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