Tests
1st tests to order
CSF microscopy
Test
In viral meningitis CSF WBC count is typically >5 cells/mm³, but may be normal. A CSF WBC count >5 cells/mm³ has a sensitivity of 90% for the diagnosis of viral meningitis.[43]
A lymphocytosis is classically described, but neutrophil predominance is commonly seen and so the differential cell count is unable to distinguish between viral and bacterial meningitis accurately.[44] A lymphocytosis may be seen in bacterial meningitis, particularly when antibiotics have been given.[45]
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: >0.45g/L in 50% and >1g/L in 16%.[43]
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 TB meningitis but may be seen in viral meningitis.
Result
may be low
CSF PCR for enteroviruses
Test
When compared with cell culture, PCR of CSF has a sensitivity of 97%.[46] It has been shown to be almost twice as sensitive as viral culture when the two techniques were compared during an outbreak.[47] PCR testing is the gold standard for confirming the diagnosis of viral meningitis.[3][16] A negative PCR does not necessarily exclude viral meningitis; some people with viral meningitis won’t have a positive PCR test.
Result
a positive result indicates the presence of enteroviruses; PCR will not identify the serotype
CSF PCR for herpes viruses
Test
PCR for herpes viruses has sensitivity and specificity of >95% for all herpes CNS infections.[33]
This is not specific for meningitis. A negative PCR does not necessarily exclude viral meningitis; some people with viral meningitis won’t have a positive PCR test.
Result
a positive result indicates the presence of herpes viruses
HIV serology/HIV reverse transcriptase (RT)-PCR
Test
Testing for HIV is recommended for all cases of meningitis. Aseptic meningitis may be a feature of primary HIV infection.[19] HIV viral load testing may enable diagnosis of acute HIV infection prior to seroconversion.
Result
may be positive
Tests to consider
CT/MRI head
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 TB 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
EEG
Test
EEG is often abnormal in encephalitis or may show seizure activity. Particular patterns may suggest a specific etiology (e.g., periodic lateralizing epileptiform discharges in herpes encephalitis).
Result
abnormal in encephalitis
Emerging tests
CSF C-reactive protein (CRP)
Test
A low CSF CRP has a negative predictive value of >97% for bacterial meningitis.[48]
Result
low CSF CRP values may exclude bacterial meningitis
serum and CSF procalcitonin
CSF lactate
Test
A low CSF lactate (<35 mg/dL) is useful in distinguishing viral from bacterial meningitis, particularly if measured prior to antibiotics being administered. Sensitivity is reduced if antibiotics have been started.[36] 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 it is compared with CSF glucose, CSF/plasma glucose quotient, CSF protein, and CSF total number of leukocytes.[3][37]
Result
low
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