The PASI is the gold-standard physician-reported instrument for quantifying psoriasis severity, first published in 1978 by Fredriksson and Pettersson. It evaluates four body regions (head, upper extremities, trunk, lower extremities) across two domains: percentage of body surface area affected (scored 0 to 6) and lesion severity for three characteristics, erythema, induration, and desquamation (each scored 0 to 4). Each region is weighted by its proportion of total body surface area: head = 0.1, upper extremities = 0.2, trunk = 0.3, lower extremities = 0.4.

A PASI score greater than 10 is considered severe psoriasis. In clinical trials, PASI 75 (a 75% reduction from baseline) is the standard efficacy benchmark, with PASI 90 and PASI 100 increasingly used for newer biologic agents. The theoretical maximum score of 72 represents complete involvement of all body regions with the most severe lesion characteristics.

The PASI has demonstrated good internal consistency and moderate interobserver variation, with high reproducibility when performed by trained evaluators. Known limitations include non-linear scale distribution, low sensitivity at lower severity ranges, inability to capture nail or genital psoriasis, and significant interobserver variability. It does not assess quality of life, which is why it is frequently used alongside the DLQI.

PASI-HD (High Discrimination) is a modification proposed by Papp et al. (2021) to address the loss of precision when regional involvement falls below 10%. In the standard PASI, any involvement from 1% to 9% receives a blanket area score of 1, creating a ceiling effect that limits detection of change in patients with mild disease. PASI-HD replaces this with a linear scale: 1% maps to an area score of 0.1, 2% to 0.2, and so on through 9% to 0.9. Scoring for involvement of 10% or greater is unchanged. The affected area can be estimated using hand landmarks: one palmar surface (including fingers) represents approximately 1% of total BSA, the thumb approximately 0.1%, the thumbnail approximately 0.02%, and the fifth fingernail approximately 0.01%.