensitivity to small, scattered patches or lesions in difficult areas (such as the scalp or face). It can be time-consuming to calculate, and some aspects of patient impact, quality of life, or localized disease may be underrepresented.