What is a skin lesion KOH exam?

A skin lesion KOH exam is a simple skin test to check if an infection in the skin is caused by fungus.

KOH stands for potassium (K), oxygen (O), and hydrogen (H). These elements make up potassium hydroxide. Besides the exam, KOH is used in fertilizers, soft soaps, alkaline batteries, and other products.

It’s also known as KOH prep or fungal smear.

A skin lesion— an abnormal change in the surface of the skin — can have many causes. Your doctor may order a KOH exam if they suspect that a fungus could be the cause of your lesion. Common fungal infections that can be caught by performing KOH examination are ringworm and tinea cruris, commonly referred to as “jock itch.”

Symptoms of a fungal infection that may be detected through a KOH exam include:

  • brittle, deformed, or thickened nails
  • itchy, red, scaly patches of skin or scalp
  • thrush (white patches in the mouth)
  • yeast infection (vaginal discharge and itching)

Your doctor may also order the test to check on the effectiveness of treatments related to fungal infection.

The test is very simple and carries no significant risks.

A skin lesion KOH exam requires no special preparation and will happen in an outpatient setting, so you won’t have to spend the night at a hospital. If your doctor is taking a sample from a bandaged piece of skin, the bandages will have to be removed.

During your appointment, your doctor will use the edge of a glass slide or another instrument to scrape off small pieces of skin from your lesion. Your doctor may use a swab to obtain fluid for testing if the lesion is in the mouth or vagina.

These scrapings are then mixed with potassium hydroxide. The potassium hydroxide destroys the healthy skin cells, leaving behind only fungal cells. Normal results of a KOH test will show no fungi present, while abnormal results will tell your doctor that you may have a fungal infection.

If the potassium hydroxide destroys all the cells from the sample, it means there’s no fungus present, and your symptoms are most likely being caused by something else. If fungal cells are present, your doctor will begin treatment for your infection.

A KOH examination is a straightforward, simple procedure that your doctor might order to determine whether you have a fungal infection on your skin. It’s a low-risk procedure, though you may experience some light bleeding in the area where your skin was scraped for the cell sample. Once your doctor has received the results of your exam and has determined that you have a fungal infection, follow-up tests are usually unnecessary, unless your doctor needs to know the type of fungus present. In that case, a fungal culture will be ordered.


Are there over-the-counter products that I can use to treat a fungal infection?

Anonymous patient


You can usually start treatment for a superficial fungal infection (one that is on the skin and not deep inside), such as ringworm or athlete’s foot, with an over-the-counter product. Antifungal products are available in many forms such as creams, ointments, shampoos, and sprays. Some common antifungals available over-the-counter are clotrimazole, miconazole, tolnaftate, and terbinafine. Talk to your doctor or pharmacist if you need help finding the right treatment for your fungal infection.

Laura Marusinec, MDAnswers represent the opinions of our medical experts. All content is strictly informational and should not be considered medical advice.
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