PASI Score Calculator — Psoriasis Severity Index

PASI combines how much skin is affected with how bad the plaques are, region by region. The body is split into head, trunk, upper limbs and lower limbs, weighted 0.1, 0.3, 0.2 and 0.4 respectively to approximate the share of total body surface each contributes. Within each region the involved area is graded 0 to 6 on a banded scale (0, under 10 percent, 10 to 29, 30 to 49, 50 to 69, 70 to 89, 90 to 100 percent), and three plaque characteristics - erythema, induration and desquamation - are each graded 0 to 4 and summed. Multiplying area grade by severity sum and applying the regional weight gives four subtotals whose sum is the PASI, running 0 to 72 although scores above 40 are almost never seen. Common bands put under 5 as mild, 5 to 10 as mild to moderate, above 10 as moderate to severe warranting systemic or biologic therapy, and above 20 as severe. Reliability is the recurring problem. Inter-rater intraclass correlations typically fall between 0.60 and 0.85, with the area component the least consistent because visual estimation of percentage involvement is difficult. Erythema is harder to grade on darker skin, where inflamed and uninflamed skin contrast less. The scale is also non-linear and insensitive at low severity, and it captures nothing about itch, nail or scalp disease, joint involvement or quality of life, which is why guidelines pair it with body surface area and the DLQI.

Torbjorn Fredriksson and Ulf Pettersson introduced the index in Dermatologica in 1978, but not as a contribution to measurement theory. They were running a trial of a new retinoid in severe psoriasis, the compound that became etretinate, and needed a quantitative way to demonstrate that plaques had changed. The four-region weighting borrowed the logic of burn surface area estimation, and the index was essentially a tool of convenience built for one study. It stayed obscure for years. Adoption came with the systemic therapy era, as cyclosporine and methotrexate trials through the 1980s and 1990s needed a shared endpoint, and accelerated sharply once biologic agents such as etanercept and infliximab entered psoriasis trials around 2003. PASI 75, meaning a 75 percent reduction from baseline, became the de facto primary endpoint accepted by the Food and Drug Administration and the European Medicines Agency. Richard Langley and Charles Ellis published a critical evaluation of PASI alongside the Physician Global Assessment and body surface area in the Journal of the American Academy of Dermatology in 2004, documenting its psychometric shortcomings even as its regulatory role hardened. When interleukin-17 and interleukin-23 inhibitors began producing near-complete clearance in the mid-2010s, PASI 75 lost its ability to separate active drugs, and trials moved to PASI 90 and PASI 100 while European consensus work led by Ulrich Mrowietz in 2011 added absolute targets such as a PASI below 3.