We have little data on the prevalence and impact of psoriasis, but estimates suggest that there is affection about 2% of the population English. There are differences in the spread between men and women, and impact is greatest in the white population in which members of other ethnic groups.
A study on people with severe form of psoriasis showed that 60% of subjects had, in previous years, temporarily stopped work as a direct result of their condition. People with serious illness could be forced to undergo each year at different periods of hospitalization of the average duration of 20 days each.
The psoriasis is generally classified as mild, moderate or severe. They used many different scales for measuring the severity of psoriasis. These scales are based, in varying degrees, on the following criteria: the proportion of body surface area affected, disease activity (degree of redness, peeling and thickness of plaques), respond to previous therapy and impact of disease on the person.
In clinical trials, the index PASI (Psoriasis Area Severity Index) is the most widely used measurement system for psoriasis. The index combines the PASI assessment of the severity of injuries and its interest in the disease in a single score that can range from 0 (no disease) to 72 (maximum degree of illness). A PASI score greater than 10 is linked to a number of indicators commonly associated with serious illness, such as the need for hospitalization.
The results of clinical trials are usually reported in terms of the number of people attaining a specific percentage reduction in the PASI score compared to their baseline (eg., PASI 75 is a reduction of 75% from baseline score).
The EMEA (European Medicines Agency) considers the achievement of a PASI 75, as a response to treatment in the form of severe psoriasis.