Investigations
1st investigations to order
blood cultures
FBC and differential
Test
Always request an FBC and differential.
Patients with bacterial meningitis may have a raised white blood cell (WBC) count, a low red blood cell count, and low platelets.
Result
WBC may be normal or mildly elevated
C-reactive protein (CRP)
Test
Always measure serum CRP.
Result
may be normal or mildly elevated
serum procalcitonin
serum urea, creatinine, and electrolytes
Test
Patients with severe bacterial meningitis often have metabolic abnormalities.
Result
may be normal
blood gases
blood glucose
Test
Take blood for glucose (to compare with CSF glucose). In viral meningitis there is a normal or mildly lowered ratio of CSF:blood glucose.
Result
CSF:blood glucose ratio may be normal or mildly lowered
CSF microscopy
Test
In early infection there may be a neutrophil predominance, but this evolves to a more typical lymphocytic picture over the first 2 days of infection. Therefore, the differential cell count is unable to distinguish between viral and bacterial meningitis accurately.[67] A lymphocytosis may be seen in bacterial meningitis, particularly when antibiotics have been given.[68]
Result
WBC count elevated; typically 5-1000 cells/mm3 for children and adults and >20 cells/mm3 for neonates
CSF Gram stain
Test
CSF Gram stain is negative in viral meningitis; a positive CSF Gram stain indicates bacterial meningitis.
Result
negative
CSF bacterial culture
Test
Bacterial culture should be negative in viral meningitis.
Result
negative
CSF protein
Test
In viral meningitis, CSF protein is typically normal or mildly elevated.
Result
normal or elevated
CSF glucose
Test
CSF glucose is usually >50% of plasma glucose. Low CSF glucose is typically seen in bacterial, fungal, and tuberculous meningitis but may be seen in viral meningitis.
Result
may be low
CSF lactate
Test
A low CSF lactate (<35 mg/dL) is useful in distinguishing viral from bacterial meningitis, particularly if it is measured prior to antibiotics being administered. Sensitivity is reduced if antibiotics have been started.[50] A meta-analysis of 25 studies found that, as a single marker, CSF lactate concentration had better diagnostic accuracy in distinguishing bacterial from aseptic meningitis when compared to CSF glucose, CSF/plasma glucose quotient, CSF protein, and CSF total number of leukocytes.[51]
Result
low
CSF, stool and throat swab PCR for viral pathogens
Test
If you suspect a viral cause based on the CSF examination, test the CSF by polymerase chain reaction (PCR) for enteroviruses, herpes simplex viruses type 1 and 2 (HSV-1 and HSV-2), and varicella zoster virus (VZV).[2]
CSF PCR is the gold standard diagnostic test for confirmation of viral meningitis.[2][52]
Isolation of virus from the CSF by cell culture is time-consuming and expensive, and so is not available in the majority of laboratories. PCR methods are able to detect enteroviruses and herpes viruses in the CSF more rapidly than cell culture and with greater sensitivity and specificity.[53][54] Bear in mind, however, that a negative PCR test does not necessarily exclude viral meningitis.
Stool and/or throat swabs can also be tested for enterovirus by PCR.[2]
If you identify a viral pathogen, stop any empirical antibiotics that may have been started.[2] This approach reduces the number of investigations performed and the duration of hospital stay.[58][59][60]
Result
may be positive
HIV serology/HIV reverse transcriptase (RT)-PCR
Test
Aseptic meningitis may be a feature of primary HIV infection.[21] HIV viral load testing may enable diagnosis of acute HIV infection prior to seroconversion.
Result
may be positive
Investigations to consider
CT/MRI head scan
Test
Although unremarkable in viral meningitis, CT or MRI of the brain may be useful to exclude cerebral abscess. Meningeal enhancement may be seen with tuberculous or bacterial meningitis. MRI is more sensitive than CT for detecting changes associated with viral encephalitis. Encephalitis due to herpes simplex typically causes lesions in the temporal lobe. Many of the other viral encephalitides also have suggestive appearances on MRI.
Result
unremarkable; may exclude abscess or bacterial meningitis
Emerging tests
CSF procalcitonin
Test
CSF testing for procalcitonin is less widely available than serum; a value >0.5 nanograms/mL has a positive predictive value of 100% for bacterial meningitis and negative predictive value of >93%, and is useful for distinguishing viral from bacterial meningitis.[42][43][44][45]
Result
elevated in bacterial meningitis
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