A 3-year-old child with suspected bacterial meningitis is being prepared for lumbar puncture. The child has no focal neurological signs, no papilledema, and GCS is 14. Cerebrospinal fluid shows WBC 1200 cells/μL (95% neutrophils), protein 180 mg/dL, and glucose 18 mg/dL (simultaneous blood glucose 90 mg/dL). Which ancillary investigation on CSF would BEST help predict bacterial etiology when Gram stain is negative?
- A CSF adenosine deaminase (ADA) level
- B CSF lactate level >35 mg/dL (>3.9 mmol/L) as a biomarker for bacterial meningitis ✓
- C CSF/serum albumin ratio for blood-brain barrier assessment
- D CSF cytology for malignant cells
Explanation
When the Gram stain is negative in suspected bacterial meningitis, CSF lactate is a highly useful adjunct. A CSF lactate level >35 mg/dL (3.9 mmol/L) has a sensitivity of ~85–90% and specificity >95% for distinguishing bacterial from aseptic (viral) meningitis — especially valuable in patients already on antibiotics. Elevated CSF lactate reflects anaerobic metabolism by bacteria and ischemic neurons due to glucose consumption. ADA in CSF is more useful for tuberculous meningitis (typically elevated in TBM). The CSF glucose ratio (CSF glucose/blood glucose <0.4) is also supportive of bacterial meningitis. Gram stain is positive in 70–80% of untreated bacterial meningitis.
Reference: Ghai Essential Pediatrics, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.