A 35-year-old woman with fever and meningeal signs has CSF pleocytosis. You are comparing the likelihood of bacterial versus viral meningitis. Which finding would most strongly favor a diagnosis of viral meningitis over bacterial meningitis?

Explanation

## Viral Meningitis: Key Discriminating Features ### The Defining CSF Profile of Viral Meningitis **Key Point:** Viral meningitis is characterized by a lymphocytic pleocytosis with **normal or near-normal glucose and protein levels**. This triad—lymphocytes + normal glucose + normal protein—is the hallmark that distinguishes it from bacterial meningitis. ### Comparative CSF Profiles | Parameter | Bacterial Meningitis | Viral Meningitis | |-----------|---------------------|------------------| | **WBC count** | 100–10,000/μL | 10–1000/μL (usually <500) | | **Predominant cell** | Neutrophils (early) | Lymphocytes (after 24 hrs) | | **Glucose (mg/dL)** | **<40 (often <25)** | **>40 (normal 40–80)** | | **CSF-to-plasma glucose ratio** | **<0.4** | **>0.6** | | **Protein (mg/dL)** | **100–500** | **50–100 (often <100)** | | **Gram stain/culture** | Often positive | Always negative | ### Why Option 0 Is the Best Discriminator **High-Yield:** A CSF lymphocyte count of 150/μL with **normal glucose AND normal protein** is virtually diagnostic of viral meningitis because: 1. **Preserved glucose metabolism** — viral infection does not consume glucose or impair BBB glucose transport 2. **Mild protein elevation** — reflects only mild inflammation, not the severe breakdown of BBB seen in bacterial disease 3. **Lymphocytic response** — indicates viral immune activation rather than acute bacterial toxicity This combination is **incompatible** with bacterial meningitis, which always shows either low glucose or markedly elevated protein (or both). ### Clinical Pearl **Clinical Pearl:** If you see a patient with meningeal signs, CSF pleocytosis, but **normal glucose and protein**, you can confidently exclude bacterial meningitis and focus on viral causes (enterovirus, HSV-2, mumps, VZV). This finding allows you to defer or withhold antibiotics in appropriate clinical contexts. ### Mnemonic **Mnemonic:** **"VIRAL = Glucose Intact, Lymphs, Absent organisms"** - **V**iral → **G**lucose **I**ntact (normal) - **I**ntact → **L**ymphocytes predominate - **R**esponse → **A**bsent organisms (negative culture) - **A**cute → **L**ow cell count (<500/μL typical) [cite:Harrison 21e Ch 383]