An autoimmune condition affecting the skin, psoriasis (PsO) is usually characterized by raised, red patches of skin that may have silvery-white scales. It’s considered a chronic disease that involves cycles in which symptoms flare for a few weeks or months, then subside or go into remission, according to the National Institutes of Health.
As a way to track the severity of these flare-ups, health providers use what’s called PASI, short for Psoriasis Area and Severity Index, says Amy Huang, M.D., a dermatologist with Medical Offices of Manhattan in New York City.
“This score is a tool that measures the severity of psoriasis and also helps assess the effectiveness of treatments,” she notes. Let’s look at how the PASI scoring system works, along with whether you can use this tool on your own to rate your PsO flares.
PASI Scoring System
According to the National Psoriasis Foundation, PsO affects more than 8 million people in the U.S., but the condition isn’t the same for everyone. Some people may have frequent and intense skin patches, called plaques, while others may have mild plaques that appear only very occasionally.
PASI is utilized as a way to determine where you might be on the spectrum, as well as how your PsO could be progressing over time, says Dr. Huang. The score ranges from 0 to 72 and incorporates both how intense your psoriasis is and how much of your body it covers.
“In simple terms, the PASI score is calculated through looking at the intensity of redness, thickness, and scaling of psoriasis plaques on the body, combined with the body surface area affected,” she says.
Let’s take a closer look at these two measures.
Intensity
One of the two measures for PASI is intensity, which assesses symptoms such as scaly patches, itching, soreness, burning, redness, and flaking. The intensity score ranges from 0 to 4, as follows:
0: absent symptoms
2: moderate symptoms
4: very severe symptoms
Symptoms are assessed for each of four body regions—neck and head, upper limbs, trunk, and lower limbs—and scored on three specific parameters: redness, thickness, and scaling. In other words, each of the four body regions receives three scores (one each for redness, thickness, and scaling), then the resulting 12 scores are added together to create the total intensity score.
That means your number will keep climbing if you have intense symptoms in multiple body regions. For example, if you have very severe scaling (which warrants a score of 4) in each of the four body regions, but the plaques are thin and have minimal redness, your intensity score would be 16. But if you also have very thick plaques that are extremely red in each of those regions, you would receive the maximum intensity score of 48.
Area
In addition to the intensity of your symptoms, the PASI score will take area into consideration, meaning how much of your skin is affected. As with intensity, each of the four body regions receives a separate score; these range from 0 to 6, as follows:
0: absence of symptoms or coverage
1: 1% to 9% coverage
2: 10% to 29% coverage
3: 30% to 49% coverage
4: 50% to 69% coverage
5: 70% to 89% coverage
6: 90% to 100% coverage
This means that the maximum area score you could get is 24 (a rating of 6 in each of the four body regions). To get the total PASI score, intensity and area are added together, for a maximum score of 72.
How Is the PASI Score Determined?
Although a PASI score can be done during a dermatology appointment, it’s also possible to get the score through photographs or by simply reporting severity and area to your doctor, according to 2023 research in the journal JAAD International.
In that study, patients who had both in-clinic assessments and also had photographs assessed by different dermatologists had similar PASI scores for each. That means you may not need an appointment if your doctor approves the use of photos for a PASI score instead. In terms of calculating the PASI score on your own, that may be challenging, says Dr. Huang.
“The score is usually calculated by trained professionals, since patients are usually unable to accurately assess areas that can’t be directly observed by them, such as the back or the scalp,” she adds.
Benefits of PASI Scoring
A top advantage of using PASI is that it’s easy to perform—you simply need a visual assessment, either in a dermatologist’s office or through a telehealth session. No equipment is needed, and it’s noninvasive since it's only a visual check.
Another benefit is that regular PASI scoring over time can be helpful for assessing the efficacy of treatment, says Dr. Huang. For instance, if you had a high PASI score—such as 50 or above—before starting treatment, and now you’re scoring in the 20s or 30s, that means you’re moving in the right direction in terms of treatment goals, she adds.
Drawbacks of PASI Scoring
While the PASI scale provides a standardized way to assess psoriasis, determining a plaque’s PASI number in intensity or area is still a subjective analysis, notes Frank Morocco, D.O., a dermatologist at OhioHealth System in Marion, OH. “PASI scoring can vary according to who is assessing the score,” he says.
Equally important, the PASI scale measures visible symptoms but not their impact on how much physical pain you feel or the amount of distress they cause you. “It does not adequately capture patient experience of their disease,” acknowledges Dr. Morocco.
For example, treatment may reduce the size and appearance of plaques, thus lowering your PASI score, but you might still be struggling with the emotional impact of having PsO, says Dr. Morocco. Research in the journal Cureus reported that it’s common for those with the condition to feel social isolation, embarrassment, decreased self-esteem, stress, depression, and anxiety.
Using only a PASI score and ignoring these emotional impacts could lead to less comprehensive treatment, including referral to a mental health professional.
Improving a PASI Score
Particularly with effective treatment, a PASI score can change over time. For instance, there’s a milestone called “PASI 75,” which means your PsO has improved by 75% compared to your score before treatment. PsO treatments may include topical treatments, oral therapies, or biologic injections.
“What type of treatment you have will depend on the severity and frequency of your psoriasis symptoms,” says Dr. Morocco. “Also, treatment may change over time as your condition improves.” For example, you may start with biologic injections but transition to topical treatments if your plaques are becoming milder and your PASI score lowers.
What’s the Difference Between a PASI Score and a PGA Score?
Another assessment used for psoriasis is the Physician Global Assessment (PGA), which also measures the severity of the condition, according to Dr. Huang.
“The PGA tends to be preferred because it’s much simpler to use,” she says. According to research in the journal Psoriasis: Targets and Therapy, PASI may be “impractical for everyday clinical practice because it is complicated to calculate, difficult to interpret, time consuming, and insensitive in mild psoriasis.” The researchers noted that PGA is more practical, especially when combined with another metric called Body Surface Area (BSA).
Rather than using complex calculations, the PGA uses a single global assessment of all lesions and assigns a score between 0 and 5, with 0 representing clear skin, 1 representing almost clear skin, and 5 representing severe psoriasis, the researchers noted. The BSA score is given as a single percentage of body area covered.
Bottom Line
As a tool, the PASI score may be informative for your health provider in determining whether your PsO treatment is working as well as it should be. However, keep in mind that it’s not the only factor that might be used to manage your condition, says Dr. Huang.
For example, your doctor will also look at how the condition is affecting your quality of life, and whether there are certain triggers making your flares more frequent and intense. As with any questions about your PsO, talk with your doctor about whether knowing your PASI score or other measures, like the PGA score, could be helpful for managing your condition over time.