The PASI score stands for Psoriasis Area and Severity Index. This tool allows researchers to put an objective number on what would otherwise be a very subjective idea: how bad is a person's psoriasis. To make up the score, the three features of a psoriatic plaque (redness) scaling and thickness are each assigned a number from 0 to 4 with 4 being worst. Then the extent of involvement of each region of the body is scored from 0 to 6. Adding up the scores give a range of 0 to 72.
Many studies quote the improvement seen in the PASI score over time as a measure of a drug's effectiveness. For example, they may note that a certain proportion of patients experienced a 75% reduction in their PASI scores over a 12-week treatment period and report this as a percentage of people achieving "PASI 75."
PASI scores are seldom used in clinical practice, although more fastidious doctors or those working at university-based clinics or specialized psoriasis treatment centers may routinely use this tool to follow their patients' progress.