Psoriasis Area & Severity Index

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The Psoriasis Area and Severity Index (PASI) is a score used by doctors and nurses to record psoriasis severity. It is widely used to assess the progress of people receiving treatment for psoriasis, particularly in research trials. PASI has become the most widely used measure of psoriasis severity internationally. It does not examine arthritis for which other scores are available.

PASI examines four body regions: i) the head and neck, ii) the hands and arms, iii) the chest, abdomen and back (trunk) and iv) the buttocks, thighs and legs. Each region is given a score to show how much of the region is affected by psoriasis (area) and a score to record how bad the psoriasis is (severity). The area score can range from 0 (no psoriasis) to 6 (all of the skin affected). The severity score for each region is reached by adding scores for redness, thickness and scale, each of which is graded from 0 to 4, giving a maximum of 12.

Some complicated arithmetic is then used to produce the final score. An area and severity score for each region is calculated by multiplying the area score by the severity score (maximum 6 x 12 = 72). The amount each region contributes to the final PASI is then weighted according to how much of the total body skin surface it represents. The head and neck contribute a tenth, the hands and arms two tenths, the trunk three tenths and the buttocks, thighs and legs four tenths. The region scores are each weighted by the given amount and then added together to give the final PASI score. Thus someone with severe extensive psoriasis affecting the face and scalp might achieve a region score of 36 for the head and neck but this would contribute only 3.6 (one tenth of 36) to the final PASI score. Although PASI can in theory be as high as 72 it is rare to get a score of more than 40. PASI scores will also vary to some degree between scorers as there are no absolute standards to indicate, for instance, how red, thick or scaly a patch of psoriasis must be to be counted as very severe (4) rather than severe (3).

PASI is used to help judge the most appropriate treatment for a person’s psoriasis. A score of more than ten is generally taken to mean that the psoriasis is “moderate-to-severe” and therefore may be suitable for the more powerful forms of treatment. A PASI level of ten or more is often required for someone to be considered for a clinical trial. This is also used in helping to decide whether some of the newer treatments are appropriate for a given person with psoriasis.

In many recent clinical trials success is measured by the proportion of people receiving a treatment who achieve a “PASI 75”. This is shorthand for a 75% reduction of the PASI score from the start to the end of the trial. If someone starts with a PASI score of 36 then the final score should be no more than 9 for that person to achieve a PASI 75 response. The important issue to remember that PASI 75 is not clearance and although your doctor may be content to see this level of improvement you may still not be happy with your skin.

PASI is a useful way of indicating how severe psoriasis is but it is not a perfect tool and many other factors are taken into account when you and your doctor are discussing what the best treatment will be for you.

We are grateful to: Dr Robert J G Chalmers MB FRCP, Consultant Dermatologist, University of Manchester for his input into this article. July 2008.