## India Ink Preparation for Cryptococcus **Key Point:** India ink (or nigrosin) preparation is the gold standard presumptive test for *Cryptococcus neoformans* in cerebrospinal fluid (CSF) and other clinical specimens. ### Mechanism of the India Ink Preparation The India ink preparation exploits the **polysaccharide capsule** of *C. neoformans*: 1. India ink particles are excluded by the thick gelatinous capsule 2. The capsule appears as a **clear halo** around the yeast cell 3. The yeast itself stains dark (due to the ink), creating a distinctive contrast 4. The halo (capsule) remains unstained and appears as a bright ring ### Sensitivity and Specificity | Parameter | Value | |-----------|-------| | Sensitivity in CSF | 50–80% | | Specificity in CSF | ~95% | | Sensitivity in serum | 10–40% | | Best specimen type | CSF (meningitis) | **High-Yield:** The India ink preparation is **rapid** (takes minutes) and requires **no culture**, making it ideal for emergency diagnosis of cryptococcal meningitis. ### Morphology on India Ink - **Size:** 4–6 μm (smaller than RBCs, which are 7–8 μm) - **Shape:** Round to oval - **Capsule width:** Often as wide as or wider than the cell itself - **Budding:** May be visible as a narrow-based bud **Clinical Pearl:** A negative India ink does NOT rule out cryptococcal meningitis — CSF cryptococcal antigen (CrAg) or culture must be performed for confirmation. **Warning:** Do not confuse the capsule halo with air bubbles — air bubbles are usually larger and lack the characteristic yeast cell in the center.
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