Diagnostic Approach

The diagnosis of meningitis relies heavily on analysis of cerebrospinal fluid (CSF) obtained via lumbar puncture. However, lumbar puncture should only be performed after CT scan in patients with concerning neurological findings or evidence of increased intracranial pressure.

CSF Analysis

• CSF opening pressure is elevated (>180 mm H2O in adults, >200 mm H2O in children) in bacterial meningitis.
• CSF cytology shows a marked pleocytosis with a neutrophil predominance in bacterial meningitis. CSF white blood cell count is typically 100-10,000 cells/μL. Viral meningitis generally shows milder pleocytosis with a lymphocytic predominance.

• CSF protein is elevated in bacterial meningitis, usually >45 mg/dL in neonates and >170 mg/dL in older children and adults. Mildly elevated protein may be seen in viral meningitis.
• CSF glucose is decreased, often <45 mg/dL or CSF:plasma glucose ratio <0.4 in bacterial meningitis due to increased cellular metabolism and inflammation. Viral meningitis generally shows normal glucose levels.

Blood Tests

• Complete blood count may show leukocytosis or neutropenia. Highly elevated or depressed white blood cell count suggests increased risk of serious bacterial infection.Markers of inflammation like CRP and procalcitonin can distinguish bacterial from viral inflammation when interpreted in context with clinical findings. However, they cannot be used alone to rule out bacterial meningitis.

Imaging

• CT or MRI should precede lumbar puncture in high risk patients to assess for mass effect, cerebral edema, or abscess that may lead to cerebrospinal fluid leakage or herniation with lumbar puncture.
• CT or MRI can also assess for complications of meningitis like infarction, hemorrhage, or hydrocephalus and guide management.

• Clinical Decision Rules
  • Rules like the Bacterial Meningitis Score or Meningitest use CSF and blood parameters to risk stratify children at low risk for bacterial meningitis who may not require empirical antibiotics. However, no findings completely rule it out.

The combination of CSF analysis, blood work, neuroimaging, and clinical judgment guide the diagnosis. Maintenance of a high index of suspicion is critical.