The Simplified Psoriasis Index (SPI): A Practical Tool for Assessing Psoriasis

Abstract

The Simplified Psoriasis Index (SPI) is a summary measure of psoriasis with separate components for current severity (SPI-s), psychosocial impact (SPI-p), and past history and interventions (SPI-i). It derives from the Salford Psoriasis Index but replaces Psoriasis Area and Severity Index (PASI) with a composite weighted severity score designed to reflect the impact of psoriasis affecting functionally or psychosocially important body sites. Two complementary versions are available, differing only in that current severity (SPI-s) is either professionally (proSPI-s) or patient self-assessed (saSPI-s). This study examined the criterion and construct validity and response distribution of proSPI-s, saSPI-s, and SPI-p in 100 patients with plaque psoriasis. A further 50 patients were assessed for test-retest reliability of these three components. Interrater reliability of proSPI-s was assessed in 12 patients, each assessed by 12 assessors (144 assessments). There was close correlation between PASI and proSPI-s (r=0.91); SPI-p was closely correlated with the Dermatology Life Quality Index (r=0.89). Strong intrarater (proSPI-s, saSPI-s, SPI-p, and SPI-i) and interrater (proSPI-s) reliability was demonstrated (all intraclass correlation coefficients >0.75). There were wide response distributions for all three components. We believe that both professional (proSPI) and self-assessed (saSPI) versions can readily be introduced into routine clinical practice.