Physical Examination
Psoriasis causes areas of the skin to become red and inflamed and develop silvery scales. It can also cause itching and burning and affect any part of the body. Sometime psoriasis can also affect the nails, causing them to become thick and pitted, and it can also lead to arthritis. Psoriasis can typically be diagnosed using a thorough physical exam and careful examination of the effected areas. Although psoriasis can resemble other skin problems, psoriatic lesions are different. Seborrheic dermatitis causes the skin to be unusually oily. Other skin disorders, such as lichen planus, ringworm and pityriasis rosea all cause skin lesions, but these tend to have a characteristic shape distinguishable from psoriasis. Because these conditions don't affect the nails or feet, skin lesions coupled with nail or joint problems strongly suggests that the patient has psoriasis.
Skin Biopsy
For a definitive diagnosis a skin biopsy is usually required, especially if the patient's skin problems are not clear enough for a positive diagnosis. With a skin biopsy, a small piece of skin from the effected area is taken and examined under a microscope. A pathologist will look at this slide and search for areas of inflammation (which is the underlying cause of psoriasis) as well as skin cells that are abnormally shaped due to rapid replication (which is how psoriasis occurs). A positive skin biopsy is the best way to diagnose psoriasis.
Distinguishing Between Types
Once psoriasis has been diagnosed, the next step is to determine what kind it is based on the characteristics and location of the skin problems. The most common psoriasis is plaque psoriasis which causes the typical red and raised skin problems with silvery scales. This can also affect the scalp where it will resemble a particularly bad case of dandruff. Guttate psoriasis, on the other hand, is marked by the development of small red and itchy dots in the shape of a water drop that can occur on your arms, legs, trunk and scalp, typically after a bacterial infection. Pustular psoriasis is marked by the rapid formation of blisters filled with pus on your fingertips, hands and feet. Finally there is erythrodermic psoriasis, which causes the skin over the entire body to become red and inflamed.
