A PASI score is a tool used to measure the severity and extent of psoriasis (Psoriasis Area and Severity Index). It takes a few minutes and experience to calculate it accurately.
A representative area of psoriasis is selected for each body region. The intensity of redness, thickness and scaling of the psoriasis is assessed as none (0), mild (1), moderate (2), severe (3) or very severe (4).
The three intensity scores are added up for each of the four body regions to give subtotals A1, A2, A3, A4.

Each subtotal is multiplied by the body surface area represented by that region.

A1 x 0.1 gives B1
A2 x 0.2 gives B2
A3 x 0.3 gives B3
A4 x 0.4 gives B4
Area

The percentage area affected by psoriasis is evaluated in the four regions of the body. In each region, the area is expressed as nil (0), 1-9% (1), 13-29% (2), 30-49% (3), 50-69% (4), 70-89% (5) or 90-100% (6).

Head and neck
Upper limbs
Trunk
Lower limbs
Calculations for area

Each of the body area scores is multiplied by the area affected.

B1 x (0 to 6)= C1
B2 x (0 to 6)= C2
B3 x (0 to 6)= C3
B4 x (0 to 6)= C4
Total score

The PASI score is C1 + C2 + C3 + C4.