What is the typical range of cerebrospinal fluid (CSF) protein levels in viral meningitis?

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

2, 1

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