CSF Findings in TB Meningitis
TB meningitis characteristically shows lymphocytic pleocytosis (5-500 cells/μL), markedly elevated protein (>1 g/L), very low glucose (<2.2 mmol/L), and very low CSF/plasma glucose ratio (<0.5), with raised opening pressure and clear or cloudy appearance. 1
Key CSF Parameters
Cell Count and Differential
- White cell count ranges from 5-500 cells/μL, which is lower than typical bacterial meningitis but overlaps with viral and fungal meningitis 1
- Lymphocytes predominate in the cell differential, though neutrophils may predominate early in the disease course before transitioning to lymphocytic predominance 1, 2
- The predominantly lymphocytic pattern helps distinguish TB meningitis from acute bacterial meningitis, which shows neutrophil predominance 3
Protein
- CSF protein is markedly raised, typically >1 g/L, which has a sensitivity of 78% and specificity of 94% for diagnosing TB meningitis 1
- Mean CSF protein in confirmed TB meningitis is approximately 1.9-2.0 g/L 4
- This marked protein elevation is more pronounced than in viral meningitis (which shows only mild elevation) and helps differentiate TB from viral causes 5, 1
Glucose
- CSF glucose is very low, typically <2.2 mmol/L (sensitivity 68%, specificity 96%) 1, 4
- CSF/plasma glucose ratio is very low, typically <0.5 (sensitivity 90%), which is more informative than absolute glucose values 1, 4
- This profound hypoglycorrhachia distinguishes TB meningitis from viral meningitis, where glucose remains normal or only slightly low with CSF/plasma ratio >0.36 5, 1
Physical Characteristics
- Opening pressure is typically raised 1, 2
- CSF appearance is usually clear or cloudy, unlike the turbid or purulent appearance of acute bacterial meningitis 1, 2
Comparative Diagnostic Table
| Parameter | TB Meningitis | Bacterial | Viral | Fungal |
|---|---|---|---|---|
| Opening Pressure | Raised | Raised | Normal/mildly raised | Raised |
| Appearance | Clear or cloudy | Turbid | Clear | Clear or cloudy |
| WCC (cells/μL) | 5-500 | >100 | 5-1000 | 5-500 |
| Cell type | Lymphocytes* | Neutrophils | Lymphocytes | Lymphocytes |
| Protein (g/L) | Markedly raised (>1) | Raised | Mildly raised | Raised |
| Glucose | Very low (<2.2 mmol/L) | Very low | Normal/slightly low | Low |
| CSF/plasma glucose ratio | Very low (<0.5) | Very low | Normal/slightly low | Low |
*Neutrophils may predominate early 1
Critical Diagnostic Pitfalls
Early Disease Presentation
- Neutrophils may predominate in early TB meningitis before the characteristic lymphocytic pattern develops, potentially causing confusion with bacterial meningitis 1, 2
- However, the clinical presentation is typically subacute with symptoms persisting for weeks, unlike the acute presentation of bacterial meningitis 6, 2
Microbiological Confirmation Challenges
- CSF acid-fast smear and culture have relatively low sensitivity, but yield increases with multiple large-volume samples 6
- PCR for TB on CSF is highly specific but has suboptimal sensitivity, so a negative test does not rule out TB meningitis 6, 2
- Decision to treat is mostly clinical because of difficulty demonstrating TB bacilli on routine smear and the time needed for culture 2
Clinical Context is Essential
- CSF findings alone cannot provide an absolute diagnosis and must be interpreted in clinical context 1
- Supporting evidence includes positive TB contact history, positive Mantoux skin test, chest radiograph findings, CT/MRI brain scan showing basal meningitis or hydrocephalus, and demonstration of TB from extraneural sources 2
- The subacute presentation with nonspecific symptoms (fever, cough, vomiting, listlessness) over weeks distinguishes TB from acute bacterial meningitis 2