Psoriasis Australia

Psoriasis area severity index (PASI)

A formal system used to measure the severity and extent of your psoriasis, before and after treatment. 

The severity of your diagnosis is determined by the Psoriasis Area and Severity Index, known as PASI. This is a formal system used to measure the severity and extent of your psoriasis, before and after treatment. 

A PASI score is an objective measure of how much of the skin is covered by psoriasis plaques and also how red, thick and scaly the plaques are. The higher the PASI score, the greater the severity of your psoriasis.

Scoring System

The scoring system used to calculate the PASI rates the intensity of the psoriasis from none to very severe and estimating the percentage of the body that is covered by the lesions. A PASI Score can range from 0-72 indicating the following:

  • 0 to 5: None to mild Psoriasis
  • 6 to 10: moderate Psoriasis
  • 11 or above: severe Psoriasis

How is the PASI score calculated?

The PASI is calculated using a complicated mathematical formula. It uses information acquired by a dermatologist during physical examination taking into account the regions the psoriasis is present on the body (area) and the intensity of the lesions.

The body is divided into four areas, each with a separate weighting.

  • Head and neck (10%)
  • Arms (20%)
  • Torso (30%)
  • Legs (40%)

A representative area of psoriasis is selected for each region and graded from 0 (none) to 4 (very severe) across three characteristics:

    •  Erythema, which refers to how red it looks
    • Scaling, which assess the coarseness of the scales
    • Thickness, which is the depth of the area affected
The three intensity scores for each region are added up to give subtotals, A1, A2, A3 and A4.
 
Each subtotal is multiplied by the body surface area for that region. 
 
  •  Head and neck – A1 x 0.1 = B1
  • Arms – A2 x 0.2 = B2
  • Torso – A3 x 03 = B3
  • Legs – A4 x 0.4 = B4

Each body region is assessed to determine how much of the area is affected by psoriasis. It is scored as:

  • Nil – 0
  • 1% to 9% – 1
  • 10% to 29% – 2
  • 30% to 49% – 3
  • 50% to 69% – 4
  • 70% to 89% – 5
  • 90% to 100% – 6
The body area score 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

The final PASI score is calculated by adding the four region scores together.

PASI = C1 + C2 + C3 + C4

The final PASI score can range from 0 to 72. 

  •  0 to 5: none to mild psoriasis
  • 6 to 10: Moderate psoriasis
  • 11 or above: Severe psoriasis