The Psoriasis Area and Severity Index

If you’re living with psoriasis, you may have heard of the Psoriasis Area and Severity Index, also known as PASI. But what is PASI, and what’s it used for? We’ll explain the index in more detail in this blog. 

The Psoriasis Area and Severity Index is a measurement tool used by medical professionals to assess the extent of a patient’s symptoms. The tool uses a combination of metrics to assess the condition and then outputs a single score; ranging from 0, where there is no psoriasis on the body, to 72, which would apply to the most severe cases of psoriasis. However, it’s considered rare for patients to have a score over 40. 

The index was created back in 1978, when there was a need to objectively measure the effectiveness of a new treatment on the market. It’s now commonly used for diagnosis, tracking progress and evaluating new medications and therapies, often during clinical trials. Scores can be recorded before, during and after the treatment period to understand how participants have responded to the treatment. A PASI score may also be used to build a patient’s case for biologics or other new forms of treatment, when making a claim through an insurance company.


How is the PASI calculated?

The score consists of two key calculations; Body Surface Area (BSA) and Severity. BSA is an assessment of how much of the skin is affected by the condition. Severity is a measure of how severe the condition is thought to be, based on a criteria of redness, thickness and scaling. Healthcare professionals must examine the body carefully to identify psoriasis lesions and complete the assessment following the steps below:

Arms (20%)

Head (10%)

Legs (40%)

Torso (30%)

  • Step 1 - Divide the body into four areas, each with a separate weighting; head (10%), arms (20%), torso (30%) and legs (40%).
  • Step 2 - Assess each area based on the severity criteria; generating a score for each parameter (redness, thickness and scaling), on a scale of 0 (none) to 4 (maximum). 
  • Step 3 - Add these three scores together for each area of the body to calculate a single severity score per area. 
  • Step 4 - Assess each area to gauge how much skin is affected by the condition, calculating a percentage score per area on a gradient of nil (0), 1-9% (1), 10-29% (2), 30-49% (3), 50-69% (4), 70-89% (5) or 90-100% (6).
  • Step 5 - Multiply the severity score (step 3) with the percentage area score (step 4) for each area. 
  • Step 6 - Multiply each result by the appropriate weighting (step 1), for example 0.1 for the head, 0.2 for the arms etc.
  • Step 7 - Add these four scores together to get the final PASI score


There are a number of online platforms which are built using this equation, allowing data to be inputted and returning a final score, to make the process more efficient. More recently mobile applications have also been created with this functionality.


How is the assessment made?

It’s likely that your doctor or dermatologist will visually assess your symptoms against the severity criteria, however there are a couple of methods for assessing the BSA. The first, known as the ‘palm method’, works on the basis that the patient's palm - up to the thumb and the middle finger knuckles - represents 1% of the body’s surface area. Each area of the body equals a different amount of palms, for example the head is roughly 10 palms, while the legs are roughly 40 palms. This method provides a reference point for measuring the area of skin affected by the condition. Some healthcare professionals on the other hand may be able to gauge this calculation by visual examination alone, which is referred to as the ‘eyeball method’.


What doesn’t PASI cover?

As PASI is an objective assessment of the appearance of the skin, it doesn't take into account other symptoms such as itching or burning sensations. In this respect, it has some limitations in understanding how the condition is affecting a patient’s overall quality of life. Despite this, PASI remains a standard assessment for medical professionals working with psoriasis patients.


Are there any other psoriasis assessment tools?

The Psoriasis Area and Severity Index is not the only assessment tool used to measure psoriasis symptoms. Others include the Dermatology Life Quality Index (DLQI) and the Physician’s Global Assessment (PGA). Your doctor or dermatologist will determine the most suitable model for your situation.


This blog is intended to provide more information about the Psoriasis Area & Severity Index and help you understand some of the science behind it. The content is not designed to facilitate self-diagnosis; if you are living with psoriasis and would like more guidance and advice on your symptoms, please speak to your GP or a healthcare professional.