Diagnosis of Skin Disorders

Sometimes a skin biopsy, in which a small piece of skin is removed for examination under a microscope, must be done.

For this simple procedure, the doctor generally numbs a small area of skin with a local anesthetic and, using a small knife (scalpel), scissors, razor blade (called a shave biopsy), or round cutter (called a punch biopsy), removes a piece of growth. The size of the piece is determined by the type of abnormal growth that is suspected, its location, and the type of tests to be done.

Skin Biopsy Procedures

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Sometimes the doctor can both diagnose and treat a small growth by removing the entire growth along with a small border of normal skin around it (sometimes called an excisional biopsy). The growth is sent to a laboratory for examination under a microscope.

If a fungal infection or scabies is suspected, a doctor may do a skin scraping. During this test, the doctor scrapes off some material from the skin, such as scale, and examines it under a microscope. Sometimes special chemicals or stains are applied to the material.

If an infection is suspected, a doctor removes a sample of material (such as a skin cells or fluids) and sends it to a laboratory, where the sample is placed in a culture medium (a substance that allows microorganisms to grow). If the sample contains bacteria, fungi, parasites, or viruses, they will often grow in the culture and can then be identified.

Doctors do a Wood light examination when they suspect certain skin disorders. The skin is illuminated with an ultraviolet light (also known as a black light) in a dark room. The ultraviolet light makes some fungi or bacteria glow brightly. The light also accentuates the skin's pigment (melanin), making pigmentation abnormalities, such as vitiligo, more visible.

Vitiligo Seen Under a Wood Light

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Vitiligo is viewed more easily under a Wood light (right).

© Springer Science+Business Media

A Tzanck test is done to help a doctor diagnose certain diseases caused by viruses, such as herpes simplex and herpes zoster. When these diseases are active, they produce small blisters (vesicles).

During a Tzanck test, a doctor removes the top of a blister with a sharp blade and then scrapes the blister with a scalpel to obtain fluid. The specimens are examined under a microscope after special stains are applied.

Diascopy is done so that a doctor can observe color changes that occur when pressure is applied to the skin.

During this test, a doctor presses a microscope slide against a growth to see whether it blanches (becomes less red) or otherwise changes color. Certain kinds of growths become less red, whereas others do not. Some skin growths (such as those caused by sarcoidosis) turn a yellowish brown color when this test is done.

Skin tests, including a use test, a patch test, a prick (puncture) test, and an intradermal test, may be done if a doctor suspects an allergic reaction is the cause of a rash.

In the use test, a suspected reaction-causing substance, known as an allergen, is applied far from the original area where the rash is (usually on the forearm). This test is useful when perfumes, shampoos, or other substances found in the home may be the cause of a rash. If redness, blistering, itching, or sometimes pain occurs in (and sometimes around) the area where the allergen is applied, the test is considered positive.

In the patch test, many small samples of common and suspected allergens are contained in patches that doctors tape to the skin (typically on the upper back). The patches are left on for 48 hours and then removed. Doctors evaluate the skin under the patches at removal and then again 72 or 96 hours later. It often takes the skin several days to produce a visible reaction. If an allergen produces a characteristic, usually itchy reaction, the person is probably allergic to it. In people who have light skin, the color of the reaction is usually red. In people who have dark skin, the color of the reaction may have less contrast with surrounding skin and thus be more subtle. Sometimes the allergens produce an irritating reaction that is not a true allergic reaction.

In the prick test, doctors place a drop of an allergen on the skin. Then, the drop is pricked with a needle to introduce a very tiny amount of the allergen into the skin. The skin is then observed for hives, which usually occur within 15 to 20 minutes. Redness (or other color changes) can help support a positive test. (See also Skin testing.)

Skin Prick Allergy Test (Illustration)

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This illustration shows the skin prick method of allergy testing. Drops of solutions that contain different allergens, such as pollen and animal dander, are placed on a skin prick device. The skin pricks are then placed on the person's forearm where they lightly prick the skin surface to allow the allergen to enter the body. If the person is allergic to any of the allergens, a hive appears at the site of application, as shown in magnified view (top).

MAURIZIO DE ANGELIS/SCIENCE PHOTO LIBRARY

In the intradermal test, tiny amounts of a diluted allergen are injected under the skin. Doctors then watch the area for the extent and firmness of swelling, which indicate an allergic reaction. Redness (or other color changes) can help support a positive test. (See also Skin testing.)

Intradermal Allergy Test

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This photo shows an intradermal allergy test. For this test, a small amount of test solution is injected into the person’s skin through a needle.

EDWIGE/SCIENCE PHOTO LIBRARY

Although rare, prick and intradermal tests can cause a severe allergic reaction, known as anaphylaxis, which can be life threatening. Therefore, these types of tests should be done only by a trained health care professional.

See allergen-specific serum IgE tests for more information about blood testing for allergies.

See provocative testing for more information about these tests for allergies.