Understanding PASI; largely inadequate, but 'it's what we're stuck with', says one M.D.

September 1, 2005

The Psoriasis Area and Severity Index (PASI) is a critical dermatologic tool -- the predominant system available for the specific purpose of measuring psoriasis severity and the one study investigators usually rely on as their primary outcome measure.

Editor's note: In March, the First Annual International Psoriasis Educational Symposium on Clinical Trials met in Scottsdale, Ariz. The day-and-a-half meeting featured speakers discussing all aspects of clinical trials in dermatology, under the auspices of the International Psoriasis Council. Kim Papp, M.D. is chairman and Craig Leonardi, M.D. helped direct the program. Alan Menter, M.D., was among the presenters.

The Psoriasis Area and Severity Index (PASI) is a critical dermatologic tool - the predominant system available for the specific purpose of measuring psoriasis severity and the one study investigators usually rely on as their primary outcome measure.

Yet, the tool is largely inadequate. It is so cumbersome that dermatologists rarely use it in clinical practice because PASI measurements can be tedious and are known to be inaccurate and inconsistent, even across psoriasis experts, according to Alan Menter, M.D., chief of dermatology, Baylor Medical Center, Dallas.

PASI's concept

Four factors determine the PASI score: erythema, scaling, induration and body surface area (BSA). PASI treats the different body areas - the head and neck, trunk, upper extremities and lower extremities - as separate anatomic regions. Dermatologists taking PASI measurements come up with scores of zero, for non-evident, to four, which is severe, for each parameter.

This, he says, leads to ambiguity among even the most experienced study investigators.

"Some people overestimate the percent of that BSA involved; some people overestimate the redness. Erythema is not easy to evaluate, especially in different racial groups," he explains. "So, it is an inaccurate measurement, and when people start clearing - when you get PASI scores below 10 - then, it is even less accurate."

Quality of life issues

To determine quality of life, dermatologists might turn to other tools, such as the Dermatology Quality of Life Index (DLQI), which asks patients questions such as, "Over the last week, has your skin prevented you from working or studying?" and "Has your skin created problems with your partner? ... with sexual activity?"

Patients answer using a one-to-five rating system, with one being "very much."

Another rating system, from a quality of life point of view, is the short form-36 (SF-36). Physicians use this rating tool for all sorts of diseases, beyond those affecting the skin.

Patient results from the SF-36 make it clear that quality of life is an important consideration when measuring psoriasis severity, Dr. Menter says.