PASI Score Calculator

PASI (Psoriasis Area and Severity Index) is the standard research and clinical tool for quantifying plaque psoriasis severity. Fredriksson and Pettersson introduced it in 1978 as a way to score treatment response in a clinical trial of a new oral retinoid for severe psoriasis, and it has since become the field's most widely used composite severity score. The body is divided into four regions -- head, arms, trunk, and legs -- and each region is scored on three clinical signs (erythema, induration, and desquamation, each 0 to 4) and one area-involvement grade (0 to 6, banding the percentage of that region's surface affected). Each region's score is (erythema + induration + desquamation) x area grade x a fixed regional weight -- 0.1 for head, 0.2 for arms, 0.3 for trunk, and 0.4 for legs -- reflecting each region's approximate share of total body surface area.

The four regional scores sum to the total PASI, which ranges from 0 (no disease) to a maximum of 72. Because the four regions are scored and weighted completely independently, a change entered for one region (say, the head) never affects any other region's component score -- only the sum they all feed into. Total PASI is conventionally read as roughly mild below 5, moderate from 5 to 10, and severe above 10; a PASI of 10 or higher is also the threshold most commonly used to define "moderate-to-severe" psoriasis for clinical trial eligibility. PASI 75, PASI 90, and PASI 100 -- reductions of 75%, 90%, and 100% from a baseline PASI -- are the standard benchmarks used to measure how well a treatment has worked; this calculator reports the target scores that would represent a 75% and 90% reduction from the entered score.