How does the Psoriasis Area and Severity Index (PASI) work?

How does the PASI work?

The PASI is a questionnaire that must be filled in by doctors. Doctors must fill in the PASI questionnaire to measure the severity of psoriatic lesions and the patient's response to treatment.

The PASI produces a numeric score ranging from 0 to 72, with higher values reflecting higher psoriasis affectation. In general, a PASI score of 5 to 10 is considered a moderate disease, and a score over 10 is considered severe.

A 75% reduction in the PASI score (PASI 75) is the current benchmark for most clinical trials in psoriasis and the criterion for efficacy of new psoriasis treatments approved by the FDA.

How do you calculate the PASI score?

To measure the PASI score, you must look at three things: redness, induration, and scaling. Then, you must know where the psoriasis is located, and how much of that area is affected.

The severity is determined by dividing the body into four regions: head (h), upper extremities (u), trunk (t) and lower extremities (l). Each region accounts for 10%, 20%, 30%, and 40% of the total Body Surface Area, also called BSA).

Each of these areas is assessed separately for erythema, induration, and scaling on a scale of 0 (none) to 4 (very severe).

Extent of psoriatic involvement is graded as follows:

• 0 = no involvement;
• 1 = 1% to 9%;
• 2 = 10% to 29%;
• 3 = 30% to 49%;
• 4 = 50% to 69%;
• 5 = 70% to 89%;
• 6 = 90% to 100%.

What is the PASI formula?

The formula that calculates the PASI is the following:

PASI = 0.1 (E_h + l_h + S_h) A_h + 0.2 (E_u + l_u + S_u) A_u + 0.3 (E_t +l_t + S_t) A_t + 0.4 (E_l +l_l +S_l) A_l

Legend:

• E = erythema
• I = induration
• S = scaling
• A = area
• h = head score
• u = upper extremities
• t = trunk score
• l = lower extremities.

What are the limitations of the PASI?

A number of limitations of the PASI have been identified, and include the following:

• The PASI has been criticized for not correlating the clinical extent of the disease with quality of life and the psychological stress caused by psoriasis. The patient's measure of the quality of life is often worse than the physician-rated clinical severity.
• There are significant inter-rater reliability issues regarding the measurement of BSA.
• PASI scores can vary substantially between experienced and inexperienced physicians, raising concerns for inter-rater reliability.
• Improvements in PASI score are not linearly related to severity or improvements in psoriasis. The extent of psoriatic involvement is measured using a scale of 1 to 6 and the areas corresponding to each score are nonlinear.
• Some severe cases may be scored low. For example, scores as low as 3 (on palms and soles) may represent psoriasis that disables a patient from work and other life activities.
• Most patients fall into a narrow band of scores, thereby decreasing the usefulness of the full range of scores (i.e., scores above 40 are rare). Validity of this scale may be overrated, in part because of the skew toward lower scores.
• There is little research on the reliability of the assessments for erythema, desquamation, and induration, together with overall PASI scores.
• Criterion validity is restricted by the lack of a gold standard measure of psoriatic severity.
• The PASI lacks sensitivity as erythema, desquamation, and induration are scored with equal weight within each of the four body regions. Thus, a reduction in scaling with a concomitant increase in skin erythema could be recorded with the same PASI score.
• Improvement of the histological phenotype of psoriasis can be underestimated by the percent improvement in PASI (e.g., reduction of T cells, loss of keratin 16 (K16) expression and reduction in epidermal thickness).
• Little work has been done to determine the clinical relevance of derived PASI scores.

What is the difference between a severe and a moderate score?

There are many differences between severe and moderate psoriasis.

The following chart shows how different severities look depending on each of the visual signs: redness, thickness, and scaling.