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Diagnosing a Skin Rash

author image Leigh A. Zaykoski
Leigh Zaykoski has been a writer and editor for six years. Her medical writing has appeared on dozens of websites. Zaykoski attended the University of Pittsburgh and Keystone College, studying microbiology and business administration. She is currently pursuing a medical writing certification.
Diagnosing a Skin Rash
A doctor examines a woman's back Photo Credit Dean Bertoncelj/iStock/Getty Images


Skin rashes are relatively common and can occur due to skin disorders, allergic reactions or exposure to chemicals and rough fabrics. If a skin rash does not go away with over-the-counter treatment methods, it should be evaluated by a physician to determine its cause. There are several methods of diagnosing a skin rash that is persistent or severe.

Physical Examination

When a patient visits a family doctor or dermatologist for a skin rash, the medical professional examines the rash and may be able to make a diagnosis based on the appearance, color and texture of the rash. The rash is examined to determine if it consists of scales, blisters, spots, sores or bumps. It is also examined to determine if it is purple, white, tan or red. The texture of the rash can be smooth, scaly or bumpy, which can help doctors make a more definitive diagnosis. The location of the rash can also be important in diagnosing a skin rash. Some rashes only appear on the face and scalp, while others can affect the chest and abdomen. If a physical examination does not allow a doctor to make a diagnosis, other tests can be performed.

Skin Scrapings and Biopsy

Skin scrapings can be taken so that the tissue can be analyzed under a microscope. This can help doctors diagnose the cause of a skin rash. When collecting skin scrapings, a medical professional cleans the affected area with alcohol wipes or sterile water. The active borders of the rash are scraped so that epidermal cells can be collected. If there are several lesions, more than one should be scraped to obtain a good sample. The scrapings are placed on a glass slide and sent to a laboratory for analysis. This analysis can reveal the cause of a skin rash so that proper treatment may be given.

Skin biopsy is a procedure in which skin cells or a skin samples are removed from the surface of the body. The samples are sent to a laboratory to diagnose the cause of skin conditions and diseases that cause skin rashes. Shave biopsy is done with a tool that looks like a razor. This tool removes a small piece of the epidermis, which is the top layer of the skin. Punch biopsy is done using a circular tool that can punch out small areas of the skin. This type of skin biopsy can get samples from below the skin's surface. Excisional biopsy is a procedure in which an entire portion of abnormal skin is removed. The skin may be removed from the epidermis right down to the fatty skin layer. The results of a skin biopsy can help a doctor form a plan to treat the cause of a skin rash.

Blood and Allergy Tests

Blood tests that check immune system function can be used to determine if a skin rash is caused by an autoimmune disease. The antinuclear antibody test is used to determine if someone has systemic lupus erythematous or another autoimmune disorder. Blood tests can also be done to detect levels of specific antibodies. If an autoimmune disease is present, these tests can determine if antibodies are attacking healthy cells, which can cause a skin rash.

Allergy skin tests can be used to determine if an allergy is causing a skin rash. During this type of test, the skin is exposed to allergens, which are substances that can cause an allergic reaction. The skin is observed by a medical professional to determine if an allergic reaction has occurred. If a specific allergen is found to cause a reaction, treatment plans can be developed so that exposure to the allergen is limited. Medications and allergy shots may also be given based on the results of an allergy skin test.

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