The Psoriasis Area and Severity Index (PASI) is an assessment tool that doctors and nurses use to record the severity of a patient's psoriasis the health condition defined by inflamed lesions and silver-colored plaques.
Physicians created the index in 1978 because they needed an objective means to measure the effectiveness of the drug Soriatane (acitretin), a psoriasis treatment. Since then, PASI has become a widely used method for diagnosing psoriasis severity and judging new psoriasis treatments, medications, and therapies.
The PASI Score System
Doctors who use the Psoriasis Area and Severity Index in diagnosing psoriasis must examine the body carefully for lesions. For the assessment, the body is divided into four distinct regions â€” the head, the upper extremities, the trunk, and the lower extremities. The head includes the scalp and neck; the armpits and genitals are included as part of the trunk; the upper extremities include the arms, hands, and palms; and the lower extremities include the legs, buttocks, feet, and soles.
The doctor performs a full assessment of all the psoriasis lesions and blemishes on each of these regions. To calculate the PASI score, the doctor will first grade the psoriasis plaques found on each body region for their combined redness, thickness, and scaliness. The severity of the plaques in each region is graded on a 0 to 4 scale, with 0 meaning no involvement and 4 meaning severe involvement. Next, he will calculate the amount of surface area on each body region that is covered by the plaques. The total surface area affected by psoriasis is graded from 0 to 6, with 0 meaning no involvement and 6 meaning greater than 90 percent of the region covered in plaques.
These grades are then fed into an equation to determine the patient's PASI score. The equation weights each region based on the amount of body surface area each represents, with the head, upper extremities, trunk, and lower extremities corresponding to approximately 10 percent, 20 percent, 30 percent, and 40 percent of the patient's skin.
The PASI score then is used as a clinical assessment of the patient's psoriasis involvement. A person free of psoriasis has a score of 0. Theoretically, the score could be as high as 72, but it is rare to find a person with a PASI score over 40.
Applying a PASI Score
The Psoriasis Area and Severity Index is used to objectively assess the effectiveness of psoriasis treatments. A psoriasis treatment is deemed effective during clinical trials if a large number of patients are judged to be "PASI 75 responder" as a result of the medication or therapy. PASI 75 means that the person's PASI score dropped by 75 percent as a result of the psoriasis treatment. For example, a patient with a baseline PASI score of 20 who improves to a PASI score of 5 or less is considered a PASI 75 responder.
Clinical dermatologists may use PASI scores as a way of diagnosing psoriasis severity and judging the progress of individual patients. PASI scores also can help dermatologists make the case to insurance companies that a patient should be covered for psoriasis treatment with biologics and other new forms of psoriasis medications, or justify other medical decisions made during the course of a patient's treatment.
However, the Psoriasis Area and Severity Index does have some drawbacks as far as doctors are concerned. In its cold calculation of the lesions present on a patient, the index does not capture the total effect of psoriasis on a person's quality of life. For example, it cannot take into account how much itching or burning a patient feels as a result of the psoriasis plaques.
Despite these shortcomings, the Psoriasis Area and Severity Index has remained the gold standard in assessing how new psoriasis treatments are evaluated.
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