Potassium hydroxide examination of skin lesion

Alternative names
Skin lesion KOH exam; Fungal wet prep

Definition
The skin lesion KOH exam is a test to diagnose a fungal infection of the skin.

How the test is performed
The lesion is scraped, using a blunt edge such as the edge of a microscope slide. The scrapings from the skin lesion are placed in a solution of potassium hydroxide (KOH) and examined under the microscope. The fungus can be seen, if present. This is different from a saline (salt water) examination, since the KOH destroys all non-fungal cells, making it much easier to see the fungal material.

How to prepare for the test
There is no special preparation for the test.

Infants and children:
The physical and psychological preparation you can provide for this or any test or procedure depends on your child’s age, interests, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child’s age:

     
  • Infant test or procedure preparation (birth to 1 year)  
  • Toddler test or procedure preparation (1 to 3 years)  
  • Preschooler test or procedure preparation (3 to 6 years)  
  • Schoolage test or procedure preparation (6 to 12 years)  
  • Adolescent test or procedure preparation (12 to 18 years)

How the test will feel
You may feel a pressure sensation when the lesion is scraped from the skin.

Why the test is performed
The test is performed when fungal infections of the skin are suspected.

Normal Values
The lack of fungus is normal and expected.

What abnormal results mean

The KOH smear shows hyphae or fungal organisms. This may indicate ringworm (tinea corporis), athlete’s foot, jock itch, or many other fungal infections.

What the risks are
There is a small risk of bleeding or infection from scraping the lesion.

Special considerations
Not applicable.

Johns Hopkins patient information

Last revised: December 3, 2012
by Gevorg A. Poghosian, Ph.D.

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