Skin Lesion KOH Exam

Written by Brian Krans | Published on July 16, 2012
Medically Reviewed by George Krucik, MD

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 is also known as KOH prep or fungal smear.

Why Is a Skin Lesion KOH Exam Ordered?

A skin lesion—an abnormal change in the surface of the skin—can have many causes. Your doctor may order a KOH exam if he or she suspects a fungus could be the cause of your lesion.

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
  • 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 risks.

How a Skin Lesion KOH Exam Is Performed

A skin lesion KOH exam requires no special preparation. 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.

These scrapings are then mixed with potassium hydroxide. The potassium hydroxide destroys all healthy cells, leaving behind only fungal cells.

What to Expect After a Skin Lesion KOH Exam

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

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