The chronic skin condition psoriasis affects approximately 7.5 million people in the United States, according to the American Academy of Dermatology. Although psoriasis generally causes only mild discomfort, severe cases can be disfiguring, and cases that affect the hands can be disabling. The appearance of the skin normally allows for an accurate diagnosis of psoriasis, but in other cases more specific tests are needed.
Symptoms
The Merck Manual says that correct diagnosis of psoriasis usually occurs after a patient's description of their symptoms and simple clinical observation of the skin lesions. Most commonly, patients have red, thick patches of skin with a silvery scale-like covering; some have pustules on the skin. The skin can crack and bleed at site of the lesions and might be mildly itchy. Other types of psoriasis can affect the fingernails and toenails, the scalp, the sensitive skin folds in the armpits, under the breasts and around the genitals and even the joints. The skin symptoms come and go in cycles called flares.
Other Possibilities
Several skin conditions that can mimic psoriasis, and your doctor will need to rule them out. Ringworm, a fungal infection of the skin, causes a red, often ring-like rash that can be scaly. Like psoriasis, lichen planus is an inflammatory condition, but it causes rows of flat bumps that are itchy. Seborrheic dermatitis causes greasy, scaly and red patches of skin, usually confined to patches of skin that are normally oily such as the scalp, face, upper back and chest. Eczema or atopic dermatitis causes red, bumpy patches of skin in characteristic places and is usually very itchy.
Skin Biopsy
A doctor can perform a skin biopsy to test for psoriasis and rule out other similar conditions. According to the Merck Manual, a skin biopsy is rarely necessary to differentially diagnose psoriasis, and it might not yield a definitive answer. After applying a local anesthetic, the doctor cuts or punches out a small sample of skin for analysis under a microscope.
Analysis
Biopsies from skin afflicted with psoriasis might show excessive numbers of keratinocytes. Keratinocytes are the most common skin cell type. Their overproduction and rapid migration to the skin's surface overwhelms the normal schedule of skin cell turn-over so that thicky scaly patches of skin form. In addition to excessive numbers of keratinocytes, the analysis of the skin biopsy might show signs of inflammation pointing to the underlying immune problem that causes hyperproliferation of keratinocytes.
Blood Test
A doctor might perform a blood test if she suspects arthritis because of psoriasis. Unlike the case with rheumatoid arthritis, the blood from patients with psoriatric arthritis lacks a particular antibody. The negative blood test combined with the presence of skin or nail lesions can diagnose psoriatric arthritis.
