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The KOH test takes its name from the chemical formula for potassium hydroxide (KOH), which is the substance used in the test. The test, which is also called a potassium hydroxide preparation, is done to rapidly diagnose fungal infections of the hair, skin, or nails. A sample of the infected area is analyzed under a microscope following the addition of a few drops of potassium hydroxide.
The primary purpose of the KOH test is the differential diagnosis of infections produced by dermatophytes and Candida albicans from other skin disorders. Dermatophytes are a type of fungus that invade the top layer of the skin, hair, or nails, and produce an infection commonly known as ringworm, technically known as tinea. It can appear as "jock itch" in the groin or inner thighs (tinea cruris); on the feet (tinea pedis); on the scalp and hair (tinea capitis); and on the nails (tinea unguium). Tinea versicolor appears anywhere on the skin and produces characteristic unpigmented patches. Tinea unguium affects the nails.
Similar symptoms of redness, scaling, and itching can be caused by other conditions, such as eczema and psoriasis. The KOH test is a quick, inexpensive test—often done in a physician's office—to see if these symptoms are caused by a dermatophyte. If a dermatophyte is found, treatment is started immediately; further tests are seldom necessary.
A yeast (candidal) infection of the skin or a mucous membrane, such as the mouth, often produces a white cheesy material at the infection site. This type of infection, known as thrush, is also identified with the KOH test.
The KOH test involves the preparation of a slide for viewing under the laboratory microscope. KOH mixed with a blue-black dye is added to a sample from the infected tissues. This mixture makes it easier to see the dermatophytes or yeast under the microscope. The KOH dissolves skin cells, hair, and debris; the dye adds color. The slide is gently heated to speed up the action of the KOH. Finally the slide is examined under a microscope.
Dermatophytes are easily recognized under the microscope by their long branch-like structures. Yeast cells look round or oval. The dermatophyte that causes tinea versicolor has a characteristic spaghetti-and-meatballs appearance.
If the KOH test is done in the doctor's office, the results are usually available while the person waits. If the test is sent to a laboratory, the results will be ready the same or following day. The KOH test is covered by insurance when medically necessary.
The physician selects an infected area from which to collect the sample. Scales and cells from the area are scraped using a scalpel. If the test is to be analyzed immediately, the scrapings are placed directly onto a microscope slide. If the test will be sent to a laboratory, the scrapings are placed in a sterile covered container.
A normal, or negative, KOH test shows no fungi (no dermatophytes or yeast).
Dermatophytes or yeast seen on a KOH test indicate the person has a fungal infection. Follow-up tests are usually unnecessary.
Crissey, John Thorne. "Common Dermatophyte Infections. A Simple Diagnostic Test and Current Management." Postgraduate Medicine February 1998: 191-192, 197-198, 200, 205.
Dermatophyte — A type of fungus that causes diseases of the skin, including tinea or ringworm.
KOH — The chemical formula for potassium hydroxide, which is used to perform the KOH test. The tests is also called a potassium hydroxide preparation.
Thrush — A disease of the mouth, caused by Candida albicans and characterized by a whitish growth and ulcers. It can be diagnosed with the KOH test.
Tinea — A superficial infection of the skin, hair, or nails, caused by a fungus and commonly known as ringworm.