Psoriasis Area and Severity Index (PASI)

The Psoriasis Area and Severity Index (PASI) is the most widely used composite score for plaque psoriasis severity. It combines assessment of erythema (redness), induration (thickness), desquamation (scaling) and affected body surface area across four regions: head, upper limbs, trunk and lower limbs. Each region is weighted according to its proportion of total body surface area. Scores range from 0 (no disease) to 72 (maximal disease), although values above 40 are rare.

In clinical practice, PASI helps standardise assessments across clinicians and over time, especially when monitoring systemic therapies and biologics. In trials, PASI “response” thresholds such as PASI 75, PASI 90 and PASI 100 (75%, 90%, and 100% improvement from baseline) are used as key endpoints, reflecting increasing levels of skin clearance. Many modern treatment guidelines define treatment goals in terms of PASI response combined with quality‑of‑life improvement—for example, aiming for PASI 90 with a DLQI of 0–1 where possible.

However, PASI has limitations. It can be time‑consuming in busy clinics, requires training for reliable scoring, and may underestimate the impact of psoriasis in high‑impact but small areas (such as nails, scalp, face or genitals). As a result, some services combine PASI with simpler measures (such as BSA and PGA) and patient‑reported tools to provide a fuller picture of disease burden.

Key references

1. Fredriksson T, Pettersson U. Severe psoriasis – oral therapy with a new retinoid. Dermatologica. 1978;157(4):238–244. (Original PASI description.)

2. Schmitt J, Wozel G. The Psoriasis Area and Severity Index is the adequate criterion to define severity in chronic plaque‑type psoriasis. Dermatology. 2005;210(3):194–199.

3. Mrowietz U et al. Definition of treatment goals for moderate to severe psoriasis: a European consensus. Arch Dermatol Res. 2011;303(1):1–10.

Reviewed: August 2026

Version ID: 2/PAS/AAR/08/26