The KOH preparation is used to identify fungal organisms in the skin, hair, or nails. The doctor obtains a tissue sample using a scalpel or another instrument. Heat and potassium hydroxide (KOH) are then applied to the sample to dissolve keratin—a fibrous protein that is a major component of skin, hair, and nails—as well as the skin cells that make keratin. Once these substances have been removed, fungal elements can be detected under a microscope.

Purpose of the KOH Preparation

  • To determine whether itchy, red, crack, blistered or scaly conditions of the skin, hair, and nails are caused by a fungal infection

Who Performs KOH Preparation

  • A physician

Special Concerns about KOH Preparation

  • If the sample is too small or is taken from an area in which there is no fungus, false-negative results may be obtained.
  • Previous use of antifungal drugs may also lead to false-negative results.

Before the KOH Preparation

  • No special preparation is required.

What You Experience

  • If your skin is the site of the suspected infection, the doctor scrapes the outer layer of abnormal skin with a scalpel.
  • When the scalp is affected, the doctor gently removes diseased hairs with a forceps and also scrapes the scalp with a scalpel.
  • For nail infections, the examiner scrapes the inner surface of the nail below the tip or clips off the portion of the nail that appears abnormal.
  • You may experience some minor discomfort while the sample is being collected.
  • Sample collection takes about 1 minute, and the results are usually available within 10 minutes.

Risks and Complications

  • There are no risks or complications associated with this test.

After the KOH Preparation

  • You may resume your normal activities after the test.

Results of KOH Preparation

  • If fungal organisms are detected under a microscope, your doctor will prescribe appropriate antifungal medication. In some cases, a fungal culture may be performed to confirm the results or identify the specific type of fungus.
  • If results are negative, your doctor may take a second sample or order a fungal culture to be done.


The Johns Hopkins Consumer Guide to Medical Tests

Simeon Margolis, M.D., Ph.D., Medical Editor

Updated by Remedy Health Media

Publication Review By: the Editorial Staff at

Published: 17 Jan 2012

Last Modified: 27 Feb 2015