Psoriasis is a common, chronic-recurrent inflammatory dermatosis. Its most typical clinical manifestation are well-demarked, erythemato-squamous plaques, namely on the extensor sites of the extremities. In this case, the diagnosis can easily be made clinically. However, this chronic plaque-type manifestation accounts for only around 70 % of cases. There are numerous other clinical manifestations, including inverse psoriasis lacking scales, acute exanthematic as well as localized manifestations, and finally pustular forms, some of which may represent pathogenetically distinct entities. In these cases, biopsies might be necessary to establish the diagnosis and to exclude numerous differential diagnoses.
Clinical assessment of psoriasis comprises both aspects accessible by physicians’ observation as well as patient-reported outcomes. The best-known tool to assess chronic plaque-type psoriasis is the Psoriasis Area and Severity Index (PASI), quantifying the involved body surface area along with the extent of redness, infiltration, and scaling. Given its limitations, numerous modifications of the PASI have been proposed. An alternative approach is to assess psoriasis more globally, e.g. by the Physician Global Assessment (PGA). The most widely used tool to document patient-reported outcomes is the Dermatology Life Quality Index (DLQI), a 10-item questionnaire that can also be used to assess other dermatoses.