Culture-Negative CSF in a Patient Treated Before Lumbar Puncture
A 47-year-old man is admitted with 2 days of fever, severe headache, vomiting, and neck stiffness. He saw a local practitioner and took oral cefixime 200 mg twice daily for two days before lumbar puncture. CSF is turbid: 1,200 white cells/µL (82% neutrophils), protein 180 mg/dL, glucose 24 mg/dL with a simultaneous blood glucose of 110 mg/dL. Gram stain shows numerous neutrophils but no organisms. Culture is negative after 48 hours. Latex agglutination for Streptococcus pneumoniae, Haemophilus influenzae type b, and Neisseria meningitidis is negative. Blood cultures taken on admission are pending. The patient remains febrile but is haemodynamically stable. The laboratory has an automated blood culture system, a 16S rRNA PCR service sent to a reference laboratory with a 5-day turnaround, and no on-site molecular meningitis panel.
1. Interpret the CSF findings and explain why the Gram stain, culture, and latex agglutination are negative despite a likely bacterial meningitis.
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The CSF is inflammatory and bacterial in pattern: 1,200 white cells/µL with 82% neutrophils, protein 180 mg/dL, and a CSF-to-blood glucose ratio of 24/110 = 0.22. That combination strongly favours bacterial meningitis rather than viral meningitis, which usually has a lymphocytic picture, normal glucose, and lower protein.
The negative microbiology does not exclude bacterial meningitis. The patient received cefixime for two days before the lumbar puncture. Partial treatment reduces viable organisms and antigen load, so Gram stain, culture, and latex agglutination can all become negative. Latex agglutination is also less sensitive than culture and does not cover every serotype or organism.
The most likely organism remains Streptococcus pneumoniae, but Neisseria meningitidis and other pathogens are possible. The correct interpretation is 'partially treated bacterial meningitis with negative cultures', not 'aseptic meningitis' and not 'no infection'. Blood cultures and molecular testing may still identify the organism.
2. What additional investigations and empirical management would you recommend?
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Additional investigations:
- Ensure blood cultures were collected before or at the time of the first hospital antibiotic dose; if not, collect now.
- Send CSF for 16S rRNA PCR and, if available, specific pneumococcal and meningococcal PCR.
- Request CSF lactate, which is often elevated in bacterial meningitis.
- Consider repeat lumbar puncture if the patient does not improve or deteriorates; send cell count, protein, glucose, Gram stain, culture, and PCR.
- Review for contiguous infection such as otitis media or sinusitis.
Empirical management:
- Start ceftriaxone 2 g intravenously every 12 hours plus vancomycin 15-20 mg/kg every 8-12 hours, adjusted for renal function, to cover penicillin-resistant pneumococci.
- Give dexamethasone with or before the first antibiotic dose if possible; if antibiotics have already started, the benefit is less certain but can still be considered.
- Add coverage for Listeria if the patient is older, immunocompromised, or pregnant; this patient does not clearly require it.
- Provide supportive care and monitor consciousness, seizures, and haemodynamic status.
Do not delay treatment while waiting for PCR.
3. How should the laboratory report this case, and what infection-control precautions are appropriate?
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The report should read something like: 'CSF: neutrophil-predominant pleocytosis, low glucose, high protein. Gram stain: no organisms seen. Culture: negative after 48 hours. Latex antigen: negative. Interpretation: findings are consistent with partially treated bacterial meningitis; negative microbiology does not exclude bacterial infection because of prior cefixime.'
Include a comment that prior antibiotics reduce culture yield and that molecular testing is pending. Report the blood culture result as soon as it becomes positive. If 16S PCR is positive, report the organism or the closest taxonomic identification with an interpretation.
Infection control:
- Use droplet precautions until N. meningitidis is excluded, because meningococcal meningitis requires droplet precautions for the first 24 hours of effective therapy.
- If pneumococcal meningitis is confirmed, standard precautions are sufficient; no contact tracing is needed.
- Notify public health if meningococcal disease is confirmed or strongly suspected, so chemoprophylaxis for close contacts can be considered.
- Ensure hand hygiene and safe handling of CSF and blood cultures.