Microscopic potassium hydroxide preparation

Applications

Diagnosis of fungal infections when findings of clinical examination and Wood lamp examination are inconclusive.

Equipment necessary

• Microscope slide and cover glass

• 20% potassium hydroxide (KOH)

• Gauze

• Microscope with 10 × and 40 × objectives

Set-up

Depending on the type of sample being tested, there are various ways to yield a specimen. One of the easiest ways in the outpatient setting is to obtain a skin scraping using a small scalpel blade. Set up the microscope and turn the power on.

Evidence

One study found a likelihood ratio of 2.86 with regard to finding fungal infection using KOH preparation.¹ This represents a clinically useful value in the context of a common diagnosis. Moreover, the set-up required for this diagnostic tool is modest (KOH and a microscope), and the procedure can be performed quickly at the point of care. Culture is the criterion standard test used to determine the presence of a fungal infection. Given that culture has a likelihood ratio that is only moderately superior (3.28) to KOH preparation, is expensive, and takes 6 weeks to yield a result, teaching microscopic KOH preparation appears to be reasonable during residency. However, a negative KOH preparation result should still prompt a culture.

Diagnostic confirmation

Fungal culture or referral to a dermatologist for biopsy or periodic acid–Schiff stain might be necessary to confirm the diagnosis.

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