BACKGROUNDA wide variety of scoring systems have been proposed to assess severity of psoriasis. Given its importance as a health issue both for patients and health care systems, it is critically important to evaluate the validity and reliability of existing outcome measures.OBJECTIVEThe objective of this systematic review was to assess the extent of validation including the validity, reliability, sensitivity to change and ease of use of available outcome measures for psoriasis.MATERIALS AND METHODSWe conducted a systematic review of all clinical studies (prospective and retrospective) investigating the severity of psoriasis patients and published between January 1980 and June 2009. The following methodological validation and quality criteria were recorded systematically: construct validity, content validity and internal consistency, intra-observer variation and inter-observer variation, sensitivity to change and acceptability/ease of use assessed as time required to perform measurement.RESULTSBased on methodological validation and quality criteria, six clinical severity scores were selected and analysed (PASI, BSA, PGA, LS-PGA, SPI and SAPASI scores). We did not find substantial evidence of construct validity for any of the psoriasis clinical severity scores. Content validity was studied by considering the PASI score as gold standard. The relative content validity was good for the LS-PGA, PGA, and SPI scores, which correlated strongly with the PASI score. The SAPASI and PASI scores showed moderate correlation. Internal consistency was good for the PASI and LS-PGA scores. The PASI, BSA, PGA and LS-PGA scores displayed limited intra-observer variation. The inter-observer variation was low for LS-PGA (ICC < 0.5) and SAPASI, moderate for PASI, SPI and PGA and high for BSA (ICC > 0.8). The PASI score and the SAPASI displayed moderate sensitivity to change.DISCUSSIONBased on this systematic review, it appears that none of the severity scores used for psoriasis meets all of the validation criteria required for an ideal score. However, we can conclude that the PASI score is the most extensively studied psoriasis clinical severity score and the most thoroughly validated according to methodological validation criteria. Despite certain limitations, use of the PASI score can be recommended for scientific evaluation of the clinical severity of psoriasis.