KOH (potassium hydroxide) preparation test for athlete's foot

Exam Overview

The KOH preparation test is used to find out whether there is a fungal infection of the skin, such as athlete's foot.

Your health professional will take samples by lightly scraping the skin of your foot with a sharp blade or the edge of a microscope slide. He or she may also take nail samples if a toenail is also infected. The skin or nail scrapings are placed on a slide with potassium hydroxide (KOH) solution and heated. This solution dissolves the skin cells but not the fungus cells. The fungus cells can then be seen with a microscope. Color stains can be used to highlight the fungi.

Why It Is Done

A KOH preparation may be done to determine the cause of cracking, scaling, peeling, or blistered skin, or if there is an area of persistent irritation (and sometimes redness) on the feet. The presence of fungi suggests that the condition is probably athlete's foot.



No fungi are present in the skin or nail scrapings. Other skin tests may be done to determine the cause of the skin irritation or nail problem.


Fungi are present in the skin or nail sample.

What To Think About

Test results on severe toe web infections may not always show fungi. In this case, a bacterial infection may hide the fungal infection.

If you have been diagnosed with athlete's foot before and the symptoms have returned, a KOH preparation test will probably not be needed. Your health professional may suggest you treat the infection with nonprescription or prescription antifungal medicine.

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

Last Updated: July 2, 2008

Author: Maria G. Essig, MS, ELS

Medical Review: Kathleen Romito, MD - Family Medicine & Anne C. Poinier, MD - Internal Medicine

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