Introduction: The Psoriasis Area and Severity Index (PASI) is the most widely used tool to assess psoriasis disease severity in clinical trials, although it can be exceedingly cumbersome for use in daily clinical practice. Because clinical trials rely on the PASI for inclusion criteria, having a PASI score on a clinic patient may be useful for determining if the patient has a level of disease severity similar to that of patients treated in clinical trials.

Purpose: The purpose of this study is to assess a simplified measure of psoriasis disease severity that is more conducive to use in general dermatology practice, the simplified PASI (SPASI).

Methods: We evaluate an area-weighted assessment of lesion severity composed of the sum of the average redness, thickness, and scaliness of all the psoriasis lesions multiplied by an estimate of total body surface area involved. The SPASI is mathematically derived from the PASI. The SPASI and PASI are not identical because of the categorical nature of area estimates used in the PASI. We use existing psoriasis-population data regarding the anatomical distribution of psoriasis lesions to create a simulated patient database. Monte Carlo analysis is then performed to determine the relation between the PASI and SPASI.

Results: For a sample population with a mean PASI score of 12.8, the mean SPASI was 14.2. Correlation between the PASI and the SPASI was high (r = 0.90). Bland-Altman analysis showed no consistent bias between the PASI and the SPASI. When attempting to identify simulated patients with a PASI score of 12 (an inclusion criterion for many clinical trials for severe psoriasis), SPASI was 97 percent sensitive and 66 percent specific.

Discussion: The SPASI is much less onerous than the PASI, requiring estimation of only four rather than sixteen independent variables. It provides a quick and practical estimate of disease severity similar to the PASI and can be used to communicate that patients have a level of disease severity similar or dissimilar to that of patients studied in clinical trials.