A 32-year-old man with HIV (CD4 count 85 cells/µL) presents with fever, headache, and neck stiffness. CSF analysis shows elevated protein (120 mg/dL), low glucose (25 mg/dL), and India ink preparation is negative. Which single feature best distinguishes cryptococcal meningitis from tuberculous meningitis in this patient?

Explanation

## Distinguishing Cryptococcal from Tuberculous Meningitis ### Clinical Context Both cryptococcal meningitis (CM) and tuberculous meningitis (TBM) present with similar CSF profiles in advanced HIV (CD4 <100 cells/µL): lymphocytic pleocytosis, elevated protein, and hypoglycorrhachia. Rapid differentiation is critical for timely antifungal vs. antituberculous therapy. ### Key Discriminating Feature **Key Point:** Positive CSF culture on Sabouraud dextrose agar (or other fungal media) within 48–72 hours is the single best discriminator. Cryptococcus neoformans grows reliably on fungal media; Mycobacterium tuberculosis requires weeks (Löwenstein–Jensen or MGIT) and is culture-negative in ~50% of TBM cases. ### Why This Matters | Feature | Cryptococcal Meningitis | Tuberculous Meningitis | |---------|------------------------|------------------------| | **CSF Culture (fungal media)** | Positive in 70–90% within 48–72 hrs | Negative (requires TB media) | | **CSF Culture (TB media)** | Negative | Positive in 50–60% (weeks) | | **India Ink Stain** | Positive in 50–80% (low CD4) | Negative | | **CSF Glucose** | Low (hypoglycorrhachia) | Low (hypoglycorrhachia) | | **CSF Protein** | Elevated | Elevated | | **CSF Cell Count** | Lymphocytic pleocytosis | Lymphocytic pleocytosis | **High-Yield:** The CSF culture on Sabouraud dextrose agar is rapid, specific, and practical — it will grow Cryptococcus in 2–3 days but will NOT grow TB, making it the single best discriminator in this clinical scenario. **Clinical Pearl:** In resource-limited settings, CSF cryptococcal antigen (CrAg) latex agglutination test is faster (same-day) and more sensitive (~95%) than India ink, and is now recommended as the primary diagnostic test for CM in advanced HIV [cite:Harrison 21e Ch 197]. ### Mnemonic **FUNGAL FAST:** Fungal cultures are fast (48–72 hrs); TB cultures are slow (weeks).