A Reevaluation of the Simplified Psoriasis Index (SPI) Using the Thai Language Version: A Study Examining the Validity, Reliability, and Interpretability of SPI When Used in Translation

Background: The Simplified Psoriasis Index (SPI) is a summary measure of psoriasis for current severity (SPI-s), psychosocial impact (SPI-p), and past history and interventions (SPI-i). Two complementary versions are available, differing only in that current severity is either professionally (proSPI-s) or patient self-assessed (saSPI-s). They have been shown to be valid, reliable, and interpretable. Objective: The current study investigated whether the validity, reliability, and interpretability of SPI could be replicated in an independent study in a different population and setting. Methods: A validated translation of SPI into Thai was used to assess the validity and reliability of SPI for assessing psoriasis in 100 Thai patients. The interpretability of proSPI-s and saSPI-s was assessed in 289 Thai patients. Results: There were close correlations between Psoriasis Area and Severity Index (PASI) and proSPI-s ( r = 0.84) and between PASI and saSPI-s ( r = 0.78). The SPI-p was closely correlated with the validated Thai version of the Dermatology Life Quality Index ( r = 0.78). Excellent intrarater (proSPI-s, saSPI-s, SPI-p, and SPI-i) reliability was demonstrated (all ICC ≥0.88). The PASI-equivalent cutoff scores for mild (PASI < 10) and severe psoriasis (PASI ≥ 20) were <9 and ≥18 for proSPI-s and <11 and ≥20 for saSPI-s. Conclusion: Our study confirms that SPI retains good validity and reliability in translation. The cutoff points we obtained were very close to those of the original study and confirmed that the originally proposed PASI-equivalent cutoff scores for mild and severe psoriasis of <9 and ≥18 for psoSPI-s and <10 and ≥20 for saSPI-s, respectively, were valid for use in a very different language and setting.