Partially Treated Bacterial Meningitis with a Negative Culture
A 24-year-old university student presents with 12 hours of fever 39.4°C, severe headache, photophobia, and neck stiffness. He received one dose of ceftriaxone 2 g IV in the emergency department 45 minutes before lumbar puncture. CSF: WBC 2,400 × 10^6/L with 88% neutrophils, RBC 30 × 10^6/L, protein 2.9 g/L, glucose 1.2 mmol/L; paired serum glucose 7.0 mmol/L. CSF lactate is 10.2 mmol/L. Gram stain shows no organisms; bacterial culture is negative at 48 hours. The patient remains febrile but stable. The laboratory scientist must interpret the CSF findings, calculate the CSF:serum glucose ratio, explain the negative culture, and advise the clinical team on additional testing and result reporting. The patient has no recent antibiotics other than the ceftriaxone, and no immunocompromise.
1. Calculate the CSF:serum glucose ratio and interpret the CSF findings.
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CSF:serum glucose ratio = 1.2/7.0 = 0.17. A ratio below 0.4, and certainly below 0.5–0.6, strongly suggests bacterial meningitis. The CSF shows a neutrophilic pleocytosis of 2,400 × 10^6/L with 88% neutrophils, high protein of 2.9 g/L, low glucose, and high lactate of 10.2 mmol/L. This combination is classic for bacterial meningitis. The RBC count of 30 × 10^6/L is far too low to explain the protein. A negative Gram stain does not exclude bacterial meningitis; sensitivity is roughly 60–80% and falls after antibiotics. The patient should be managed as probable bacterial meningitis despite the negative Gram stain and culture.
2. Why is the culture negative, and what additional testing should be performed?
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Ceftriaxone given 45 minutes before lumbar puncture can rapidly sterilize CSF; culture yield falls markedly within hours, and the Gram stain becomes negative. A negative culture at 48 hours therefore does not exclude bacterial meningitis. The pretreated CSF still shows the inflammatory pattern, and lactate is high. Additional testing should include multiplex PCR for Streptococcus pneumoniae, Neisseria meningitidis, and Haemophilus influenzae; 16S rRNA PCR; and possibly CSF antigen testing. PCR is less affected by antibiotics. Hold the culture for 5 days. If PCR is negative, the diagnosis remains clinical and CSF-based; do not stop appropriate antibiotics solely because cultures are negative.
3. What should be reported, and what infection-control actions are required?
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Report the CSF results as critical and call the clinician, documenting the read-back. Add a comment that the CSF pattern is consistent with bacterial meningitis and that cultures may be negative because of pre-lumbar-puncture ceftriaxone; recommend PCR and continued therapy. If meningococcal disease is suspected or confirmed, notify infection control and public health promptly, institute droplet precautions, and arrange chemoprophylaxis for close contacts according to local policy. For pneumococcal meningitis, standard precautions apply. Repeat CSF analysis is not routinely needed if the patient is improving, but persistent fever or deterioration should prompt reassessment and imaging.