A 28-year-old man presents to the emergency department with a 2-day history of fever, severe headache, neck stiffness, and photophobia. On examination, he has a positive Kernig sign and Brudzinski sign. Blood cultures are pending. What is the investigation of choice to confirm bacterial meningitis and guide antimicrobial therapy?

Explanation

## Diagnostic Approach to Bacterial Meningitis **Key Point:** Lumbar puncture with CSF analysis (Gram stain, culture, and sensitivity) is the gold standard and investigation of choice for diagnosing bacterial meningitis and identifying the causative organism for targeted therapy. ### Why CSF Analysis is Definitive | Feature | Bacterial Meningitis | Viral Meningitis | |---------|---------------------|------------------| | **Cell count** | 100–10,000 (predominantly PMN) | 10–1,000 (initially PMN, then lymphocytes) | | **Protein** | Markedly elevated (100–500 mg/dL) | Mildly elevated (50–100 mg/dL) | | **Glucose** | Low (<40 mg/dL or CSF:serum <0.4) | Normal (>40 mg/dL or CSF:serum >0.4) | | **Gram stain** | Positive in 60–90% of cases | Negative | | **Culture** | Gold standard; identifies organism | Negative (viral culture rarely used) | **High-Yield:** Gram stain positivity allows immediate organism identification (cocci vs. rods, Gram-positive vs. Gram-negative), enabling empiric therapy adjustment within hours before culture results return (48–72 hours). ### Timing of Lumbar Puncture **Clinical Pearl:** In a patient with clinical meningitis signs (fever, headache, neck stiffness, Kernig/Brudzinski positive) and no contraindications (normal mental status, no focal neurological signs, no papilledema), lumbar puncture should be performed immediately after blood cultures are drawn—do not delay for imaging. **Warning:** CT scan before LP is indicated only if there are signs of increased intracranial pressure or focal neurological findings. Delaying LP for imaging in uncomplicated meningitis increases mortality risk. ### Culture and Sensitivity **Key Point:** CSF culture with antibiotic susceptibility testing is essential for: - Confirming the diagnosis - Identifying the organism (Neisseria meningitidis, Streptococcus pneumoniae, Listeria monocytogenes, Gram-negative rods) - Guiding de-escalation from empiric broad-spectrum therapy to organism-specific therapy **Mnemonic:** **NNLG** — the common bacterial meningitis pathogens in adults: **N**eisseria meningitidis, **N**eisseria (meningococcus), **L**isteria monocytogenes, **G**ram-negative rods (E. coli, Klebsiella). [cite:Harrison 21e Ch 381]