A patient’s Psoriasis Area and Severity Index (PASI) is a measure of overall psoriasis severity and coverage. It is a commonly-used measure in clinical trials for psoriasis treatments.
PASI consists of two major steps:
1) calculating the BSA (Body Surface Area) covered with lesions and
2) assessment of the severity of lesions.
The second step in turn consists of assessing lesions' erythema (redness), induration (thickness) and scaling.
All calculations are combined into a single score (PASI Score) in the range of 0 (no psoriasis on the body) and up to 72 (the most severe case of psoriasis).
Typically, the PASI would be calculated before, during and after a treatment period in order to determine how well psoriasis responds to the treatment.