Characteristic CSF Findings in Viral Meningitis

CSF Cell Count and Differential

CSF white cell count in viral meningitis is typically elevated between 5-1000 cells/μL, which is generally lower than in bacterial meningitis 1
Lymphocytes predominate in most cases of viral meningitis, though a predominance of neutrophils may be seen in early viral meningitis, especially with enteroviral infections 1, 2
Even with neutrophil predominance early in the disease course, viral meningitis rarely presents with CSF white cell counts exceeding 2000 cells/μL 1

Opening pressure is typically normal or only mildly raised in viral meningitis, unlike bacterial meningitis where it is often significantly elevated 1, 2
CSF appearance is typically clear in viral meningitis, compared to turbid or cloudy in bacterial meningitis 2
CSF protein is mildly raised in viral meningitis, but typically less than 0.6 g/L (values above this level are more suggestive of bacterial meningitis) 1, 2
CSF glucose is usually normal or only slightly decreased in viral meningitis 2
CSF/plasma glucose ratio is typically normal or only slightly decreased, generally remaining above 0.36 (ratios below this level strongly suggest bacterial meningitis) 1, 3
CSF lactate is a valuable biomarker - levels below 3.5 mmol/L (or 35 mg/dL) strongly suggest viral rather than bacterial etiology 1

No single CSF parameter provides absolute differentiation between viral and bacterial meningitis; results must be interpreted in clinical context 1
CSF lactate has high sensitivity (93%) and specificity (96%) for distinguishing between bacterial and viral meningitis when measured before antibiotic administration 1
PCR testing of CSF for specific viral pathogens (HSV 1 & 2, VZV, and enteroviruses) can identify approximately 90% of viral causes 4
If initial CSF findings are non-diagnostic, a second lumbar puncture performed 24-48 hours later is recommended 4