Fungus Smear and Stain: PAS, GMS, and Calcofluor White in Fungal Diagnostics
Fungus Smear
Applies To
Calcofluor White, Gomori Methenamine Silver (GMS) Stain, Periodic Acid Schiff (PAS)
Test Commonly Includes
Smear only. Fungal culture and KOH preparation must be ordered separately for a complete diagnostic workup.
Patient Care & Preparation
- Only trained medical professionals should collect specimens
- Avoid contamination with skin flora
Specimen
Same specimen used for fungal culture from the infection site. Includes conjunctival scrapings, corneal scrapings, or swabs from wet conjunctiva.
Container
Glass microscope slide
Reason to Reject Sample
- Insufficient specimen volume
- Slide lacks visible cellular material
Special Instructions
Provide exact anatomical site and clinical diagnosis. These tests are often performed in histology or cytology departments, not microbiology.
Reference Range
No yeast or hyphal elements observed
Use
Used with fungal culture to aid in the diagnosis of fungal infections. Essential for early visual confirmation of fungal elements.
Limitations
A negative smear result does not rule out fungal infection. Culture or molecular methods may still be necessary.
Methodology
- PAS (Periodic Acid-Schiff) Stain: Stains fungal elements pink-red on a green background
- GMS Stain: Black fungal structures on a green background
- Calcofluor White: Fluorescent dye binds to fungal chitin and cellulose; fungi fluoresce green under UV light
Additional Information
- Calcofluor White is more sensitive for keratomycosis than culture
- Calcofluor White can also detect Acanthamoeba keratitis
- Do not use ethanol fixation for Acanthamoeba — it causes cysts to dry and detach
- Gram staining does not sufficiently highlight Actinomyces or Nocardia; modified acid-fast staining is needed
Caution
Use proper handling techniques—ethanol fixation should be avoided when suspecting Acanthamoeba.
References
- Jacobs, Demott, Finley, Horvat, Kasten.JR, & Tilzer, Laboratory Test Handbook, Lexi-Comp Inc., 1994
- Hageage GJ, Harrington BJ, Lab Med, 1984
- Marines HM, Osato MS, Ophthalmology, 1987
- Gray LD, Roberts GD, Infect Dis Clin North Am, 1988
- Koneman EW, Practical Laboratory Mycology, 3rd ed., 1985
