Although most cases of psoriasis can be reliably diagnosed by simple physical examination, some cases can be difficult to diagnose, because the appearance can be similar to other skin conditions. Currently, there is no blood test that can conclusively diagnose psoriasis. A skin biopsy is the most specific test in the diagnosis of psoriasis.

Physical examination —Your doctor will begin by carefully examining your skin, scalp, fingernails, and toenails for reddening or scaling skin that is characteristic of psoriasis. Small pits on the fingernails are also signs of psoriasis. Your doctor will also examine your joints for signs of psoriatic arthritis.

Skin biopsy —To confirm the diagnosis, your doctor may also take a small sample of skin (a biopsy) to test for psoriasis.

Severity of Psoriasis

Psoriasis affects people with varying degrees of severity. The majority of people with psoriasis have what is considered mild psoriasis, while the remaining have moderate to severe psoriasis. Treatment depends on the severity of the condition.

If you do have psoriasis, your doctor will assess the severity and grade it according to the following guidelines:

Mild psoriasis —If the psoriasis covers less than 3% of your body, it is considered mild. Usually, people with mild cases develop isolated patches of psoriasis on the knees, elbows, scalp, hands, and feet.

Moderate psoriasis —If the psoriasis covers 3% to 10% of your body, it is considered a moderate case. Psoriasis may appear on the arms, legs, torso, scalp, and other areas.

Severe psoriasis —When psoriasis covers more than 10% of your body, it is considered severe. Large areas of skin may be covered with psoriasis plaques or pustules, or widespread erythrodermic psoriasis can cause severe peeling of the skin. People with severe psoriasis are more likely to develop psoriatic arthritis.