What Is a KOH Prep Test?

KOH Prep Test: Uses, Side Effects, Procedure, Results

woman scratching her skin on her arm

Maria Fuchs / Getty Images

In This Article

A KOH prep test is a simple, non-invasive procedure for diagnosing fungal infections of the skin or nails. Cells are obtained from the affected area, placed on a slide with a solution consisting of potassium hydroxide (KOH), and examined under a microscope to look for signs of a fungus. Also called a potassium hydroxide skin lesion exam, fungal smear, or skin scraping, the KOH prep test is quick, accurate, and virtually painless.

Purpose of Test

A KOH test may be recommended for someone who has symptoms of a fungal infection of the skin, hair, or nails, such as:

  • 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 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 look nearly identical.

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 cause 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

The KOH prep test carries little risk aside from a small chance of bleeding or infection as a result of scraping the skin to obtain a sample.

After the test, your doctor will provide instructions for how to care for the area from which the sample was taken.

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

  • Increased redness
  • Swelling
  • Heat
  • Pain

There are no specific contraindications.

During the Test

If you see your doctor for a rash that they suspect upon visual examination might be due to a fungal infection, they will likely do a KOH prep test on the spot in the office. You will not have to make a separate appointment or do any type of preparation beforehand.

The test itself will take no more than a few minutes and will proceed as follows:

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.

Was this page helpful?

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.
  1. Ponka D, Baddar F. Microscopic potassium hydroxide preparationCan Fam Physician. 2014;60(1):57.

  2. Government of South Australia. Fungal infections of the hair, skin, or nails.

  3. Faergemann J. Atopic dermatitis and fungi. Clin Microbiol Rev. 2002;15(4):545-63.

  4. Low CY, Rotstein C. Emerging fungal infections in immunocompromised patients. F1000 Med Rep. 2011;3:14. doi:10.3410/M3-14

  5. Gilson RL, Crane JS. Scabies (Sarcoptes Scabiei). Treasure Island, FL. StatPearls Publishing. Updated December 13, 2019.

  6. Hainer BL. Dermatophyte infections. Am Fam Physician. 2003;67(1):101-8.

  7. MedlinePlus. Candida infection of the skin. Updated December 1, 2018.

  8. Consultant360. Performing in-office KOH prep tests. September 2013.

  9. MedlinePlus. Tinea infections. Updated December 19, 2018.

  10. MedlinePlus. Fungal culture test. Updated April 15, 2019.

Additional Reading

  • 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.