What Is a KOH Prep Test?

What to expect when undergoing this test

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, 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.

Woman scratching her skin on her arm
Maria Fuchs / Getty Images

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

A KOH test can confirm the presence of fungi, including dermatophytes. Dermatophytes are fungi that require keratin for growth. Diseases caused by dermatophytes include athlete's foot, jock itch, nail infections, and ringworm. They commonly cause skin infections of the feet, the genitals, and, particularly in kids, the scalp.

The KOH test may be used after a clinical examination and a Wood lamp examination, which uses ultraviolet light to look closely at the skin.

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.

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 healthcare provider will provide instructions for how to care for the area from which the sample was taken.

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

  • Increased redness
  • Swelling
  • Heat
  • Pain

There are no specific contraindications for this test.

During the Test

If you see your healthcare provider 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 healthcare provider 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 healthcare provider 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. A KOH test can confirm the presence of dermatophytes, which include epidermophyton, trichophytan, and microsporum. It can also test for Candida albicans. This same yeast that causes oral thrush and vaginal infections can cause raised, itchy skin rashes as well.

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

Your healthcare provider 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 skin biopsy may be necessary if the results from the KOH test are inconclusive.

Based on the results of the KOH test, your healthcare provider may recommend an over-the-counter or prescription antifungal treatment. Follow-up appointments may be recommended to make sure that the treatment is working.

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 healthcare provider to prescribe the most appropriate treatment and get your rash cleared up quickly.

Frequently Asked Questions

  • What is the purpose of the KOH test?

    The KOH prep test is used for diagnosing fungal infections on the skin or nails. The test examines a scraping of cells from the affected area under a microscope to look for signs of a fungus. 

    The KOH test is also called a potassium hydroxide skin lesions exam, a fungal smear, or a skin scraping. 

  • How is a skin scraping done?

    A skin scraping is used to test skin cells for pathogens. The test involves a healthcare provider using a small scalpel or the edge of a glass slide to gently scrape off skin cells. 

    Depending on where the cells are taken from it may feel slightly uncomfortable, though it should not be painful. The area may bleed a little and tender following the test. 

  • How long does it take to get results from KOH test?

    You should get the results of the KOH test pretty quickly. The KOH test is performed in the doctor’s office.

    After taking a sample of skin cells, the healthcare provider adds a few drops of potassium hydroxide (KOH) solution to the drops on a slide. The solution kills all cells except for fungus.

    Your doctor will look at the slides under a microscope while you are still in the office to determine what medication to prescribe. 

10 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process 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.

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

  3. Achterman RR, White TC. A foot in the door for dermatophyte research. PLoS Pathog. 2012;8(3):e1002564. doi:10.1371/journal.ppat.1002564

  4. Hainer BL. Dermatophyte infections. Am Fam Physician.

  5. U.S. National Library of Medicine. Wood lamp examination.

  6. Consultant360. Performing in-office KOH prep tests.

  7. MedlinePlus. Candida infection of the skin.

  8. MedlinePlus. Tinea infections.

  9. MedlinePlus. Fungal culture test.

  10. MedlinePlus. Skin lesion KOH exam. U.S. National Library of Medicine.

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

  • Ponka D, Baddar F. Microscopic potassium hydroxide preparation. Can Fam Physician

By Heather L. Brannon, MD
Heather L. Brannon, MD, is a family practice physician in Mauldin, South Carolina. She has been in practice for over 20 years.