In a patient with severe retro‑orbital and occipital headache and altered mental status suggestive of bacterial meningitis, how should I test for it?

Testing for Bacterial Meningitis

In a patient with suspected bacterial meningitis, immediately obtain blood cultures, then perform lumbar puncture for CSF analysis including cell count with differential, protein, glucose, Gram stain, and culture—but only after assessing for contraindications that would require CT imaging first. 1

Pre-Lumbar Puncture Assessment

Before performing lumbar puncture, evaluate for clinical features that indicate increased risk of herniation and warrant CT imaging first: 1

• Severely altered mental status (Glasgow Coma Scale <10)
• Focal neurologic deficits (excluding cranial nerve palsies alone)
• New-onset seizures
• Severely immunocompromised state (organ transplant recipients, HIV-infected patients)

Critical caveat: If lumbar puncture must be delayed for CT imaging, obtain blood cultures and immediately start empiric antibiotics before sending the patient for imaging—do not wait for diagnostic confirmation, as delays in treatment increase mortality. 1

Blood Cultures

Obtain blood cultures before administering the first antibiotic dose. 1

• Blood cultures are positive in 75% of pneumococcal meningitis, 50-90% of H. influenzae meningitis, and 40-60% of meningococcal meningitis 1
• Yield decreases by 20% if antibiotics are given before collection 1

Cerebrospinal Fluid Analysis

Essential CSF Studies (Grade A Recommendation)

Perform these tests on all CSF samples: 1

• Opening pressure measurement (typically elevated in bacterial meningitis)
• Cell count with differential (best diagnostic parameter with area under curve 0.95) 1
• Protein concentration (elevated, often >120 mg/dL in bacterial meningitis) 2
• Glucose concentration (typically <34 mg/dL in bacterial meningitis) 2
• CSF-to-blood glucose ratio (ratio <0.23 suggests bacterial meningitis) 2
• Gram stain (sensitivity 60-90%, excellent specificity) 1, 3
• Culture (positive in 60-90% of bacterial meningitis cases) 1

Expected CSF Profile in Bacterial Meningitis

The typical pattern includes: 4, 2

• Pleocytosis with neutrophil predominance (WBC >12,000 cells/mm³, neutrophils >11,000 cells/mm³)
• Low glucose (<34 mg/dL)
• Low CSF-to-blood glucose ratio (<0.23)
• Elevated protein (>120 mg/dL)

Additional Diagnostic Tests

When Initial Tests Are Negative

If CSF culture and Gram stain are negative, use PCR for pathogen identification (Grade A recommendation). 1

• PCR has additive value when standard tests are negative 1
• Particularly useful if antibiotics were administered before lumbar puncture 5
• Can detect S. pneumoniae, N. meningitidis, H. influenzae, and other pathogens 5

Tests with Limited Value

• Latex agglutination testing: Little incremental value in diagnosis 1
• Immunochromatographic antigen testing: Unclear incremental value 1
• Serum CRP and procalcitonin: Associated with bacterial infections in children but cannot make the diagnosis alone 1

CSF Lactate

CSF lactate has good sensitivity and specificity for differentiating bacterial from aseptic meningitis. 1

• Value is limited in patients who received antibiotic pretreatment 1
• Also limited in patients with other CNS diseases 1

Impact of Prior Antibiotic Treatment

Antibiotic pretreatment significantly affects diagnostic yield: 1

• CSF culture yield decreases by 10-20% 1
• Blood culture yield decreases by 20% 1
• Gram stain yield decreases slightly 1
• However, CSF parameters (cell count, glucose, protein) remain useful 6

Critical Timing Considerations

Strive to initiate treatment within one hour of presentation in all patients with suspected bacterial meningitis, regardless of whether CT imaging or lumbar puncture is performed first. 1

• Cranial imaging before lumbar puncture causes significant delays in antibiotic administration 1
• These delays are associated with poor outcomes 1
• The sequence should be: blood cultures → empiric antibiotics (if LP delayed) → CT (if indicated) → lumbar puncture 1

Common Pitfalls

• Traumatic lumbar puncture can falsely elevate white cell count 6
• Normal CSF analysis does not rule out bacterial meningitis in patients with high clinical suspicion—consider repeat lumbar puncture 4
• Waiting for CT results before starting antibiotics increases mortality—start treatment immediately if LP is delayed 1
• Relying solely on clinical features has limited diagnostic accuracy—CSF analysis remains the principal diagnostic tool 1