The Psoriasis Area and Severity Index

Information about the PASI score as used in psoriasis evaluation.

A patient’s Psoriasis Area and Severity Index (PASI) is a measure of overall psoriasis severity and coverage. It is a commonly-used measure in clinical trials for psoriasis treatments. Typically, the PASI would be calculated before, during and after a treatment period in order to determine how well psoriasis responds to the treatment under test (a lower PASI means less psoriasis, generally).

Not all PASIs are the same, however. In clinical trials, it is common that a “modified PASI” is used as the main measurement, and how “modified” it is will depend on the researchers and what they are looking for, specifically. The only way to learn exactly how the PASI has been modified is to read whatever study you’re interested in, the entire measurement protocol should be written up in detail.

As far as I know, what follows is the “generic” PASI, with no modifications.

Skin Sections

For the PASI, the body is divided into four sections. Each of these areas is scored by itself, and then the four scores are combined into the final PASI. The four areas are: the Legs, which have 40% of a person’s skin; the Body (trunk area: stomach, chest, back, etc.) at 30%; the Arms (20%); and the Head (10%).


For each Skin Section, measure the amount of skin involved, as a percentage of the skin just in that part of the body (not the whole body — see below), and then assign it a score from 0 to 6:

< 10%1

So, if your head is 37% covered, your area score for your head — Ahead — would be 3. Find the area score for the other three Skin Sections — Alegs, Abody and Aarms.


The severity is measured by four different parameters: Itching, Erythema (redness), Scaling and Thickness (psoriatic skin is thicker than normal skin). Again, each of these is measured separately for each Skin Section. These are measured on a scale of 0 to 4, from none to “maximum,” according to the following chart:


So, if your head psoriasis itches moderately, that would mean that Ihead would be 2. If it’s only somewhat red, your Ehead score would be 1. Also calculate your Shead (scaling on the head) and Thead (thickness of the head psoriasis) scores, as well as all four scores (I, E, S and T) for the other three Skin Sections.

Toting Up the Index

When all 20 of the above scores are figured out, you’re ready to calculate your PASI. For each Skin Section, add up the four severity scores, multiply the total by the area score, and then multiply that result by the percentage of skin in that section, as follows:

Head: (Ihead+Ehead+Shead+Thead) x Ahead x 0.1 = Totalhead

Arms: (Iarms+Earms+Sarms+Tarms) x Aarms x 0.2 = Totalarms

Body: (Ibody+Ebody+Sbody+Tbody) x Abody x 0.3 = Totalbody

Legs: (Ilegs+Elegs+Slegs+Tlegs) x Alegs x 0.4 = Totallegs

Finally, the PASI is Totalhead+Totalarms+Totalbody+Totallegs. That’s it. This PASI will range from 0 (no psoriasis) to 96 (covered head-to-toe, with complete itching, redness, scaling, and thickness).

Problems with PASI

There are some obvious problems with using PASI as a clinical trial measurement. The first is that the severity scores appear to be highly subjective. While it really isn’t possible to measure itchiness without relying on the patient’s opinion, there should be (and actually may be, I don’t know) more distinct “categories” than those listed above. For example, it’s possible to take an actual measurement of the thickness of the skin, so why not rate the thickness in terms of the patient’s own normal skin, instead of calling it “moderate” or “severe?” Erythema could probably be fairly-accurately measured using a simple photographic filter and a reference card. Scaliness? I don’t know. The point is that it all seems to open to judgement calls, and not objective measure.

The second problem I read about in an article by Darren M. Ashcroft and Alain Li Wan Po. It has to do with how small the scales used are. “Good” measurements in trials tend to have a lot of possible values, and more is better. Why not use the actual percentage of coverage, instead of converting it into a number that runs from 0 to 6? Sure, it means that PASI would run up to 1600, instead of just 96, but who really cares? It’s just a number. Same for the severity numbers — just 0 to 4? Almost everyone in this day and age understands “on a scale of 0 to 10” very well. Okay, “Dave’s PASI” now runs from 0 to 4000.

A simple example can really illustrate the problem with the small scales used in the generic PASI: Say two patients just have psoriasis on the legs. One has 2.5% of his legs covered, the other has nearly 10% of hers. In terms of PASI, both will have an area score of 1 for the legs — they’re the same. However, convert these numbers to overall body coverage for just a moment. He has 1% total coverage, and she’s got 4%. According to a “common man’s” measure of psoriasis (the “palm method” from the NPF), anything less than 2% is “mild,” while between 2% and 10% is “moderate.” Now the two patients are counted differently. Of course, these two systems are attempting to measure different things, but I can’t see any obvious reason for the PASI’s groupings.

The third problem is simply that the PASI is complicated. 20 measurements, and then a bunch of calculating. Fine for clinical trials, but not for the average psoriatic, which is why we don’t typically go around swapping PASI numbers. Beyond that, where, exactly, do the “legs” end and the “body” begin? Same for the arms and head. Someone who’s done PASIs before might know the “official” distinctions, but not me.

Again, that “common man’s” measure is much easier for most of us, but it ignores the itching, erythema, etc.. Also, how does one accurately measure the percentage of coverage of just the arms, anyway? (Perhaps that’s the reason for the PASI’s groupings, after all — the inherit inexactness of skin coverage percentage?)

Which brings us to one last thing I wanted to mention. If you want to figure out your own PASI, just for kicks, and you’re not fortuitously involved in a clinical trial, here’s how to figure out the percentages when measuring the area of skin for each Skin Section: Use the NPF’s “palm method” on the different Skin Sections, and then multiply by a single factor to get the Area measurments you need for calculating your PASI:

Skin SectionFactor

For example, if you use the “palm method” on just your legs, and come up with 8% of total body skin, you’d multiply by 2.5 to get an Alegs of 20%. A single palm-sized patch on your scalp by the “palm method” is actually a full 10% of your head (Ahead). If you’re nearly 100% covered in some particular section, it’d probably be easier for you to measure the clear skin by the “palm method,” and then subtract (if you’re 3% clear on your legs, you’re 37% covered, or if 5% clear on the body, you’re 25% covered, for two quick examples).


Many, many thanks to Alberto, who taught me the “generic” PASI method in a Newsgroup message.