What is the diagnosis for a patient with meningitis, lymphocytic (lymphocytes) predominance, and hypoglycemia (low glucose), with low cerebrospinal fluid (CSF) glucose and low serum glucose?

Diagnosis: Tuberculous (TB) Meningitis

Given the presentation of lymphocytic predominance with low CSF glucose (2.5 mmol/L, which is at the lower limit of normal) in the context of concurrent low serum glucose, the most likely diagnosis is tuberculous meningitis (Answer D). 1

Key Diagnostic Reasoning

Why TB Meningitis is Most Likely

• Lymphocytic predominance is characteristic of TB meningitis, though neutrophils may predominate early in the disease course 1, 2
• The CSF glucose of 2.5 mmol/L is critically important: TB meningitis typically presents with CSF glucose <2.2 mmol/L (sensitivity 68%, specificity 96%) 1
• The low serum glucose is a critical confounding factor: The CSF/plasma glucose ratio is more diagnostically useful than absolute CSF glucose values 3, 4
• **In TB meningitis, the CSF/plasma glucose ratio is typically <0.5** (sensitivity 90%), which is very low compared to viral meningitis where the ratio remains >0.36 4, 1

Why Other Options Are Less Likely

Bacterial meningitis (Option A):

• Would typically show neutrophil predominance (80-95%), not lymphocytic predominance 3
• While approximately 10% of bacterial meningitis cases can present with lymphocytic predominance, this is the exception 3
• The clinical context matters: if this patient has a subacute presentation over weeks, bacterial meningitis becomes even less likely 2, 5

Viral meningitis (Option B):

• CSF glucose is normal or only slightly low in viral meningitis 4
• The CSF/plasma glucose ratio in viral meningitis remains normal or slightly low but stays above 0.36 4
• A CSF glucose >2.6 mmol/L makes bacterial meningitis unlikely, but this patient's glucose is at 2.5 mmol/L, which is borderline 3
• Viral meningitis would not typically cause significantly low CSF glucose, especially with concurrent low serum glucose 4

Aseptic meningitis (Option C):

• This is a non-specific term that could include viral, fungal, or partially treated bacterial meningitis 3
• The specific pattern of lymphocytic predominance with low glucose points more specifically toward TB meningitis 1

Critical Clinical Pitfall

The absolute CSF glucose value can be misleading when serum glucose is abnormal. 3, 4 You must calculate the CSF/plasma glucose ratio:

• If the ratio is <0.5, TB meningitis is highly likely 1
• If the ratio is <0.36, bacterial meningitis becomes more likely 4
• If the ratio is normal or >0.36, viral meningitis is more probable 4

Additional Supporting Features to Look For

TB meningitis typically presents with: 1, 2

• Raised opening pressure 1
• Markedly elevated CSF protein (typically >1 g/L; sensitivity 78%, specificity 94%) 1
• CSF white cell count 5-500 cells/μL 1
• Subacute presentation with symptoms persisting for weeks 2, 5
• Evidence of pulmonary TB on chest X-ray 6

If CSF protein is <0.6 g/L, bacterial meningitis is unlikely 3, which would further support TB meningitis if protein is markedly elevated.

Immediate Action Required

Empiric anti-tuberculous therapy should be initiated immediately if clinical suspicion is supported by these CSF findings, as waiting for definitive microbiological confirmation (AFB smear, TB culture) can lead to disastrous outcomes given their low sensitivity 2, 5, 6. Treatment should include at least four first-line drugs: isoniazid, rifampin, pyrazinamide, and streptomycin or ethambutol 2.