## Diagnosis of Cryptococcal Meningitis in HIV **Key Point:** Cryptococcal antigen (CrAg) detection in cerebrospinal fluid (CSF) and/or serum is the most specific and rapid test for diagnosing cryptococcal meningitis in HIV-positive patients with low CD4 counts. ### Why Cryptococcal Antigen Detection? **High-Yield:** Cryptococcal antigen testing offers: - Sensitivity: 95–100% in CSF, 90–95% in serum - Specificity: >99% - Rapid turnaround (within hours) - Can be positive even when India ink is negative - Latex agglutination or lateral flow assay formats - Cost-effective and widely available ### Diagnostic Tests for Cryptococcal Meningitis | Test | Sensitivity | Specificity | Timing | Utility | |------|-------------|-------------|--------|----------| | **CrAg (CSF/serum)** | 95–100% (CSF) | >99% | Hours | **Gold standard** | | **India ink stain** | 50–80% | High | Minutes | Rapid but insensitive | | **Fungal culture** | 90–95% | 100% | 1–2 weeks | Confirmatory, slow | | **Gram stain** | <10% | High | Minutes | Poor sensitivity | **Clinical Pearl:** A positive serum CrAg in an HIV patient with CD4 <100 cells/μL warrants CSF examination and antifungal therapy initiation, even if asymptomatic (cryptococcal antigenemia). ### Diagnostic Algorithm ```mermaid flowchart TD A[HIV + CD4 <100<br/>+ Fever/Headache]:::outcome --> B[Lumbar puncture]:::action B --> C[CrAg in CSF<br/>and Serum]:::action C --> D{CrAg Positive?}:::decision D -->|Yes| E[Cryptococcal meningitis<br/>Start amphotericin B]:::urgent D -->|No| F[India ink stain<br/>Fungal culture]:::action F --> G{Culture positive?}:::decision G -->|Yes| E G -->|No| H[Consider other diagnoses<br/>TB, toxo, CMV]:::outcome ``` **Mnemonic: CRAG** — **C**ryptococcal **R**apid **A**ntigen **G**old standard. [cite:Harrison 21e Ch 197; Mandell Ch 265]
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