A quantitative definition of severe psoriasis for use in clinical trials.

Clinical trials of new agents for severe psoriasis require clear, well-characterized and objective measurement criteria to define the study population. The purpose of this paper is to offer parameters defining 'severe' psoriasis for use in clinical trials. If patients have at least 10% body surface area (BSA) involved and a psoriasis area and severity index (PASI) score > or =12, they have severe disease. While this definition assures inclusion of only patients with severe disease, a disadvantage is that some patients with truly severe disease (as defined by the impact of their disease on their quality of life) will be excluded. It is necessary to accept this trade off in order to have objective criteria, consistent across centers involved in clinical trials, for identifying patients affected by severe psoriasis.