Characteristic CSF Picture of Tuberculous (TB) Meningitis
The typical cerebrospinal fluid (CSF) picture in tuberculous meningitis includes lymphocytic pleocytosis (5-500 cells/μL), markedly elevated protein (typically >1 g/L), very low glucose (<2.2 mmol/L), and a very low CSF/plasma glucose ratio (<0.5). 1
Key CSF Parameters in TB Meningitis
- Opening pressure is typically raised in TB meningitis 1
- CSF appearance is usually clear or cloudy, unlike the turbid or purulent appearance seen in bacterial meningitis 1
- CSF white cell count ranges from 5-500 cells/μL, with a predominance of lymphocytes (though neutrophils may predominate early in the disease) 1, 2
- CSF protein is markedly elevated, typically >1 g/L, with a sensitivity of 78% and specificity of 94% for diagnosing TB meningitis 1, 3
- CSF glucose is very low, with values typically <2.2 mmol/L (sensitivity 68%, specificity 96%) 1, 3
- CSF/plasma glucose ratio is very low, typically <0.5 (sensitivity 90%) 1, 3
Comparison with Other Types of Meningitis
| Parameter | Tuberculous | Bacterial | Viral | Fungal |
|---|---|---|---|---|
| Opening Pressure | Raised | Raised | Normal/mildly raised | Raised |
| Appearance | Clear or cloudy | Turbid, cloudy | 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 | Raised | Mildly raised | Raised |
| Glucose | Very low | Very low | Normal/slightly low | Low |
| CSF/plasma glucose ratio | Very low | Very low | Normal/slightly low | Low |
*May be neutrophilic early in the disease 1
Diagnostic Pearls and Pitfalls
- TB meningitis typically presents subacutely with symptoms persisting for weeks before diagnosis 4, 5
- Multiple, large-volume CSF samples increase the yield of acid-fast smear and culture, which otherwise have relatively low sensitivity 4
- A CSF protein >1 g/L has a sensitivity of 78% and specificity of 94% for diagnosing TB meningitis 3
- A CSF glucose <2.2 mmol/L has a sensitivity of 68% and specificity of 96% for TB meningitis 3
- A CSF/serum glucose ratio <0.5 has a higher sensitivity (90%) than absolute glucose values alone 3
- PCR testing of CSF for TB is highly specific but has suboptimal sensitivity, meaning a negative test cannot rule out TB meningitis 4
- The combination of clinical presentation and characteristic CSF findings often necessitates empiric treatment before microbiological confirmation 5
- In one study, patients with confirmed TB meningitis had significantly higher CSF protein, lower CSF glucose, higher CSF cell count, and lower percentage of CSF lymphocytes compared to those with probable TB meningitis 2
Clinical Context
- TB meningitis develops when a small intracranial tuberculoma (Rich focus) ruptures, causing predominantly basal meningitis 5
- This can result in hydrocephalus, cranial nerve palsies, and ischemic brain injury secondary to tuberculous vasculitis 5
- The primary source of TB is usually the lung 5
- Treatment should be initiated as soon as clinical suspicion is supported by initial CSF studies 4