KOH Prep Test to Diagnose Fungal Skin Infections

woman scratching her skin on her arm
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A KOH test is used to diagnose fungal infections of the skin or nails

The skin or nail is scraped with a scalpel or glass slide causing dead skin cells to fall off onto a glass slide. A few drops of a Potassium Hydroxide (KOH) solution are added to the slide and the slide is heated for a short time. The KOH dissolves the material binding the skin cells together releasing the fungus. The slide is then viewed under the microscope looking for fungal elements.

Why Is a KOH Preparation Test Necessary?

A KOH test is recommended for patients exhibiting symptoms of fungal infections. Possible symptoms that would suggest these skin conditions include:

  • A localized rash
  • Ring-shaped, red patches with scaly edges
  • Severe itching of the skin
  • Rashes that fail to respond to or worsen with corticosteroid medications
  • Rashes that persist for months

There are a series of possible risk factors that may further exacerbate the symptoms listed above, that would indicate the necessity of a KOH test:

  • A history of skin disorders such as eczema, hay fever or allergic asthma
  • A history of exposure to animals (especially cats, but also dogs, horses, cows, and pigs)
  • A history of low immune system responses (immunosuppression)

Without running a KOH preparation test, your doctor has no way to conclusively attribute your skin condition to a fungal infection or to other conditions that exhibit similar symptoms (such as scabies). Patients with skin conditions might suffer needlessly for years, while the doctor tries, sometimes unsuccessfully, to provide treatment based on visual characteristics of the skin condition.

A KOH test can confirm cases of the most common dermatophytes (fungi that require keratin for growth), such as EpidermophytonTrichophyton, and Microsporum species, which commonly cause skin infections of the feet, the genitals, and, particularly in kids, the scalp. 

Potassium Hydroxide (KOH) solution is alkaline and has the ability to dissolve keratin that is scraped from the outer layer of the skin. This then allows for the identification of organisms such as dermatophytes or scabies under a microscope to get the right diagnosis and consequently, the most effective treatment.

About the Procedure 

An In-Office KOH prep test is done right in your doctor's office. The procedure is fairly quick and simple, but you should be aware of the following:

  • There are no special instructions for preparing for the test and the procedure should take no more than a few minutes to conduct. 
  • You may feel pressure when the doctor or nurse scrapes your skin, but there is no pain associated with the procedure itself. There is a small risk of bleeding or infection from scraping the skin, so be sure to follow your doctor's aftercare instructions on how to keep the area clean.
  • Your doctor scrapes the problem area of your skin with a handheld scalpel or the edge of the microscope slide.
  • The scrapings from the skin are then placed on a microscope slide and given a squirt of potassium hydroxide (KOH) solution.
  • KOH destroys all non-fungal cells, and so when the liquid is examined under the microscope, your doctor is able to see if there is any fungus present in the liquid.
  • Normal results indicate that there is no fungus present in your skin sample. Abnormal results mean that fungus is present.
  • Your doctor will be able to share the results with you immediately and provide you with treatment options before you leave the appointment.
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    Article Sources

    • Elewski BE, et al. Fungal diseases. In: Bolognia JL, Jorizzo JL, Schaffer JV, eds. Dermatology. 3rd ed. Philadelphia, PA: Elsevier Saunders; 2012: chap 77.
    • U.S. National Library of Medicine. Skin lesion KOH exam.