What are the key differences in cerebrospinal fluid (CSF) findings between tubercular meningitis (TBM) and viral meningitis?

Key Differences in CSF Findings Between Tubercular Meningitis and Viral Meningitis

The most significant differences in cerebrospinal fluid (CSF) between tubercular meningitis (TBM) and viral meningitis include the markedly elevated protein levels, very low glucose levels, and low CSF/plasma glucose ratio in TBM, while viral meningitis typically shows only mildly raised protein and normal to slightly reduced glucose levels. 1

Opening Pressure and Appearance

• TBM: Raised opening pressure (>20 cm CSF), clear or cloudy appearance 2, 1
• Viral meningitis: Normal to mildly raised opening pressure, typically clear appearance 2

CSF Cell Count and Differential

• TBM: Moderately elevated white cell count (typically 5-500 cells/μL) with predominant lymphocytes 2, 1
  • Early in disease course, neutrophils may predominate 2, 1
  • Mean cell count of approximately 71/μL has been reported 3
• Viral meningitis: Mild to moderate pleocytosis (typically 5-1000 cells/μL) with lymphocyte predominance 2
  • Early enteroviral meningitis may show neutrophil predominance, but total count rarely exceeds 2000 cells/μL 2

CSF Protein

• TBM: Markedly elevated protein levels, typically >1 g/L 2, 1, 4
  • Mean protein values of 1.91-2.10 g/L have been reported 4, 3
  • Protein >1 g/L has 78% sensitivity and 94% specificity for diagnosing TBM 4
• Viral meningitis: Mildly elevated protein levels 2
  • Typically <0.6 g/L 2

CSF Glucose

• TBM: Very low glucose levels, typically <2.2 mmol/L 2, 1, 4
  • CSF glucose <2.2 mmol/L has 68% sensitivity and 96% specificity for TBM 4
  • Very low CSF/plasma glucose ratio, typically <0.5 2, 1, 4
  • CSF/plasma glucose ratio <0.5 has 90% sensitivity for TBM 4
• Viral meningitis: Normal to slightly reduced glucose levels 2
  • CSF/plasma glucose ratio usually normal or only slightly reduced 2
  • CSF glucose typically >2.6 mmol/L 2

Additional Diagnostic Parameters

• CSF adenosine deaminase (ADA) activity is significantly higher in TBM (mean 13.62 ± 8.45 IU/L) compared to non-tubercular meningitis (6.51 ± 2.41 IU/L) 5
  • ADA cutoff value of 6.97 IU/L has 85% sensitivity and 88% specificity for TBM 5
• CSF lactate levels are elevated in TBM and show an increasing trend with disease severity 6
  • In bacterial meningitis, CSF lactate >35 mg/dL has high sensitivity and specificity (93% and 96%) for distinguishing from viral meningitis 2

Clinical Pearls and Pitfalls

• No single CSF parameter provides absolute diagnosis; interpretation must be in clinical context 2, 1
• CSF findings may overlap between different types of meningitis 2
• Early TBM may show neutrophil predominance, mimicking bacterial meningitis 2, 1
• Prior antibiotic treatment can alter typical CSF findings 2
• Multiple, large-volume CSF samples increase diagnostic yield for TBM 7
• CSF findings should be interpreted alongside clinical presentation, imaging findings, and other laboratory tests 2