## Discriminating CSF Features: Bacterial vs Viral Meningitis ### Key Distinguishing Parameter **Key Point:** The CSF-to-plasma glucose ratio is the single most reliable discriminator between bacterial and viral meningitis. A ratio <0.4 (or absolute CSF glucose <40 mg/dL in the presence of normal blood glucose) is highly specific for bacterial meningitis. ### CSF Profile Comparison | Feature | Bacterial Meningitis | Viral Meningitis | |---------|---------------------|------------------| | **Cell count** | 100–10,000/μL (often >1000) | 10–1000/μL (rarely >1000) | | **Cell type** | Neutrophil predominance (early) | Lymphocyte predominance (after 24 hrs) | | **Protein** | Markedly elevated (100–500 mg/dL) | Mildly elevated (50–100 mg/dL) | | **Glucose** | **Low (<40 mg/dL)** | **Normal or mildly low** | | **CSF-to-plasma glucose ratio** | **<0.4 (highly specific)** | **>0.6 (normal)** | | **Gram stain/culture** | Often positive (60–90%) | Negative | ### Why This Ratio Matters **High-Yield:** The low CSF glucose in bacterial meningitis reflects: 1. Bacterial consumption of glucose via glycolysis 2. Impaired glucose transport across inflamed blood–brain barrier 3. Reduced CSF production of glucose This mechanism is **not present** in viral meningitis, where glucose metabolism remains intact. ### Clinical Pearl **Clinical Pearl:** A CSF glucose <40 mg/dL with blood glucose >100 mg/dL is virtually pathognomonic for bacterial meningitis. This finding should prompt immediate empiric antibiotics even before culture results. ### Timing Note **Tip:** Early bacterial meningitis may show lymphocytic predominance (first 24 hours), but the low glucose persists. Conversely, viral meningitis may show neutrophil predominance early, but glucose remains normal—making the glucose ratio the most stable discriminator. [cite:Harrison 21e Ch 383]
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