Psoriasis Area and Severity Index (Fredriksson 1978).

What it is and when to use it

The PASI (Psoriasis Area and Severity Index) is the standard tool for quantifying the severity of plaque psoriasis and monitoring treatment response. It combines the extent of affected body surface with the intensity of three clinical signs —erythema, induration (thickness) and desquamation (scaling)— weighted across four body regions (head, trunk, upper limbs and lower limbs). It is the reference outcome measure in clinical trials and in psoriasis guidelines such as those from the AAD-NPF and EuroGuiDerm/EDF for assessing candidates for systemic and biologic therapy.

How to interpret it

PASI scores range from 0 to 72; higher values indicate greater severity. There are no universal thresholds, but by convention a score below roughly 5-10 suggests mild disease, 10 to 20 moderate and above 20 severe; a PASI ≥10 is one of the common criteria for considering systemic therapy. Treatment response is expressed as percentage reduction from baseline: PASI 75 (≥75% reduction) was the classic endpoint, while PASI 90 and PASI 100 (near-complete or complete clearance) are the current targets with modern biologic therapies.

Limitations and when not to use it

PASI was validated for chronic plaque psoriasis and is unreliable in pustular, erythrodermic or guttate forms. It has limited sensitivity in mild or very localized disease and notable interobserver variability, especially in scoring induration. It does not assess involvement of high-impact special areas (scalp, nails, palms, soles, genitals) nor symptom burden or quality of life; for this reason it is complemented by BSA, IGA and DLQI. It does not measure psoriatic arthritis or comorbidities.

Frequently asked questions

What does PASI 75 mean?

PASI 75 means at least a 75% reduction in the PASI score compared with the baseline value before treatment. It has been the classic therapeutic endpoint in clinical trials.

What PASI score is considered severe psoriasis?

There is no universal cut-off, but a PASI above 20 is often considered severe; a PASI ≥10 typically already justifies considering systemic treatment, especially when combined with a high DLQI.

Is PASI suitable for scalp or nail psoriasis?

Not adequately. PASI underestimates special areas; for scalp, nails, palms, soles or genitals, dedicated indices such as PSSI, NAPSI or ppPASI are used instead.