KOH (Potassium Hydroxide) Preparation

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. Color stains may be used so that the fungus is easier to see.

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

• Reference Ringworm of the scalp or beard Opens New Window. 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 blisterlike bumps with pus (kerions); and/or hair loss.
• Reference Ringworm of the skin Opens New Window. With this infection, a person has patches of skin that are itchy, red, or scaly, with blisterlike bumps on the edges.
• Reference 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.

Normal

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.

Abnormal

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

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 Reference PDF Opens New Window document?) to help you prepare for this test.

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