What is severe psoriasis?

When establishing the severity of disease, it has important impact on the patient quality of life than the area of skin affected by psoriasis.
All patients with psoriasis want to get rid of this condition. Some have one form extended psoriasis but others have psoriatic lesions only on the scalp or only at the knees or elbows. Some patients have a single board on penis.
Which of these has the most severe form of psoriasis?
According to the FDA, severe form of psoriasis is defined when psoriatic plaques affecting more than 20% of total body surface area.

But who is the most severely affected - a patient with psoriatic plaques in 25% of the body that manages to conceal the lesions  under clothing or an patient who has psoriasis plaque on only 1% of the body surface but who hides in the house and refuse invitations  in society, to shame?
In clinical practice, when determining severity of the condition, dermatologists always take into account the impact of psoriasis on quality of life.
Now, a group of experts to validate this concept. In a consensus report, they concluded that physical and mental disabilities should be taken into account when assessing the severity of psoriasis. Psoriasis affects the lives of patients in all aspects: physical, emotional, social, sexual and financial.
Patients affected by psoriasis have trouble sleeping, walking or using arms. They are usually sad, embarrassed and ashamed of their situation. They lose a lot of time and money for various treatments more or less effective. They are socially isolated and generally avoid to meet with other people in public.
This type of patients need "aggressive therapy" even when the skin is affected by small portions. The consensus group shared psoriasis patients in two groups according to the needs of local therapy compared to those who require systemic therapy and phototherapy.
Candidates for local therapy usually have psoriasis in less than 5% of body surface area. Nevertheless some patients with psoriasis on less than 5% of body surface area should be candidates for systemic therapy, phototherapy, or both because of the difficulties related to localization, coexistence psoriatic arthritis or damage physical or mental functions.