KOH test is an easy microbiology test used for the identification of fungal elements or skin fungus from a yeast infection, candida infection and hyphae in skin scraping ,nail samples and other biological samples such as mucous membranes from vaginal infection test , bronchial aspirate or sputum.It has a relatively low sensitivity (20- 70%) hence should not be used as a confirmatory test.
Principle of KOH TEST
Potassium hydroxide facilitates the microscopic observation of fungal elements when mixed with the specimen by dissolving obscuring cellular material including debris making the fungal elements more visible when viewed using a microscope.
Instrumentation,reagents and supplies for KOH Preparation
- 10% Potassium hydroxide
- Alcohol swab
- Glass slides
- Scalpel or Curette
Procedure for skin specimen
- Briefly explain the procedure to the patient
- Label the glass slide with patient name or number depending on your registration system
- With the help of an alcohol wipe or swab,clean the skin to be sample in order to remove contaminating microorganisms
- If there is a characteristic dermatophyte ring present on the skin,collect the samples from the outer margin of the ring especially at its junction with the normal skin.Otherwise,collect samples from areas where skin appears to be scaling
- Use the edge of the glass slide or the scalpel to scrape off the surface skin scales.Don’t use swab because the swab fibers may resemble hyphae resulting in false positive result
- Collect sample on a labelled glass slide then apply a coverslip
Procedure for Nail samples collection
- Cleanse the nail with alcohol
- Using a curette or scalpel,collect nail debris onto the labelled glass slide
- Apply cover slide
How to prepare 10% KOH solution
- Weight exactly 10 gram Potassium hydroxide pellets
- Then transfer it into a screw-cap bottle
- Now add 50 ml of distilled water and mix until it is completely dissolved then filled up the remaining distilled water to the 100 ml mark.
- Transfer to a plastic container and label your bottle as 10 % KOH solution and remember it is corrosive
Quality Control of 10 % KOH solution
- Quality control should be done on the 10% KOH preparation by checking the appearance of the reagent.
- The 10% KOH solution should be clear to proceed the test.If the solution is not clear,Discard and obtain or prepare a new solution of potassium hydroxide then proceed with the test
Potassium hydroxide treatment
- Place a drop or 50µl of 10% KOH on the glass slide containing the skin or nail specimen at the edge of the coverslip.
- Allow the 10% KOH solution to diffuse under the coverslip
- Gently heat the slide using a butane lighter
- Using a pen or any applicator stick,apply a light pressure on top of the coverslip in order to spread the skin or nail material.The specimen should be flat and transparent for microscopic viewing
- Observe under the microscope starting with low power objective (10x) with reduced light looking for fungal elements such as yeast cells and hyphae
- Change to high power objectives (40x) in order to verify fungal elements seen with the low power 10x objective
- Yeast elements can be difficult to differentiate from cotton fibers and other debris.True hyphae crisscross epidermal cells in a random fashion and the stands are usually of a uniform diameter
How to report result
- Report the presence or absence of yeast or hyphae
- KOH is corrosive hence all personal protective equipment should be made available
Possible Limitation of procedure
- The KOH preparation should be immediateluy setup after sampling for optimal recovery of fungal elements(KOH smear or KOH mount)
- Also small quantities of hyphae and yeast may be missed in KOH preparation thus inability to see fungal elements doesn’t completely exclude yeast or fungal infection remembering that this test method has a relatively low sensitivity
- In case fungal infection is suspected and KOH Preparation shows negative,further test can be carried out or sample may be culture.