Psoriasis is a chronic condition that can cause thick, scaly patches, or plaques, to form on the skin. The Psoriasis Area and Severity Index (PASI) score is a measurement of the discoloration, thickness, scaling, and coverage of these plaques. A doctor can use it to measure the severity and extent of psoriasis and observe the effectiveness of psoriasis treatments.

Psoriasis is a condition that causes skin cells to develop rapidly, which can result in patches that may itch and cause discomfort. Evidence suggests that more than 8 million people in the United States have psoriasis.

Although there is no cure for psoriasis, treatments are available to reduce the symptoms.

Keep reading to learn more about the PASI score, including how to calculate it and the benefits and shortcomings of the system.

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The PASI score is a measurement tool that dermatologists commonly use to assess the severity of psoriasis. Using the tool also allows them to monitor the progression of the condition and evaluate the effectiveness of treatment.

The scoring involves rating the symptoms of psoriasis from none to very severe and estimating the percentage of the body that they affect.

Researchers also use PASI scores to determine the effectiveness of psoriasis medications in clinical trials.

The range of absolute PASI scores is 0–72, with higher scores indicating a greater severity of psoriasis. A score of 0 indicates no psoriasis, while a score higher than 10 suggests severe psoriasis. The scoring system includes a section for intensity and another for body coverage:


The intensity section of the score measures discoloration, thickness, and scaling. A doctor can rate each of these three symptoms using the below scoring system:

  • 0: absent symptoms
  • 1: mild symptoms
  • 2: moderate symptoms
  • 3: severe symptoms
  • 4: very severe symptoms


The area section shows the extent to which psoriasis affects the following:

  • head and neck
  • upper limbs
  • trunk
  • lower limbs

Doctors assign one of the below scores to each region:

  • 0: absence of symptoms
  • 1: 1–9% coverage
  • 2: 10–29% coverage
  • 3: 30–49% coverage
  • 4: 50–69% coverage
  • 5: 70–89% coverage
  • 6: 90–100% coverage

The British Association of Dermatology explains that people can calculate PASI scores as follows:

  • A scores: Add together the three intensity scores for each of the four body regions to get four A scores.
  • B scores: These scores range from 0–6 and correspond to the percentage of each body region that psoriasis affects. There will be a B score for each of the four body regions.
  • C scores: Multiply the A score by the B score for each body region to get four C scores.
  • D scores: Multiply each C score by the amount of body surface area that the region represents. This amount is 0.1 for the head and neck, 0.2 for the arms, 0.3 for the trunk, and 0.4 for the legs. This gives four D scores.
  • PASI score: Add together the four D scores.

People can also calculate their PASI score using online calculators or other online resources. However, this score will likely be inaccurate, so a person should only use a PASI score from a trained professional.

A PASI score guides doctors in prescribing treatment for individuals. When PASI scores do not decrease in response to treatment, a doctor can prescribe a medication that may be more effective.

PASI scores are also an invaluable tool in helping researchers develop and test medications that reduce psoriasis symptoms. The scores correlate with quality of life, which is a strong indicator of treatment outcomes.

An older study found a consistent correlation between PASI scores and quality-of-life scores in people receiving treatment for moderate-to-severe psoriasis. This correlation means that if a certain therapy resulted in at least a 75% reduction in PASI scores, it also led to a substantial improvement in quality of life.

The shortcomings of PASI scores are the unreliability and inconsistency that stem from the subjectivity involved in rating symptoms and estimating percentages.

For example, one doctor may consider psoriasis to affect 65% of a body region, whereas another doctor may report an 80% coverage. If some doctors or researchers calculating the PASI score are inexperienced, this could contribute further to the variation among scorers.

The authors of a 2020 study reviewing the reliability of PASI scores note that the subjectivity in measuring PASI ratings led to a variation in scores. They also point out that PASI scoring does not provide a complete picture of a person’s condition. For instance, it does not consider the symptoms of itching and pain.

When comparing the reliability of other psoriasis scoring systems, the study authors note that the PASI system had the most validation and is currently the best measurement tool. However, they recommend the development of a more thorough and objective assessment tool.

In clinical trials, researchers often use PASI percentage response rates to indicate treatment outcomes. For example, PASI 75 means that a person’s PASI score has decreased 75% from baseline, which indicates a significant improvement in the condition.

Historically, the goal of treatment has been to achieve PASI 75. However, with treatment options improving, researchers are focusing more on PASI 90 or PASI 100, equating to a 90% or 100% reduction in PASI scores, respectively.

By regularly checking in with their dermatologist, following their advice, avoiding potential triggers, and following treatment guidelines, a person may be able to reduce their symptoms and achieve a PASI 75, 90, or 100.

A PASI score is a measurement tool that a doctor may use to assess the severity and extent of psoriasis. They may also use it to establish a baseline from which they can monitor the progression of the psoriasis and evaluate the effectiveness of treatment.

A doctor can calculate the score by measuring the discoloration, thickness, scaling, and coverage of the skin condition. A PASI score involves some subjectivity, so it is not a perfect means of assessing a person’s condition. Despite this drawback, it is the most valid assessment tool currently available.