KOH (potassium hydroxide) preparation for fungal nail infections

Examination Overview

It is not always possible to accurately diagnose a fungal nail infection based on appearance alone. A potassium hydroxide (KOH) preparation test can help determine whether you have a fungal nail infection.

Your health professional will collect a sample of skin and/or nail fragments (debris) under the infected nail. If a sample of debris cannot be removed, he or she can take a nail sample by lightly scraping the nail near the infected area or by using a small blade to shave off a piece of nail.

Nail or debris samples are placed on a slide with a potassium hydroxide (KOH) solution and gently heated. The solution slowly dissolves the skin and nail cells, leaving the fungus cells. The fungus cells can then be seen with a microscope. Colour stains can be used to make the fungi easier to see.

Why It Is Done

If you have discoloured, damaged, broken, or thickened nails or have a buildup of skin and nail fragments (debris) under a nail, a KOH test can help confirm whether a fungal nail infection is present.

Results

Normal

No fungi are present in the debris or nail sample. However, the test may be done again using more samples taken from other parts of the infected nail. This is because there may no longer be fungi on the edge of the infected area, where it is easiest to scrape for a sample, or in the dead tissue, even if you have a fungal nail infection.

Abnormal

Fungi are present in the debris or nail sample.

What To Think About

About 50% of all nail diseases are fungal nail infections.¹ Because of this and the risks associated with oral antifungal medication, your health professional will probably confirm that you have a fungal infection using a KOH preparation test if you are considering taking antifungal pills (oral treatment).

If your test is normal, your health professional may do other tests to determine the cause of the nail damage.

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Author:	Douglas Dana Sabra L. Katz-Wise	Last Updated: October 11, 2006
Medical Review:	Patrice Burgess, MD - Family Medicine Tom Bailey, MD - Family Medicine Randall D. Burr, MD - Dermatology
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