Psoriasis definitely affects the health-related quality of life (HRQoL) of those suffering from this disease. Various measure of HRQoL include the Koo-Mentor Instrument among others.
The degree to which psoriasis affects a patient's HRQoL does not clearly correlate with their baseline level of disease. Some patients cope better than others. What does correlate well is the improvement of HRQoL seen with improvement in PASI score during treatment. In order to see a large impact in HRQoL, a large improvment in PASI score, such as PASI 75 or greater must be achieved. What this means is that a 75% reduction in psoriasis severity is sort of a threshold below which large changes in a patient's health-related quality of life are less likely. Many biologic drugs are indeed capable of achieving this level of improvement especially newer drugs such as Ustekinumab.
source: Importance of PASI-75 response for HRQoL highlighted By Eleanor McDermid03 October 2008Arch Dermatol Res 2008; Advance online publication