A 32-year-old woman presents with a 3-day history of fever, headache, and neck stiffness. Lumbar puncture is performed. CSF analysis shows: WBC 250/μL (85% lymphocytes), protein 120 mg/dL, glucose 55 mg/dL (serum glucose 110 mg/dL), Gram stain negative, and bacterial culture negative after 24 hours. What is the most appropriate next investigation to confirm the diagnosis?

Explanation

## Diagnostic Approach to Viral Meningitis ### CSF Profile Analysis The CSF findings are consistent with **aseptic meningitis** (negative Gram stain and bacterial culture after 24 hours): - **Lymphocytic pleocytosis** (85% lymphocytes) — characteristic of viral meningitis - **Elevated protein** (120 mg/dL) — non-specific but supportive - **Low-normal glucose** (CSF:serum ratio = 55/110 = 0.5) — can occur in viral meningitis, especially enterovirus and HSV - **Negative bacterial culture** — rules out bacterial meningitis **Key Point:** Once bacterial meningitis is excluded (negative Gram stain and culture), the next step is to identify the viral etiology using CSF PCR, which is the most sensitive and specific test for viral meningitis. ### CSF PCR for Viral Pathogens | Pathogen | Frequency | CSF PCR Sensitivity | Clinical Notes | |----------|-----------|-------------------|----------------| | **Enterovirus** | 50–80% of viral meningitis | 90–95% | Most common; self-limited | | **HSV-1** | 5–10% | 95–98% | Can cause encephalitis; needs acyclovir | | **HSV-2** | 5–10% | 95–98% | Associated with genital herpes | | **VZV** | 5% | 90–95% | Dermatomal rash may be absent | | **EBV, CMV** | <5% | Variable | Immunocompromised patients | **High-Yield:** CSF PCR has largely replaced viral culture (slow, low sensitivity) and serology (not useful acutely). PCR results are available within 24–48 hours and guide treatment decisions—particularly identifying HSV, which requires empiric acyclovir even during the diagnostic workup. **Clinical Pearl:** Empiric acyclovir should already be initiated in this patient during the diagnostic phase (before PCR results) because HSV meningitis/encephalitis is treatable and delay increases morbidity. PCR confirmation allows continuation or discontinuation based on results. ### Why This is the Most Appropriate Next Step 1. **Diagnostic yield:** CSF PCR identifies the viral etiology in >90% of cases 2. **Guides therapy:** Positive HSV/VZV PCR → continue acyclovir; negative → can discontinue 3. **Prognostic value:** Identifies self-limited enterovirus vs. treatable HSV 4. **Rapid turnaround:** Results within 24–48 hours **Mnemonic:** **HEVZ** — the common viral meningitis pathogens: **H**SV-1/2, **E**nteroviruses, **V**ZV, **Z**ika (in endemic areas). [cite:Harrison 21e Ch 381]