Dermatologists can diagnose many skin conditions simply by observation. Signs to look for include the size, shape, color, and location of the lesion and the presence or absence of other signs and symptoms. Sometimes, the physician must take a small piece of lesion tissue for biopsy and then observe the tissue pattern under a microscope. This examination is usually done under local anesthesia and a scalpel, scissors or biopsy drill is used to remove a piece of skin. The size of the specimen depends on the location and type of lesion and the type of examination to be performed. When there is a possible skin infection, such as a fungal infection, bacterial infection, viral infection or mites, the doctor scrapes the specimen from the skin for microscopic examination. Sometimes a special stain is also needed. Specimens may also be sent to a laboratory for culture. The technician places the specimen in a culture medium (a substance that allows microorganisms to grow). If there are bacteria, fungi or viruses within the specimen, they can grow in the medium and can be identified. Wood’s lamps are often used to examine suspected skin infections. The doctor uses an ultraviolet light (what is commonly referred to as a black light) to irradiate the patient’s skin in a dark room during the examination. The UV lamp causes the fungus or bacteria to fluoresce in a variety of colors. The UV light also deepens the pigmentation of the skin, so some skin conditions with abnormal pigmentation, such as vitiligo, can be treated more clearly. Skin tests: Skin tests include patch tests, prick (pinprick) tests, and intradermal tests. If the doctor suspects that the rash is caused by an allergy, a skin test can be performed to find the cause. USE test: Place the suspected substance away from the site of the rash (usually on the forearm). The USE test is useful when allergies are suspected to be caused by fragrances, shampoos, or other substances in the room. Patch test: A number of samples of the suspected allergen are placed on the skin with tape (usually on the back). 48 hours later the tape with the allergen is removed from the back and the skin’s reaction is observed; the skin’s reaction is evaluated again after 96 hours. Generally, it takes a few days for the skin to react to the suspected allergen. If the skin becomes red and itchy, it is an indication that the skin may be allergic to the substance. Note, however, that sometimes the skin reaction may be caused by physical irritation of the skin by the suspected substance and is not a true allergic reaction. Prick test: A drop of the allergen solution is placed on the skin and then the skin is pricked with a needle so that a small amount of the allergen solution enters the skin. 30 minutes later, the skin may become erythematous or (and) windy. Intradermal test: A drop of the allergen solution is injected under the skin. If erythema and edema appear at the injection site, an allergic reaction has occurred.