## KOH Mount for Fungal Corneal Ulcer Diagnosis **Key Point:** Potassium hydroxide (KOH) preparation is the gold standard rapid stain for identifying fungal elements in corneal scrapings because it dissolves bacterial and epithelial cell debris while preserving fungal cell walls. **High-Yield:** KOH mount is superior to Gram stain for fungal identification because fungi have thick, rigid cell walls composed of chitin and β-glucans that resist Gram staining but are highlighted by KOH digestion. ### Microscopic Appearance on KOH Mount - **Filamentous fungi (Aspergillus, Fusarium):** Septate or non-septate hyphae with branching patterns - **Yeast (Candida):** Budding yeast cells and pseudohyphae - **Acanthamoeba:** Trophozoites and cysts (though not true fungi) ### Comparison of Staining Methods for Fungal Detection | Stain | Sensitivity | Appearance | Use | | --- | --- | --- | --- | | **KOH mount** | High | Clear fungal elements on transparent background | First-line rapid diagnosis | | **Gram stain** | Low | Fungi may not stain or stain weakly | Not recommended for fungi | | **Giemsa stain** | Moderate | Stains fungal elements but less specific | Better for parasites (Acanthamoeba) | | **Calcofluor white** | Very high | Bright fluorescence under UV light | Gold standard but requires fluorescence microscopy | | **Culture** | Gold standard | Organism identification + sensitivity | Confirmatory, takes days | **Clinical Pearl:** Although KOH is rapid and widely available, **Calcofluor white** staining under fluorescence microscopy is more sensitive and is preferred in specialized centers. However, KOH remains the most practical first-line stain in routine practice. **Mnemonic:** **KOH** = **K**eratitis **O**rganisms **H**ighlighted (dissolves background, reveals fungi) [cite:Khurana Comprehensive Ophthalmology Ch 4] 
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