Doctors can identify many skin disorders simply by looking at the skin. Revealing characteristics include size, shape, color, and location of the abnormality as well as the presence or absence of other symptoms or signs. To check the distribution of a skin problem, the doctor usually asks the person to undress completely, even though the person may have noticed an abnormality on only a small area of skin.

Sometimes, a biopsy, in which a small piece of skin is removed for examination under a microscope, must be performed. 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 (shave biopsy), or round cutter (punch biopsy), removes a piece of skin the size of which is determined by the type of lesion, its location, and the type of tests to be performed.

When an infection (such as from fungi, bacteria, viruses, or mites) is suspected, a doctor may scrape off some material from the skin and examine it under a microscope (skin scraping), sometimes after applying special chemicals or stains. The material can also be sent to a laboratory, where the specimen is placed in a culture medium (a substance that allows microorganisms to grow). If the specimen contains bacteria, fungi, or viruses, they will often grow in the culture and can then be identified.

A Wood's light examination is used when certain skin infections are suspected. 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.

Skin Tests: Skin tests, including a "use" test, a patch test, a prick (puncture) test, and an intradermal test, may be performed if a doctor suspects an allergic reaction as the cause of a rash.

The use test, in which a suspected substance is applied far from the original area where the rash occurred (usually on the forearm), is useful when perfumes, shampoos, or other substances found in the home may be the cause.

In the patch test, many small samples of common and suspected causative agents, known as allergens, are applied to the skin (typically on the upper back) under adhesive tape. The skin under the patches is evaluated first after 48 hours, when the patches are removed, and then again at 96 hours. It often takes the skin several days to produce a visible reaction. If the substance produces a characteristic red, usually itchy rash, the person is probably allergic to it. Sometimes the substances produce an irritation that is not a true allergic reaction.

In the prick test, a drop of an extract of the suspected substance is placed on the skin. Then, the drop is pricked or punctured with a needle to introduce a very tiny amount of the substance into the skin. The skin is then observed for redness, hives, or both, which usually occur within 30 minutes.

In the intradermal test, tiny amounts of a substance are injected under the skin. The area is then watched for redness and swelling, which indicate an allergic reaction.

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 performed only by a trained health care practitioner.

Last full review/revision October 2006 by Robert J. MacNeal, MD