What is a skin lesion KOH exam?
A skin lesion KOH exam is a simple skin test to check if an
infection in the skin is caused by fungus.
KOH stands for potassium (K), oxygen (O), and
hydrogen (H). These elements make up potassium hydroxide. Besides the exam, KOH is used in fertilizers,
soft soaps, alkaline batteries, and other products.
It’s also known as KOH prep or fungal smear.
Why is a skin lesion KOH exam ordered?
A skin lesion — an abnormal change in the surface of the skin
— can have many causes. Your doctor may order a KOH exam if they suspect that a
fungus could be the cause of your lesion. Common fungal infections that can be
caught by performing KOH examination are ringworm and tinea cruris, commonly referred to as “jock itch.”
Symptoms of a fungal infection that may be
detected through a KOH exam include:
- brittle, deformed, or thickened
- itchy, red, scaly patches of skin
- thrush (white patches in the
- yeast infection (vaginal
discharge and itching)
Your doctor may also order the test to check
on the effectiveness of treatments related to fungal infection.
The test is very simple and carries no
How a skin lesion KOH exam is performed
A skin lesion KOH exam requires no special
preparation and will happen in an outpatient setting, so you won’t have to
spend the night at a hospital. If your doctor is taking a sample from a
bandaged piece of skin, the bandages will have to be removed.
During your appointment, your doctor will use
the edge of a glass slide or another instrument to scrape off small pieces of
skin from your lesion. Your doctor may use a swab to obtain fluid for testing
if the lesion is in the mouth or vagina.
These scrapings are then mixed with potassium
hydroxide. The potassium hydroxide destroys the healthy skin cells, leaving
behind only fungal cells. Normal results of a KOH test will show no fungi
present, while abnormal results will tell your doctor that you may have a
What to expect after a skin lesion KOH exam
If the potassium hydroxide destroys all the
cells from the sample, it means there’s no fungus present, and your symptoms
are most likely being caused by something else. If fungal cells are present,
your doctor will begin treatment for your infection.
A KOH examination is a straightforward,
simple procedure that your doctor might order to determine whether you have a
fungal infection on your skin. It’s a low-risk procedure, though you may
experience some light bleeding in the area where your skin was scraped for the
cell sample. Once your doctor has received the results of your exam and has
determined that you have a fungal infection, follow-up tests are usually
unnecessary, unless your doctor needs to know the type of fungus present. In that
case, a fungal culture will be ordered.