PASI is the workhorse severity score for plaque psoriasis. It has been in use since 1978 and remains the primary endpoint in almost every psoriasis trial, which means a PASI number is the common language between a dermatologist, a payer, and a clinical trial protocol. Its weakness is that it is labour-intensive to compute by hand, which is why calculators exist.
The formula
The body is divided into four regions — head, trunk, upper extremities, and lower extremities — each weighted by its share of body surface area: 0.1, 0.3, 0.2, and 0.4 respectively. Within each region the clinician scores three signs — erythema, induration, and desquamation — from 0 (none) to 4 (very severe), giving a sign subtotal of 0 to 12. The area of involvement in that region is scored 0 to 6. The region score is the sign subtotal multiplied by the area score and the region weight. PASI is the sum of the four region scores, giving a theoretical range of 0 to 72.
A worked example
Suppose the head has mild erythema, no induration, and mild scaling (1+0+1 = 2) covering 10–29% of the region (area 2). The head contribution is 2 × 2 × 0.1 = 0.4. If the trunk has moderate signs (2+2+2 = 6) over 30–49% (area 3), its contribution is 6 × 3 × 0.3 = 5.4. If the arms have mild signs (1+1+1 = 3) over less than 10% (area 1), that is 3 × 1 × 0.2 = 0.6, and the legs have moderate signs (2+2+2 = 6) over 50–69% (area 4), giving 6 × 4 × 0.4 = 9.6. The total PASI is 0.4 + 5.4 + 0.6 + 9.6 = 16.0, which is severe disease.
Interpreting the result
A PASI of 10 or more, combined with a DLQI above 10, defines moderate-to-severe psoriasis and is the usual threshold for biologic therapy under the EuroGuiDerm 2023 guideline. In trials, the response is expressed as a percentage reduction from baseline: PASI 75 is the minimum acceptable response, PASI 90 is the modern target, and PASI 100 is complete clearance. The absolute score matters for treatment eligibility; the percentage change matters for measuring response.
Notes and source
Source: Fredriksson T, Pettersson U, "Severe psoriasis — oral therapy with a new retinoid", Dermatologica, 1978; and the EuroGuiDerm 2023 guideline on systemic therapy for psoriasis. PASI is an observer-rated score and is subject to inter-rater variability, so the same clinician should score a patient over time where possible. Educational use only — treatment decisions require clinical assessment.