Fungal Wet Mount (KOH, Calcofluor)

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Subject: Fungal Wet Mount (KOH, Calcofluor)

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Definition

  • A direct examination for fungal elements may provide a rapid detection of fungal infection and is recommended for most types of specimens submitted for fungal culture.

Use

  • This test is used for the direct detection of fungal forms in patient specimens. The specimen is processed to form a liquid suspension of the patient sample.

    • Solid specimens, such as tissues, should be minced to facilitate suspension.

    • The specimen may be suspended in saline or a 10% KOH solution. KOH may improve liquefaction of the specimen and also lyses host cells and keratin; fungal cells are resistant to KOH digestion.

    • A cover slip is added for examination with regular or phase-contrast light microscopy.

    • Calcofluor white, a fluorogenic dye that binds to specific polysaccharide bonds found in fungal cell walls, may be added to the KOH solution to improve microscopic visualization of fungi.

  • Turnaround time: 24 hours.

  • Specimens should be collected and transported according to guidelines for fungal culture of the specimen type.

Interpretation

  • Expected result: Negative.

  • Positive: Provides evidence of fungal infection. Fungal elements may be characterized on the basis of morphology (e.g., budding yeast, aseptate hyphae, conidia-forming structures consistent with Aspergillus species).

  • Negative: Fungal infection is not ruled out by a negative wet mount examination.

Limitations

  • The morphology of objects must be examined carefully to exclude artifacts or nonspecific absorption of calcofluor dye to nonfungal objects, such as capillaries.

 
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