Assessing the validity and response distribution of the simplified psoriasis index in patients receiving phototherapy

Background/Objectives: The simplified psoriasis index (SPI) is a three-part multidimensional tool incorporating disease severity, psychosocial impact and historical course completed by the health-care professional (professional SPI, [proSPI]) or the patient (self-assessment SPI, [saSPI]). We aimed to assess the validity and response distribution of proSPI and saSPI in patients with psoriasis undergoing phototherapy. Methods: The validity and response distribution of SPI was assessed by recording saSPI and proSPI in patients with psoriasis before and after a course of phototherapy. Recruitment ended once 100 complete data sets were available for analysis. Results: Altogether 52 of the 100 patients evaluated were male and most (93) underwent narrowband UVB phototherapy. There was a close correlation between the proSPI-current severity score (proSPI-s) with the psoriasis area and severity index (PASI) score (r = 0.76, r = 0.86) before and after treatment, respectively. Although pretreatment correlation between the saSPI-current severity score (saSPI-s) and PASI was weak (r = 0.39), a more close correlation was noted at the end of treatment (r = 0.50). A moderate correlation was observed between the SPI-psychosocial impact score (SPI-p) and the dermatology life quality index (DLQI), both before and after phototherapy (r = 0.64, r = 0.73). The SPI had wide response distributions in all three domains. Conclusions: Both versions of SPI demonstrated wide response distributions and the proSPI-s in particular was shown to have good validity with PASI.