A CSF sample from a patient with acute meningitis shows WBC 400 cells/µL with 80% neutrophils, glucose 55 mg/dL (blood glucose 100), and protein 90 mg/dL. CSF lactate is measured at 5.0 mmol/L. What does this lactate value suggest?

• A Bacterial meningitis, supporting antimicrobial therapy despite borderline glucose and protein ✓
• B Viral meningitis, since lactate is always normal in bacterial infection
• C Cerebral venous sinus thrombosis rather than infection
• D Sample contamination, so the result should be discarded

Explanation

B CSF lactate above approximately 4 mmol/L (about 36 mg/dL) favours bacterial over aseptic meningitis, with pooled sensitivities and specificities around 95%. Anaerobic glycolysis by bacteria and activated leukocytes drives the elevation. Here the mildly elevated neutrophils and lactate push the picture toward partially treated bacterial meningitis, a common scenario in Indian practice where antibiotics are started before lumbar puncture. Viral meningitis typically keeps lactate below 2 mmol/L, which kills option B.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.