CSF Cell Count in TB Meningitis
In TB meningitis, the CSF typically shows a lymphocytic pleocytosis with 10-500 cells/μL (though most commonly 5-500 cells/μL), with lymphocytes predominating after the first 10 days of illness. 1, 2
Cell Count Range and Characteristics
Total white cell count ranges from 10-500 cells/μL, which is intermediate between bacterial meningitis (typically >1,000 cells/μL) and viral meningitis (usually <100 cells/μL) 1, 2, 3
The pleocytosis is generally lower than bacterial meningitis but higher than viral meningitis, making this range diagnostically useful 2
In one prospective study, the mean CSF cell count was 71 cells/μL with 67% lymphocytes 4
Critical Timing Consideration: The Neutrophil-to-Lymphocyte Shift
During the first 10 days of illness, neutrophils may predominate (60-80%), which can mislead clinicians into suspecting bacterial meningitis 5
After the first 10 days, mononuclear cells (lymphocytes, lymphoid cells, monocytoid cells, and macrophages) become predominant 5
By the third week, plasma cells (up to 20%) may be found in 30% of cases 5
Neutrophil predominance (>50%) in early TB meningitis has 54% sensitivity and 98% specificity for diagnosis, making it a useful early marker despite being counterintuitive 6
Lymphocytic Predominance Pattern
Lymphocytic predominance (>50% lymphocytes) is the characteristic finding after the early phase 1, 3, 6
The American College of Physicians notes that lymphocytic predominance is characteristic of TB meningitis, though neutrophils may predominate early in the disease course 1
Important Clinical Pitfall
Approximately 10% of bacterial meningitis cases present with lymphocytic predominance (≥50% lymphocytes or monocytes), particularly when the total CSF white cell count is ≤1,000/mm³ 7, 8
Lymphocytic predominance occurred in 32% of bacterial meningitis cases with CSF white cell count ≤1,000/mm³, making cell differential alone insufficient to distinguish TB from bacterial meningitis 8
Listeria monocytogenes can present with lymphocytic predominance and accounts for 20-40% of bacterial meningitis in immunocompromised patients, elderly, diabetics, and those on immunosuppressive therapy 9
Additional Distinguishing CSF Features
Beyond cell count, other CSF parameters help confirm TB meningitis:
CSF protein is typically markedly elevated (>1 g/L), with mean values around 2.10 g/L 1, 6, 4
CSF glucose is characteristically low (<2.2 mmol/L) with 68% sensitivity and 96% specificity 1, 6
**CSF-to-plasma glucose ratio is typically <0.5**, which is more diagnostically useful than absolute glucose values and helps distinguish TB from viral meningitis (where ratio remains >0.36) 1, 2
Mean CSF glucose in confirmed cases was 2.05 mmol/L 4
Diagnostic Algorithm Based on Cell Count
When evaluating CSF with elevated white cells:
If cell count is 10-500 cells/μL with lymphocytic predominance AND CSF/plasma glucose ratio <0.5 AND protein >1 g/L: strongly consider TB meningitis 1, 2, 6
If neutrophils predominate but cell count is in the 10-500 range AND clinical history >5 days: still consider early TB meningitis 1, 6, 5
If cell count >1,000 cells/μL with neutrophil predominance: bacterial meningitis is more likely, though TB cannot be excluded 7, 2
CSF lactate <2 mmol/L effectively rules out bacterial disease and supports TB or viral etiology 7, 9
Management Implication
Start empiric anti-tuberculous therapy immediately when clinical suspicion is supported by CSF findings showing lymphocytic pleocytosis (10-500 cells/μL), low CSF/plasma glucose ratio (<0.5), and elevated protein (>1 g/L), without waiting for microbiological confirmation 1, 3
If bacterial meningitis cannot be definitively excluded based on cell count alone, empiric antibiotics including ampicillin (for Listeria coverage if >50 years or immunocompromised) should be started alongside anti-tuberculous therapy 9