How to Calculate PASI

PASI is an acronym for Psoriasis Area and Severity Index. This is a measure of the extent of psoriasis on the body and its severity. Psoriasis is a common skin condition that results in redness and irritation, and often results in thick scaly skin.

Instructions

• 1

  Divide the body into sections: legs, trunk, arms and head.

• 2

  Measure the amount of skin affected in each section of the body, using the following scale:

  0 percent = 0; Less than 10 percent = 1; 10 to 29 percent = 2; 30 to 49 percent = 3; 50 to 69 percent = 4; 70 to 89 percent = 5, and 90 percent or more = 6. This results in four scores, one for each body section.

• 3

  Evaluate the itchiness of each section of the body, using the following scale: None = 0; some = 1; moderate = 2; severe = 3 and maximum itchiness = 4. This also results in four scores.

• 4

  Evaluate redness, scaling and thickness for each section of the body, using the same scale as for itchiness to get four scores for each section of the body.

• 5

  Add up the scores for each section of the body from Steps 3 and 4. Each section will have four scores, for itchiness, redness, scaling and thickness.

• 6

  Multiply the total for each section from Step 5 by the area affected for each section from Step 2.

• 7

  Multiply the score for the head from step 6 by 0.1, the score from the arms by 0.2, the score from the trunk by 0.3 and the score from the legs by 0.4.

• 8

  Add up the results from Step 7. This is the PASI.