CSF analysis in patients with suspected meningitis

The results of cerebrospinal fluid (CSF) analysis are crucial in diagnosing bacterial meningitis and directing therapy. CSF should be sent for cell count; lactate, protein and glucose concentrations; Gram stain and bacterial culture. Other tests that may be required include antigen assays, and nucleic acid amplification tests (eg polymerase chain reaction [PCR]) of blood or CSF.

In adults, the measurement of CSF lactate is a rapid test that can distinguish bacterial meningitis from aseptic meningitis; bacterial meningitis is more likely if CSF lactate is 3.5 mmol/L or moreBuch 2018Sakushima 2011. Bacterial meningitis is also more likely when the CSF–blood glucose ratio is 0.4 or less, or when the CSF white blood cell count is 500 cells per microlitre or moreStraus 2006.

In children, normal CSF does not usually contain neutrophils. The number of lymphocytes may vary depending on the age of the child.

Gram stain of CSF may enable an immediate aetiological diagnosis of bacterial meningitis.

Other tests that may be required, particularly if antibiotic therapy was started before a lumbar puncture was performed, are detailed below.

• Pneumococcal antigen assays of CSF are a rapid test for the diagnosis of pneumococcal meningitis and are useful when the CSF culture is negative in patients who have received antibiotics.
• Nucleic acid amplification testing (eg PCR) of blood or CSF for N. meningitidis and Streptococcus pneumoniae is useful for patients who have a lumbar puncture performed after starting antibiotics.
• PCRs for other organisms that cause meningitis (including enterovirus, parechovirus, herpes simplex virus and varicella zoster virus) or, if available, a multiplex PCR covering common bacterial and viral pathogens.

If there is a concern of encephalitis, add PCR for flaviviruses (specifically Japanese encephalitis, Murray Valley and Kunjin viruses) to the tests ordered for meningitis.