What is the PASI Score?
The Psoriasis Area and Severity Index (PASI) score is the most commonly used tool by doctors to measure the severity and extent of psoriasis. It is the gold standard measurement in clinical trials and is the most validated objective measurement of psoriasis severity in use.
The PASI score varies from 0 to 72, with higher numbers representing more severe disease.
To use the PASI, the body is divided into four areas:
- Head: scalp and neck
- Upper extremities: arms, hands, palms
- Trunk: armpits, torso, and genitals
- Lower extremities: buttocks, legs, and feet
First, psoriasis characteristics in each body area are graded by how severe the lesions are using an assessment of redness, thickness and scaliness. Each body area and will receive a grade for redness, thickness and scaling from 0 (no involvement) to 4 (severe involvement).
Next, this tool measures how much of the body surface area is affected by psoriasis in each region. Each region gets a grade from 0 (no involvement) to 6 (greater than 90% involvement).
Grades for each part of the body for redness, thickness, scaliness, and body surface area affected are used to calculate the PASI score using an equation that weights each body area based on the body surface area it represents.
If someone has no psoriasis, they will have a PASI score of 0. It is rare to have a score over 40, although theoretically, you can have a score as high as 72.
You may also hear the terms PASI 50, PASI 75, PASI 90 and PASI 100. These terms describe how much psoriasis improves in response to therapy. For example, PASI 75 means there is a 75% or greater improvement in the PASI score from the baseline score (before the therapy was started). PASI 100 means the person has achieved complete resolution of psoriasis. PASI 75 or higher is deemed an effective response to therapy in clinical trials although we are beginning to see more and more treatments with good PASI 90 and PASI 100 scores.
The PASI is not perfect and has several disadvantages. Some disadvantages include that it does not consider quality of life, does not predict how severe the disease is from the patient’s point of view, and does not tell us right away if someone has mild, moderate, or severe psoriasis.