Table of ContentsView AllTable of ContentsWhat Does the KOH Prep Test Look For?How Is the KOH Prep Test Done?Interpreting Results of a KOH TestRisks and Contraindications
Table of ContentsView All
View All
Table of Contents
What Does the KOH Prep Test Look For?
How Is the KOH Prep Test Done?
Interpreting Results of a KOH Test
Risks and Contraindications
A KOH prep test is a simple, non-invasive procedure for diagnosingfungal infectionsof the skin or nails. Cells are collected from the affected area, placed on a slide with a solution consisting of potassium hydroxide, and examined under a microscope to look for signs of a fungus.
Also called a potassium hydroxide skin lesion exam, fungal smear, or skin scraping, the KOH prep test is quick, accurate, and virtually painless.
This article discusses how and why the KOH prep test is done, what it feels like, and how the results are interpreted.
Maria Fuchs / Getty Images
A KOH prep test canconfirm the presence of fungicalled dermatophytes, which include epidermophyton, trichophytan, and microsporum. Dermatophytes are a type of fungi that require keratin for growth.
A KOH prep test can also test forCandida albicans.This yeast causesoral thrushandvaginal infectionsand can cause raised, itchy skin rashes too.
A KOH test may be recommended for someone who has symptoms of a fungal infection of the skin, hair, or nails, such as:
Common skin conditions that can be diagnosed using the KOH prep test include:
These conditions commonly cause skin infections of the feet, the genitals, and the scalp (mostly in kids).
The KOH test may be used after a clinical examination and aWood lamp examination, which uses ultraviolet light to look closely at the skin.
KOH prep tests are not done for every rash. Instead, they are typically ordered when there’s a question as to the cause of the rash.
A healthcare provider will likely do a KOH prep test in the office if they examine your skin and think that you could have a fungal infection. You won’t have to make a separate appointment or do any type of preparation beforehand.
The test itself will take no more than a few minutes and will proceed as follows:
Potassium hydroxide solution is alkaline and destroys all non-fungal cells. When the liquid is examined under the microscope, your healthcare provider is able to see if there is any fungus present.
How the KOH Prep Test Feels
Usually, the procedure is painless. You may feel pressure when the healthcare provider scrapes your skin. Sometimes it’s slightly more uncomfortable if the sample is taken from under a nail or other tender area.
The skin that was scraped may be slightly tender for a few hours after the test. Beyond that, people usually don’t have any other issues.
A KOH test does not specifically identify the type of fungus, it only shows that fungus is present. In most cases, it’s not necessary to know exactly the type of fungus causing the rash; your dermatologist or healthcare provider can make an educated judgment and prescribe treatment, such as over-the-counter or prescription antifungal treatment.
Your healthcare provider will share the results with you immediately and provide you with treatment options before you leave the appointment.
Next Steps
If your dermatologist decides it’s necessary, a fungal culture can be done to identify the specific type of fungus. Results of a fungal culture can take weeks, so this test is done only when knowing the specific strain of fungus is needed to develop a more precise treatment plan.
A skin biopsy may be necessary if the results from the KOH test are inconclusive.
Varieties of Fungal and Yeast Infections
The KOH prep test carries little risk aside from a small chance of bleeding or infection as a result of scraping the skin to obtain a sample.
After the test, your healthcare provider will provide instructions on how to care for the area where the sample was taken.
There are no specific contraindications for this test.
Call your healthcare provider know if you develop signs of infection in the scraped area, including:Increased rednessSwellingHeatPain
Call your healthcare provider know if you develop signs of infection in the scraped area, including:
Summary
9 SourcesVerywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.Ponka D, Baddar F.Microscopic potassium hydroxide preparation.Can Fam Physician. 2014;60(1):57.MedlinePlus.Candida infection of the skin.Government of South Australia.Fungal infections of the hair, skin, or nails.Achterman RR, White TC.A foot in the door for dermatophyte research.PLoS Pathog. 2012;8(3):e1002564. doi:10.1371/journal.ppat.1002564MedlinePlus.Tinea infections.Hainer BL.Dermatophyte infections.Am Fam Physician. 2003;67(1):101-109.MedlinePlus.Skin lesion KOH exam.Consultant360.Performing in-office KOH prep tests.MedlinePlus.Fungal culture test.
9 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.Ponka D, Baddar F.Microscopic potassium hydroxide preparation.Can Fam Physician. 2014;60(1):57.MedlinePlus.Candida infection of the skin.Government of South Australia.Fungal infections of the hair, skin, or nails.Achterman RR, White TC.A foot in the door for dermatophyte research.PLoS Pathog. 2012;8(3):e1002564. doi:10.1371/journal.ppat.1002564MedlinePlus.Tinea infections.Hainer BL.Dermatophyte infections.Am Fam Physician. 2003;67(1):101-109.MedlinePlus.Skin lesion KOH exam.Consultant360.Performing in-office KOH prep tests.MedlinePlus.Fungal culture test.
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
Ponka D, Baddar F.Microscopic potassium hydroxide preparation.Can Fam Physician. 2014;60(1):57.MedlinePlus.Candida infection of the skin.Government of South Australia.Fungal infections of the hair, skin, or nails.Achterman RR, White TC.A foot in the door for dermatophyte research.PLoS Pathog. 2012;8(3):e1002564. doi:10.1371/journal.ppat.1002564MedlinePlus.Tinea infections.Hainer BL.Dermatophyte infections.Am Fam Physician. 2003;67(1):101-109.MedlinePlus.Skin lesion KOH exam.Consultant360.Performing in-office KOH prep tests.MedlinePlus.Fungal culture test.
Ponka D, Baddar F.Microscopic potassium hydroxide preparation.Can Fam Physician. 2014;60(1):57.
MedlinePlus.Candida infection of the skin.
Government of South Australia.Fungal infections of the hair, skin, or nails.
Achterman RR, White TC.A foot in the door for dermatophyte research.PLoS Pathog. 2012;8(3):e1002564. doi:10.1371/journal.ppat.1002564
MedlinePlus.Tinea infections.
Hainer BL.Dermatophyte infections.Am Fam Physician. 2003;67(1):101-109.
MedlinePlus.Skin lesion KOH exam.
Consultant360.Performing in-office KOH prep tests.
MedlinePlus.Fungal culture test.
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