Bacterial meningitis vs Viral meningitis
How to tell Bacterial meningitis and Viral meningitis apart when a board question makes them look alike.
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CSF glucose below 40 mg/dL or CSF:serum glucose ratio below 0.4 points to bacterial meningitis.
Side by side
Acute fever, nuchal rigidity, and toxic appearance with neutrophilic CSF, low CSF glucose ratio, and high protein.
Meningeal symptoms with preserved mental status and lymphocytic CSF profile with normal CSF glucose.
Bacterial invasion causes intense neutrophilic inflammation, blood-brain barrier disruption, and impaired CSF glucose availability.
Viral CNS infection usually causes lymphocytic inflammation with less severe glucose consumption and lower intracranial pressure burden.
CSF typically shows WBC often above 1000/uL with neutrophil predominance, protein above 100 mg/dL, glucose below 40 mg/dL or CSF:serum glucose ratio below 0.4, and opening pressure often above 25 cm H2O.
CSF usually shows 50 to 500 WBC/uL with lymphocyte predominance, protein about 50 to 100 mg/dL, normal glucose, and positive viral PCR such as enterovirus or HSV.
The trap
After partial outpatient antibiotic exposure, CSF can become lymphocyte-predominant and mimic viral meningitis. Low CSF glucose ratio, very high protein, or markedly elevated opening pressure should still trigger immediate empiric bacterial coverage.
The repair move
Next best step: draw blood cultures and start empiric IV vancomycin plus ceftriaxone immediately, adding ampicillin if age is above 50 or immunocompromised. Give dexamethasone 10 mg IV before or with the first antibiotic dose when pneumococcal disease is possible, then tailor therapy when CSF culture and PCR results return.
Try a free question
CoreA 21-year-old woman presents with fever, photophobia, and neck stiffness for 12 hours. BP 108/68 mm Hg, HR 112/min, temp 38.9 C. CSF has 1800 WBC/uL, protein 170 mg/dL, glucose 30 mg/dL, and serum glucose 102 mg/dL. What is the most likely diagnosis?
Show answer and explanation
Answer: A. Bacterial meningitis. Neutrophilic pleocytosis with low CSF glucose ratio and high protein is most consistent with bacterial meningitis.
Why the other choices are tempting
- B. Viral meningitis is less likely because CSF glucose is significantly low and protein is markedly elevated.
- C. Idiopathic intracranial hypertension usually has normal CSF studies aside from pressure elevation.
- D. Subarachnoid hemorrhage would show xanthochromia or RBC-predominant CSF, not this infectious profile.
- E. Neurosarcoidosis does not typically present with this acute toxic febrile syndrome.
Flashcards
Tap a card to flip it.
Which CSF glucose pattern strongly favors bacterial over viral meningitis?
CSF glucose below 40 mg/dL or CSF:serum glucose ratio below 0.4 strongly supports bacterial meningitis.
What CSF cell profile is typical of viral meningitis?
Lymphocyte-predominant pleocytosis with normal glucose is typical of viral meningitis.
Quick answers
What is the fastest way to tell Bacterial meningitis from Viral meningitis?
CSF glucose below 40 mg/dL or CSF:serum glucose ratio below 0.4 points to bacterial meningitis.
What is the key clue for Bacterial meningitis?
Acute fever, nuchal rigidity, and toxic appearance with neutrophilic CSF, low CSF glucose ratio, and high protein.
What is the key clue for Viral meningitis?
Meningeal symptoms with preserved mental status and lymphocytic CSF profile with normal CSF glucose.
What is the common trap with Bacterial meningitis vs Viral meningitis?
After partial outpatient antibiotic exposure, CSF can become lymphocyte-predominant and mimic viral meningitis. Low CSF glucose ratio, very high protein, or markedly elevated opening pressure should still trigger immediate empiric bacterial coverage.