Chapter 40. KOH Preparation

• Suspected fungal or candidal infection.

• Glass microscope slide and coverslip.

• 10–20% potassium hydroxide (KOH).

• Microscope.

• #15 scalpel blade or other device to scrape skin (edge of microscope slide, cytobrush, or foman blade).

• Matches.

• Minimal discomfort and bleeding with aggressive scraping.

• This technique is most commonly used in diagnosing tinea corporis or tinea pedis.

• Although a lower sensitivity is found with this diagnostic test for tinea capitis, spores within hair shafts can be visualized in “black dot” type.

• Samples should be collected from the advancing edge or margins of skin lesions with the edge of a scalpel.

• • In children, use the dull edge if they are moving or use a cytobrush.

  • The foman blade, a 2-sided, spatula-type instrument, is less likely to cut the skin of a moving infant.

• Shavings of nails left in KOH for several hours may assist in diagnosis of onychomycosis.

• KOH is used to digest the proteins, lipids, and epithelial debris in the specimen.

• Gentle cleaning of the area to be tested with an alcohol wipe before this examination may remove confusing oil drops and excess debris from the slide.

• Parents and patients should be aware that scraping may cause minimal bleeding and mild discomfort.

• Patient should be positioned in good lighting with access to lesion to be tested.

• Obtain a skin or nail specimen with gentle scraping of the areas to be tested.

• Place scale, roof of vesicle, or nail shavings on a glass slide.

• Scrape with the edge of the #15 blade, the edge of a glass microscope slide, or a foman blade.

• Apply 1–2 drops of KOH over the specimen.

• Place a coverslip over specimen and press firmly.

• Consider gentle heating of the underside of the slide for 5–10 seconds until the epithelial cells and debris dissipate.

• The hyphae can be seen under low power (Figure 40–1), but better observation of both hyphae and spores is obtained by use of a dry high objective with reduced illumination.

• The juncture lines of epithelial cells may be mistaken for hyphae. Cell walls have irregular linearity.

• Positive preparations appear as septate and branching hyphae. A positive KOH preparation should reveal definite hyphae traversing epidermal cells.

• Fabric fibers (usually twisted and uniform) and hairs are larger than hyphae.

• If a tinea or candidal infection is strongly suspected and this examination is negative, consider performing a culture.

• This is a diagnostic examination that is underutilized due to lack of experience with interpretation. The identification of spores and hyphae becomes more straightforward the more often the test is done.

• As indicated after appropriate treatment.