KOH (potassium hydroxide) preparation for oral candidiasis (thrush)

The KOH preparation test is used to determine whether oral candidiasis (thrush) is present in a person's mouth. A sample of skin is taken from the person's mouth by lightly scraping the white patches.

The sample is then placed on a slide with potassium hydroxide (KOH) solution and gently heated. This solution slowly dissolves the skin cells but not the yeast cells. The yeast cells can then be seen with a microscope. Color stains may be used so that the yeast cells are easier to see.

If a person has white patches in the mouth or on the tongue, a KOH test may be done to determine whether the person has thrush.

Findings of a KOH test may include the following.

Normal

No yeast or other fungi are present in the sample from the person's mouth.

Abnormal

Yeast or other fungi are present in the sample from the person's mouth.

Generally, a KOH test is not needed to diagnose thrush. A doctor usually can diagnose thrush by looking at the white patches in a person's mouth. But if thrush does not respond to prescribed medicines, a KOH test may be done to determine whether drug-resistant strains of yeast are causing the infection.

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