From the Guidelines
A lumbar puncture to confirm bacterial meningitis should show elevated CSF lactate concentration, as it has been found to be a widely available, cheap, and rapid diagnostic test with better diagnostic accuracy than CSF WBC count, as noted in a meta-analysis of 25 studies with 1692 patients 1. The diagnosis of bacterial meningitis is typically confirmed by the presence of certain characteristics in the cerebrospinal fluid (CSF) obtained through a lumbar puncture.
- Elevated white blood cell count (typically >1000 cells/mm³) with neutrophil predominance
- Decreased glucose concentration (less than 40 mg/dL or less than 40% of serum glucose)
- Elevated protein levels (typically >100 mg/dL)
- Increased opening pressure (>20-25 cm H₂O)
- CSF lactate concentration, which has been found to be a useful diagnostic marker, with a sensitivity of 98% in patients who have not received antibiotic pretreatment 1 Gram stain of the CSF may reveal bacteria, and CSF culture is the gold standard for identifying the causative organism. The CSF typically appears cloudy or turbid rather than clear, reflecting the inflammatory response to bacterial infection in the meninges. While awaiting culture results, empiric antibiotic therapy should be started immediately after CSF collection, typically with a third-generation cephalosporin (ceftriaxone 2g IV every 12 hours or cefotaxime 2g IV every 4-6 hours) plus vancomycin (15-20 mg/kg IV every 8-12 hours), with consideration of ampicillin (2g IV every 4 hours) for patients at risk for Listeria monocytogenes. It is essential to note that the yield of CSF culture decreases when a patient is treated with antibiotics before lumbar puncture, as shown in a retrospective study of 231 children, where 82% of CSF cultures were positive 1. Additionally, PCR on CSF has been found to have incremental value compared to CSF culture and Gram stain, with reported sensitivities of 79–100% for S. pneumoniae, 91–100% for N. meningitidis, and 67–100% for H. influenzae 1. The use of CSF lactate concentration as a diagnostic marker has been supported by recent studies, making it a valuable tool in the diagnosis of bacterial meningitis 1.
From the Research
Lumbar Puncture Results for Bacterial Meningitis
To confirm bacterial meningitis, a lumbar puncture should show the following results:
- A low CSF/blood glucose ratio, with an optimal cut-off of 0.36 2
- A CSF white blood cell count of 500/muL or higher 3
- A CSF lactate level of 31.53 mg/dL or more 3
- Neutrophil-predominant pleocytosis 2
- A decreased glucose level in CSF 2
CSF Analysis
CSF analysis is a crucial diagnostic tool for bacterial meningitis. The results of CSF analysis can help differentiate between bacterial and aseptic meningitis. For example:
- A high absolute neutrophil count in CSF is more indicative of bacterial meningitis, especially after 24 hours of illness 4
- A high percentage of polymorphonuclear cells in CSF is also more indicative of bacterial meningitis 4
Traumatic Lumbar Puncture
In cases of traumatic lumbar puncture, the CSF white blood cell count may need to be corrected to avoid misdiagnosis. A correction factor can be used to adjust the CSF WBC count, but this may also lead to misclassification of some cases of bacterial meningitis 5
Repeat Lumbar Puncture
Repeat lumbar puncture may be necessary in some cases to confirm the diagnosis of bacterial meningitis or to exclude relapsing or persistent infection. The results of repeat lumbar puncture can help guide treatment and management of the patient 6