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 doctor 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. Color stains can be used to make the fungi easier to see.

Why It Is Done

If you have discolored, 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. But 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 medicine, your doctor will probably confirm that you have a fungal infection using a KOH preparation test if you are considering taking antifungal pills (oral treatment). Many insurance companies now require testing to verify a fungal infection before paying for medicines.

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

Complete the medical test information form (PDF) (What is a PDF document?) to help you prepare for this test.

Citations

1. Verma S, Heffernan MP (2008). Superficial fungal infection: Dermatophytosis, onychomycosis, tinea nigra, piedra. In K Wolff et al., eds., Fitzpatrick's Dermatology in General Medicine, 7th ed., vol 2, pp. 1807–1821. New York: McGraw Hill.