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KOH Test

KOH Test


The KOH (potassium hydroxide) test is the microscopic examination of a skin, nail, or hair sample for the presence of a fungus that infects these structures. The test takes its name from the chemical formula for potassium hydroxide (KOH), the substance used in the test to clear skin cells, protein, and cellular debris, making the fungi easier to observe. A sample from the infected area is analyzed under a microscope following the addition of a few drops of potassium hydroxide. A stain may be added to make the fungi more easily visible.


The KOH test is used for the rapid, differential diagnosis of infections produced by dermatophytes from skin disorders such as eczema and psoriasis which may appear similar. Dermatophytes are a type of fungus that invade the skin, hair, or nails, and produce an infection commonly called ringworm (tinea). There are three genera of fungi commonly implicated in human skin, nail, and hair infections. These are Trichophyton spp. (found in skin, nail, and hair infections), Epidermophyton spp. (skin and nail infections), and Microsporum spp. (skin and hair infections). The KOH test may also be used on scrapings from the mouth or vagina for the rapid identification of Candida spp., the cause of yeast infections of the mouth (thrush) and the vagina (vaginitis). If a dermatophyte or yeast infection is found, antibiotic treatment can be started immediately, and further testing is not usually required. The KOH prep cannot identify the specific causative organism. When fungus is found, the specimen can be submitted for fungal culture to identify the causative agent(s).


There are no specific precautions for patients who require this test. Health care professionals who perform this test should be properly trained in skin scraping, nail, and hair follicle removal procedures, the identification of fungi and yeasts by microscopy, and the safe handling of specimens.


Ringworm of the skin produces red or gray, scaly patches of itchy skin. Hair infection results in brittle hair shafts that fall out easily. Deeper infections may be ulcerative, discolored, and purulent. The health care provider selects an infected area from which to collect the sample. A scalpel or edge of a glass slide is used to gently scrape skin scales from the infected area. For hair samples, a forceps is used to remove hair shafts and follicles from the infected site. If the test is to be performed immediately, the scrapings are placed directly onto a microscope slide and are covered with 10% or 20% potassium hydroxide. If the test will be sent to a laboratory, the scrapings are placed in a sterile covered container for later testing.

After adding KOH, the slide should be allowed to stand for five minutes in order to dissolve skin cells, hair, and debris. If the sample remains unclear after five minutes, it may be left for an additional five to 10 minutes. Dimethyl sulfoxide can be added to the KOH to enhance clearing, and lactophenol cotton blue stain can be added to make the fungi easier to see. The slide may be gently heated to enhance the digestive action of the KOH. If a fluorescent microscope is available, calcofluor white stain may be added to the KOH preparation. This will cause the fungi to become fluorescent, making them easier to identify.

Dermatophytes are easily recognized under the microscope by their long branch-like tubular structures called hyphae. Fungi causing ringworm infections produce septate (segmented) hyphae. Some show the presence of spores formed directly from the hyphae (arthroconidia). Yeast infections of the skin can also be identified by the KOH test. Yeast cells appear round or oval and budding forms may be seen.

Fungal infections of the skin are described by the site of infection. Tinea cruris is a fungal infection occurring in the groin or inner thigh, tinea pedis on the feet, tinea capitis on the scalp and hair, and tinea unguium on the nails. Tinea versicolor refers to a fungal infection of the skin caused by Malassezia furfur. This organism cannot be cultured, and is recognized by characteristic yellowish skin patches and the microscopic appearance of the organism. The fungus produces curved hyphae and round yeast forms that give it a spaghetti-and-meatball appearance under the microscope.


There is no special preparation for the patient prior to sample collection.


The patient will experience slight discomfort from sample collection. The skin site or area exposed when the nail is removed may need to be covered by a gauze bandage.


There are usually no complications for the patient associated with this test.


A normal, or negative, KOH test shows no fungi (no dermatophytes or yeast). Dermatophytes or yeast seen on a KOH test indicate the patient's symptoms are caused by a fungal infection. Follow-up tests are usually unnecessary.

Health care team roles

Physicians, nurses, or physician assistants usually collect the skin, nail, or hair samples from the patient. The KOH test procedure may be performed by the physician or by a clinical laboratory scientist/medical technologist. If fungal cultures are required, they are performed by a clinical laboratory scientist/medical technologist who specializes in microbiology.

Patient education

The health care professional who collects the sample also explains the testing procedure to the patient, and explains why the test is required. The physician or nurse will also explain the treatment regimen, as well as the importance of adhering to the treatment as required, if a fungal infection is diagnosed.


Tinea— A superficial infection of the skin, hair, or nails, caused by a fungus and commonly known as ringworm.



Fischbach, Frances. "Diagnosis of Fungal Disease." In A Manual of Laboratory & Diagnostic Tests. 6th ed. Philadelphia: Lippincott Williams & Wilkins, 2000, pp.521-525.

Forbes, Betty A., Daniel F. Sahm, and Alice S. Weissfeld. "Laboratory Methods in Basic Mycology." In Bailey & Scott's Diagnostic Microbiology. 10th ed. St. Louis: Mosby, 1998, pp.870-961.

Sacher, Ronald A., Richard A. McPherson, with Joseph M. Campos. "Direct Visualization of Infectious Agents." In Widmann's Clinical Interpretation of Laboratory Tests. 11th ed. Philadelphia: F. A. Davis Company, 2000, pp.613-618.


The American Society for Clinical Laboratory Science. 6701 Democracy Blvd., Ste. 300, Bethesda, MD 20817. (301) 657-2768. 〈〉.

The American Society for Microbiology. 1752 N St. N.W., Washington, DC 20036. (202) 737-3600. 〈〉.


"Dermatologic Disorders: Fungal Skin Infections." In The Merck Manual of Diagnosis and Therapy. 17th ed., internet edition, 〈〉, 2001.

"Dermatologic Disorders: Special Diagnostic Methods." In The Merck Manual of Diagnosis and Therapy. 17th ed., internet edition, 〈〉, 2001.

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