Instruments to measure and to monitor the severity of psoriasis over time are needed for research and for optimal patient care. Scoring psoriasis has moved from an earlier time when clinical categories were adopted without concern about their reliability; for example, from clearance to more recent semi-quantitative scores, such as the Psoriasis Area and Severity Index (PASI), that carry the allure of being objective and quantitative "hard" data but actually translate a subjective judgement into a number. The PASI score has never been standardized, and data on interrater and intrarater reliability are limited. Better clinimetrics of disease severity are needed. The next generation of instruments should reflect the major concern of patients and treating physicians relative to safe and effective long-term disease control for a lifelong condition.
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