What is the PASI score my dermatologist uses ?

[Fred]

First things first the PASI score is not the same as the Psoriasis Score you see in the menu bar above of the forum.

The Psoriasis Area and Severity Index (PASI) is the most widely used tool for the measurement of severity of psoriasis. PASI combines the assessment of the severity of lesions and the area affected into a single score in the range 0 (no disease) to 72 (maximal disease).

The calculation works like this:
The body is divided into four sections (head (H) (10% of a person's skin); arms (A) (20%); trunk (T) (30%); legs (L) (40%)). Each of these areas is scored by itself, and then the four scores are combined into the final PASI. For each section, the percent of area of skin involved, is estimated and then transformed into a grade from 0 to 6:

   0% of involved area, grade: 0
   < 10% of involved area, grade: 1
   10-29% of involved area, grade: 2
   30-49% of involved area, grade: 3
   50-69% of involved area, grade: 4
   70-89% of involved area, grade: 5
   90-100% of involved area, grade: 6

Within each area, the severity is estimated by three clinical signs: erythema (redness), induration (thickness) and desquamation (scaling). Severity parameters are measured on a scale of 0 to 4, from none to maximum.

The sum of all three severity parameters is than calculated for each section of skin, multiplied by the area score for that area and multiplied by weight of respective section (0.1 for head, 0.2 for arms, 0.3 for body and 0.4 for legs).

There are other ways of scoring hence the Psoriasis Score on the forum. its just a way of monitoring how well your treatment is going.