Bacterial meningitis vs Viral meningitis
How to tell Bacterial meningitis and Viral meningitis apart when a board question makes them look alike.
Bacterial CSF is neutrophilic with low glucose; viral CSF is lymphocytic with normal glucose.
Side by side
Fever, neck stiffness, toxic appearance, neutrophilic CSF, low glucose, and high protein.
Meningeal symptoms with lymphocytic CSF, normal glucose, and moderate protein elevation.
Bacterial invasion drives intense neutrophilic inflammation and glucose consumption.
Viral infection drives lymphocytic inflammation without major glucose depletion.
Bacterial: CSF WBC above 1000 with neutrophil predominance, protein above 100 mg/dL, glucose under 40 mg/dL (CSF:serum ratio under 0.4), opening pressure above 25 cm H2O, and Gram stain or culture positive. Procalcitonin and CSF lactate above 3.5 mmol/L are supportive.
Viral: CSF WBC 25 to 500 with lymphocyte predominance, protein 50 to 100 mg/dL, normal glucose, normal-to-mildly elevated opening pressure, and enterovirus or HSV-2 PCR positive most commonly. Partially treated bacterial can mimic this profile except glucose remains low.
The trap
Partially treated bacterial meningitis can shift the CSF differential toward lymphocytes within hours of an outside antibiotic dose, so a lymphocyte-predominant pleocytosis is not safe. CSF glucose under 40 mg/dL, a CSF:serum glucose ratio under 0.4, or protein above 200 mg/dL keeps bacterial meningitis on the list regardless of cell type. Empiric vancomycin plus ceftriaxone (plus ampicillin if older than 50 or immunocompromised) should not wait for cultures.
Watch it explained
The repair move
Read the CSF as a single signature: cell type, glucose, protein, and CSF:serum glucose ratio together. A low CSF:serum glucose ratio under 0.4 outweighs cell predominance every time. Start empiric antibiotics within one hour of clinical suspicion; the LP can be deferred for CT first if there is papilledema, focal deficit, immunocompromise, or GCS below 11, but the antibiotics cannot. Add dexamethasone before or with the first antibiotic dose when pneumococcus is on the list.
Try a free question
CoreA febrile patient has headache and neck stiffness. CSF shows neutrophilic pleocytosis, high protein, low glucose, and elevated opening pressure. Which diagnosis is most likely?
Show answer and explanation
Answer: A. Bacterial meningitis. Neutrophils with low CSF glucose and high protein point to bacterial meningitis.
Why the other choices are tempting
- B. Viral meningitis usually has lymphocytes and normal glucose.
- C. HSV encephalitis causes altered behavior/seizures and temporal-lobe involvement.
- D. Subarachnoid hemorrhage has sudden severe headache and blood/xanthochromia.
- E. MS is demyelinating disease, not acute infectious meningitis.
Flashcards
Tap a card to flip it.
What is the fastest separator for Bacterial meningitis vs Viral meningitis?
Bacterial CSF is neutrophilic with low glucose; viral CSF is lymphocytic with normal glucose.
What clue should make you choose Bacterial meningitis?
Fever, neck stiffness, toxic appearance, neutrophilic CSF, low glucose, and high protein.
Quick answers
What is the fastest way to tell Bacterial meningitis from Viral meningitis?
Bacterial CSF is neutrophilic with low glucose; viral CSF is lymphocytic with normal glucose.
What is the key clue for Bacterial meningitis?
Fever, neck stiffness, toxic appearance, neutrophilic CSF, low glucose, and high protein.
What is the key clue for Viral meningitis?
Meningeal symptoms with lymphocytic CSF, normal glucose, and moderate protein elevation.
What is the common trap with Bacterial meningitis vs Viral meningitis?
Partially treated bacterial meningitis can shift the CSF differential toward lymphocytes within hours of an outside antibiotic dose, so a lymphocyte-predominant pleocytosis is not safe. CSF glucose under 40 mg/dL, a CSF:serum glucose ratio under 0.4, or protein above 200 mg/dL keeps bacterial meningitis on the list regardless of cell type. Empiric vancomycin plus ceftriaxone (plus ampicillin if older than 50 or immunocompromised) should not wait for cultures.