What Is a KOH Prep Test?

A simple test used to diagnose fungal infections of the skin

woman scratching her skin on her arm

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In This Article

A KOH prep test is used to diagnose fungal infections of the skin or nails. Skin cells obtained from the affected area are placed on a slide with potassium hydroxide (KOH) solution and examined under a microscope looking for fungal elements. It is a very simple, non-invasive, and nearly painless test that is quick and accurate.

This test is sometimes called a potassium hydroxide skin lesion exam, fungal smear, or skin scraping.

Purpose of the Test

A KOH test is recommended for patients exhibiting symptoms of fungal infections of the skin, hair, or nails. 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)

KOH prep tests are not done for every rash. Instead, they are typically ordered when there's a question as to the cause of the rash.

Many skin conditions cause rashes that can be, visually, nearly identical. This is where the KOH prep test comes in.

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. Dermatophytes are fungi that require keratin for growth. They include:

  • Epidermophyton
  • Trichophyton
  • Microsporum

These species commonly cause skin infections of the feet, the genitals, and, particularly in kids, the scalp.

A KOH test can also test for Candida albicans. This same yeast that causes oral thrush and vaginal infections can cause raised, itchy skin rashes as well.

Risks and Contraindications

KOH prep test is a very simple, safe test that carries little risk. There are no specific contraindications.

There is a very small chance 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.

Let your doctor know if, after the test, you develop signs of infection in the scraped area, including:

  • Increased redness
  • Swelling
  • Heat
  • Pain

During the Test

A KOH prep test is done right in your doctor's office. The procedure is fairly quick and simple.

There are no special instructions for preparing for the test and the procedure should take no more than a few minutes to conduct.

The affected skin or nail is gently scraped with a small scalpel or the edge of a glass slide. The scrapings from the skin are placed on a microscope slide and a few drops of a potassium hydroxide (KOH) solution are added. The slide is heated for a short time and then examined under the microscope.

Potassium hydroxide (KOH) solution is alkaline and has the ability to dissolve keratin that is scraped from the outer layer of the skin. As the KOH dissolves the material binding the skin cells together, any fungus present is released. This allows for the identification of organisms such as dermatophytes.

You may feel pressure when the doctor or nurse scrapes your skin. In some cases, the scraping may feel slightly uncomfortable if it is taken from under a nail or if the area from where the sample is being obtained is tender. Generally, though, there is no pain associated with the procedure itself.

After the test, the skin may be slightly tender where it was scraped, but this will fade over the course of a few hours. Most people, though, do not notice any after-effects.

Interpreting Results

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. This means your skin rash is not being caused by a fungal infection, but rather something else.

Abnormal results mean that fungus is present and your skin rash is being caused by a fungal infection.

Common skin conditions that can be found using the KOH prep test include:

Your doctor will be able to share the results with you immediately and provide you with treatment options before you leave the appointment.

A KOH test does not specifically identify the type of fungus, only shows that fungus is present. In most cases, it's not necessary to know exactly the type of fungus causing the rash; your dermatologist can make an educated judgment and prescribe treatment.

A fungal culture can be done if your dermatologist decides it is necessary. A fungal culture allows the fungus to grow so that the specific type can be identified. Results of a fungal culture can take weeks, so this test is done only in cases where knowing the specific strain of fungus is necessary to develop a more precise treatment plan.

A Word From Verywell

A KOH prep test, also called a potassium hydroxide preparation test, is done in cases where a fungal infection is suspected. The test is quick, accurate, and in most cases, pain-free. Knowing if a skin rash is caused by a fungus, or not, allows your physician to prescribe the most appropriate treatment and get your rash cleared up quickly.

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Article Sources

Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial policy to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  • Medline Plus. Skin lesion KOH exam. U.S. National Library of Medicine. Updated October 2, 2019.

  • Ponka D, Baddar F. Microscopic potassium hydroxide preparation. Can Fam Physician. 2014 Jan;60(1):57.

  • Elewski BE, et al. Fungal diseases. In: Bolognia JL, Jorizzo JL, Schaffer JV, eds. Dermatology. 3rd ed. Philadelphia, PA: Elsevier Saunders; 2012: chap 77.