Direct Microscopy-KOH Smear: Introduction, Uses, and Keynotes

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Introduction

Direct microscopic examination is one of the most rapid and inexpensive diagnostic methods for detecting fungal elements in clinical specimens. Potassium hydroxide (KOH) smear is a routine technique in mycology that helps visualize fungal structures such as hyphae, yeast cells, and sclerotic bodies directly from patient samples. It is widely used as a first-line screening test for suspected fungal infections before culture or molecular testing.

Uses of Direct Microscopy-KOH Smear

Melanized septate fungal hyphae in KOH mount of BAL sample
Fig. Melanized septate fungal hyphae in KOH mount of BAL sample
Yeast cells and budding yeasts in KOH mount of Urine
Fig. Yeast cells and budding yeasts in a KOH mount of Urine
Yeast cells, budding yeasts, and pseudohyphae in KOH mount of sputum microscopic examination at a magnification of 1600X
Fig. Yeast cells, budding yeasts, and pseudohyphae in KOH mount of sputum, microscopic examination at a magnification of 1600X
Yeast cells and budding yeasts in KOH mount of urinarary sediment microscopy at a magnification of 1600X
Fig. Yeast cells and budding yeasts in KOH mount of urinary sediment microscopy at a magnification of 1600X
Septate acute-angled branching hyphae of Aspergillus in KOH mount of sputum
Fig. Septate acute-angled branching hyphae of Aspergillus in a KOH mount of sputum
Fungal elements in KOH mount of urine sample at a magnification of 1600X
fIG. Fungal elements in the KOH mount of the urine sample at a magnification of 1600X
Chlamydospores of Candida albicans and hyphae in KOH mount of sputum microscopy at a magnification of 1600X
Fig. Chlamydospores of Candida albicans and hyphae in KOH mount of sputum microscopy at a magnification of 1600X
Heavy load of fungal elements in KOH mount of sputum specimen (Mag. 1600X)
Fig. Heavy load of fungal elements in KOH mount of sputum specimen (Mag. 1600X)
In a potassium hydroxide (KOH) skin scraping for chromoblastomycosis, a sclerotic body is a characteristic, dark-brown, thick-walled, round, and multi-cellular fungal cell that appears as a "medlar body" or "muriform cell".
Fig. In a potassium hydroxide (KOH) skin scraping for chromoblastomycosis, a sclerotic body is a characteristic, dark-brown, thick-walled, round, and multicellular fungal cell that appears as a “medlar body” or “muriform cell,” as shown in this image.

Keynotes on Direct Microscopy-KOH Smear

  1. Principle: KOH clears keratin and cellular debris, making fungal elements more visible under the microscope.
  2. Concentrations: 10% KOH for skin/hair, 20–40% KOH for nails; heating can enhance clearing.
  3. Visualization: Lactophenol, cotton blue, calcofluor white, or Parker ink can be added for contrast.
  4. Advantages: Simple, inexpensive, rapid, and highly useful in low-resource settings.
  5. Limitations: Cannot identify species; sensitivity depends on fungal load and observer experience.
  6. Best Use: Serves as an initial diagnostic step for suspected fungal infections in dermatology, ophthalmology, pulmonology, and general medicine.

Further Readings

  1. https://www.medicalnewstoday.com/articles/322331
  2. https://www.youtube.com/watch?v=LUwNQI_0BWU&t=1
  3. https://pmc.ncbi.nlm.nih.gov/articles/PMC8654087/
  4. https://flabslis.com/blogs/koh-test-procedure
  5. https://www.healthline.com/health/skin-lesion-koh-exam
  6. https://drkothiwalaskineva.com/medical-dermatology/koh-mount-for-fungal-diseases/
  7. https://pmc.ncbi.nlm.nih.gov/articles/PMC11642458/
  8. https://en.wikipedia.org/wiki/KOH_test
  9. https://journals.lww.com/ijpd/fulltext/2020/21040/mount_the_menace____potassium_hydroxide_in.26.aspx
  10. https://diag.vn/en/thongtinyte/koh-in-nail-and-skin-mycology-diagnosis/
  11. https://www.linkedin.com/posts/dip-ghosh-2329b3291_clinicalmycology-medicallaboratory-fungalinfections-activity-7359249932584722434-79Yj
  12. https://iacdworld.org/wp-content/uploads/2016/12/kohpreparation.pdf

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