KOH (Potassium Hydroxide) Preparation

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KOH (Potassium Hydroxide) Preparation

Examination Overview

Doctors use the KOH preparation to find out whether a fungal infection is present on the nails, skin, scalp, or beard.

A doctor or nurse takes a sample of skin by lightly scraping the infected area that is scaling or flaking.

The doctor or nurse places the nail, skin or hair sample on a slide with potassium hydroxide (KOH) solution and gently heats it. This solution slowly dissolves the skin cells but not the fungus cells. The fungus cells are then visible with a microscope. Colour stains may be used so that the fungus is easier to see.

Why It Is Done

A doctor may do a KOH test to determine whether a person has a fungal infection. Fungal infections may cause:

  • Ringworm of the scalp or beard. With this infection, a person has flakes of dead skin (dandruff) on the hair; broken, crusted, or matted hair; redness or irritation of the scalp or beard; swollen areas and blister-like bumps with pus (kerions); and/or hair loss.
  • Ringworm of the skin. With this infection, a person has patches of skin that are itchy, red, or scaly, with blister-like bumps on the edges.
  • Athlete's foot. With this infection, a person has peeling, cracking, and scaling on the bottoms of their feet or between the toes, or blistered skin on their feet.


Findings of a KOH test may include the following.


No fungi are present in the nail, skin or hair samples.

Other tests may be done to find out the cause of the skin infection.


Fungi are present in the nail, skin or hair samples.

What To Think About

Your doctor may be able to find out whether you have a fungal infection by the appearance of the rash and may not need to do a KOH test.

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


By Healthwise Staff
Primary Medical Reviewer Kathleen Romito, MD - Family Medicine
Primary Medical Reviewer Anne C. Poinier, MD - Internal Medicine
Specialist Medical Reviewer Alexander H. Murray, MD, FRCPC - Dermatology
Last Revised May 4, 2011

This information does not replace the advice of a doctor. Healthwise, Incorporated disclaims any warranty or liability for your use of this information.