CSF Protein in Viral Meningitis
In viral meningitis, CSF protein is typically mildly elevated but remains below 0.6 g/L (600 mg/dL), which is distinctly lower than the marked elevations seen in bacterial or tuberculous meningitis. 1
Typical CSF Protein Range
- The characteristic protein level in viral meningitis shows mild elevation above the normal upper limit of 0.4 g/L but typically stays well below 0.6 g/L. 2, 1
- This contrasts sharply with bacterial meningitis, where protein is significantly raised (often >1.2 g/L), and tuberculous meningitis, where protein is markedly elevated. 2
- The mild protein elevation in viral meningitis occurs alongside other characteristic findings: lymphocytic pleocytosis (5-1000 cells/μL), normal or slightly low glucose, and clear CSF appearance. 1
Clinical Context and Diagnostic Utility
When interpreting CSF protein, you must consider it within the complete CSF profile rather than in isolation:
- A CSF protein >1.2 g/L (120 mg/dL) is a strong predictor of bacterial rather than viral meningitis, with 99% certainty when combined with other parameters. 2
- The combination of mildly elevated protein with preserved CSF/plasma glucose ratio (>0.36) and lymphocytic predominance strongly favors viral etiology. 2, 1
- CSF lactate <2 mmol/L effectively rules out bacterial disease and supports viral meningitis when protein is only mildly elevated. 2, 1
Important Caveats
Several clinical scenarios can complicate interpretation:
- Early in viral meningitis (especially enteroviral), neutrophils may predominate rather than lymphocytes, but the total white cell count rarely exceeds 2000 cells/μL. 1
- In 5-10% of viral CNS infections (particularly HSV encephalitis), initial CSF may be completely normal including protein levels; repeat lumbar puncture at 24-48 hours will typically show characteristic abnormalities. 2, 1
- A traumatic lumbar puncture falsely elevates protein by approximately 0.1 g/dL for every 100 red blood cells in the CSF, requiring correction. 2
- HSV encephalitis can be hemorrhagic with elevated red cell counts in ~50% of cases, which may elevate the protein measurement. 2, 1
Distinguishing from Other Etiologies
The protein level helps differentiate viral from other causes when combined with other CSF parameters:
| Meningitis Type | Protein Level | Glucose | Cell Type |
|---|---|---|---|
| Viral | Mildly raised (<0.6 g/L) | Normal/slightly low | Lymphocytes |
| Bacterial | Raised | Very low | Neutrophils |
| Tuberculous | Markedly raised | Very low | Lymphocytes |
| Fungal | Raised | Low | Lymphocytes |
The key distinguishing feature is that viral meningitis produces only mild protein elevation, whereas tuberculous meningitis causes marked elevation despite both showing lymphocytic pleocytosis. 2