What is a KOH Fungal Smear test? 

Fungi are tiny organisms found everywhere in the environment. They grow anywhere from walls, food, clothes and paper. Fungi are even present (as spores) in air. There are millions of fungal species in the world; however, only a few can cause infections in humans. Fungal infections are mostly superficial. These include:

Fungi can also grow on hair and scalp. Regardless of type, all fungal infections can easily spread from one person to another.

In rare cases, fungi may cause serious infections, involving the lungs, bloodstream, brain or other organs in the body. Though serious fungal infections are more likely in people with a weak immune system.

The KOH test is a simple test performed to diagnose a fungal infection. KOH stands for potassium hydroxide. When mixed in a tissue sample (skin, hair or nails), KOH dissolves the non-fungal cells, leaving only the fungal cells behind. The sample is then directly examined under a microscope to detect and identify fungal elements to determine the right treatment.

  1. Why is a KOH Fungal Smear test performed?
  2. How do you prepare for a KOH Fungal Smear test?
  3. How is a KOH Fungal Smear test performed?
  4. What do KOH Fungal Smear test results mean?

Your doctor may perform this test if he/she suspects that you have a fungal infection.

Symptoms of fungal infections generally include:

Specific symptoms on the basis of the location of the infection are:

  • Nails:

    • Discoloured nails (brown, yellow or white)
    • Thickened or hard nails
    • Brittle or cracked nails
  • Feet (tinea pedis or ‘athlete’s foot’):
    • Red, itchy, swollen, peeling skin between the toes and sometimes on the sole or heel
    • Blisters on the feet
  • Groin (tinea cruris or ‘jock itch’):
    • Red, scaly, itchy spots in the inner thighs
  • Scalp (tinea capitis):
    • Red, itchy, scaly, circular, bald patches on the scalp
  • Beard (tinea barbae):
    • Red, itchy, scaly rash on the cheeks, chin and upper neck
    • Pus-filled spots or crusting
    • Hair loss
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You do not need any preparation before this test.

Your doctor will need a sample of the affected tissue for microscopic examination. Samples are obtained in the following way:

  • Nails

    • If you are wearing nail paint, the lab technician will first remove it
    • He/she will wipe the affected nail with 70% alcohol, clip a generous portion of the nail and place it in a sterile container.
    • Scrapings of tissue (keratin) present under the nail will also be collected for examination.
  • Hairs
    • A minimum of 10-12 affected hairs with an intact base will be collected using forceps.
  • Skin
    • If the rash is in the shape of a ring, the sample will be collected from the margin of the ring where the infection is active (at the junction of normal and affected skin). In any other case, scaling skin will be collected.
    • Your doctor or lab technician will gently scrape the surface of the affected skin using a scalpel.
    • The scrapings will be placed between two glass slides, wrapped and then stored in a sterile container.

The samples will be labelled and transported as early as possible to the laboratory.

To perform the test, technicians place the sample on a clean glass slide and adds a drop of 20% KOH. The sample is then examined for fungal elements under the microscope.

Sometimes a special solution called calcofluor white is added to the sample along with KOH. This gives a fluorescent colour to the fungi and helps identify them more easily under the microscope.

It is a rapid and safe test. However, there is a slight risk of bleeding, pain or infection when the skin is scraped.

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Normal results: 

Absence of fungus in the sample is reported as normal or negative.

Abnormal results:

An abnormal result means that some fungus was present in the given sample. 

However, the result can be uncertain sometimes, so the doctor may order a fungal culture test. Test results are confirmed on the basis of physical examination and the results of the KOH test and culture test.

Common fungal infections detected by the KOH test are:

  • Tinea corporis (ringworm on the body)
  • Tinea cruris
  • Tinea capitis
  • Tinea barbae 
  • Tinea pedis
  • Onychomycosis

Disclaimer: All results must be clinically correlated with the patient’s complaints to make a complete and accurate diagnosis. The above information is provided from a purely educational point of view and is in no way a substitute for medical advice by a qualified doctor.


  1. Center for Disease Control and Prevention [internet], Atlanta (GA): US Department of Health and Human Services; Fungal Diseases
  2. Students Health Center Manuals [internet]. Student Health Center. University of California Santa Cruz. U.S.A. Skin Prep (KOH) For Fungus or Scabies
  3. MedlinePlus Medical Encyclopedia: US National Library of Medicine; Skin lesion KOH exam
  4. University of Iowa. Department of Pathology. Laboratory Services Handbook [internet]. KOH Prep (Fungal Stain, KOH with Calcofluor White)
  5. David Ponka. Microscopic potassium hydroxide preparation. Can Fam Physician. 2014 Jan; 60(1): 57. PMID: 24452564
  6. Philippines Heart Center [Internet]. KOH for Fungus
  7. Sachin M Kurade et al. Skin scraping and a potassium hydroxide mount. Indian Journal of Dermatology, Venereology and Leprology. Volume 72. p238-241
  8. Jacob Oren Levitt et al. The Sensitivity and Specificity of Potassium Hydroxide Smear and Fungal Culture Relative to Clinical Assessment in the Evaluation of Tinea Pedis: A Pooled Analysis. Dermatol Res Pract. 2010; 2010: 764843. PMID: 20672004
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