KOH (potassium hydroxide) preparation for ringworm

Exam Overview

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

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

He or she takes a sample of hair from the scalp or beard area by:

  • Rubbing a clean new toothbrush, wet gauze, or a sample dish (media plate) on the infected area.
  • Pulling 10 to 12 broken hairs from the infected area. This is difficult, because the infected hairs are often broken at the root, and long hairs are usually not infected.

The doctor or nurse places the 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. Color 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:

  • Ringworm of the scalp or beard if 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 blisterlike bumps with pus (kerions); or hair loss.
  • Ringworm of the skin if a person has patches of skin that are itchy, red, or scaly, with blisterlike bumps on the edges.

Results

Findings of a KOH test may include the following.

Normal

No fungi are present in the skin or hair samples.

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

Abnormal

Fungi are present in the 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.

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Last Updated: March 24, 2009

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