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

Diagnosis: Tuberculous (TB) Meningitis

The most likely diagnosis is TB meningitis (option D), as the combination of lymphocytic predominance with a low CSF/plasma glucose ratio (despite concurrent hypoglycemia) is pathognomonic for TB rather than viral or typical bacterial meningitis. 1

Critical Diagnostic Reasoning

Why TB Meningitis is the Answer

The absolute CSF glucose of 2.5 mmol/L is misleading in isolation when serum glucose is also low. 1 The key diagnostic feature here is calculating the CSF/plasma glucose ratio:

• In TB meningitis, the CSF/plasma glucose ratio is typically <0.5, which is extremely low 1
• If the patient's serum glucose is also low (hypoglycemia), a CSF glucose of 2.5 mmol/L likely represents a ratio well below 0.5, strongly indicating TB meningitis 1
• The European Society of Clinical Microbiology and Infectious Diseases explicitly states that absolute CSF glucose values are misleading when serum glucose is abnormal, and the ratio is diagnostically superior 1

Why NOT the Other Options

Bacterial meningitis (option A) is excluded because:

• Bacterial meningitis shows neutrophil predominance (80-95%), not lymphocytic predominance 1, 2
• While some bacterial cases can show lymphocytes (Listeria, partially treated), this patient's presentation doesn't suggest these scenarios 3

Viral meningitis (option B) is excluded because:

• Viral meningitis maintains a CSF/plasma glucose ratio >0.36, even when CSF glucose is slightly low 1, 2
• The CSF glucose in viral meningitis is typically normal or only slightly low, not significantly depressed 2
• With concurrent hypoglycemia, viral meningitis would show a preserved ratio, not the severely depressed ratio seen here 1

"Aseptic meningitis" (option C) is a non-specific term:

• This term encompasses viral, TB, fungal, and other non-bacterial causes 3
• It's not a specific diagnosis and doesn't represent the most accurate answer given the clinical picture

Supporting Diagnostic Features of TB Meningitis

Classic CSF profile in TB meningitis includes: 1, 4

• Lymphocytic predominance (present in this case)
• CSF glucose <2.2 mmol/L with sensitivity 68% and specificity 96% (this patient has 2.5 mmol/L, borderline)
• CSF/plasma glucose ratio <0.5 (critical finding here)
• Markedly elevated protein, typically >1 g/L
• CSF WBC count 5-500 cells/μL
• Raised opening pressure

The subacute presentation is typical: 4, 5

• TB meningitis characteristically presents with symptoms persisting for weeks before diagnosis
• A clinical history >5 days is independently predictive with 93% sensitivity 1

Critical Clinical Pitfall to Avoid

Never interpret CSF glucose in absolute terms when the patient has abnormal serum glucose. 1 This is the most common diagnostic error in this scenario:

• A CSF glucose of 2.5 mmol/L might seem "lower normal" in isolation
• However, if serum glucose is also low (hypoglycemia as stated), the ratio becomes the critical discriminator
• Always calculate CSF/plasma glucose ratio: normal is approximately 0.6-0.7 (two-thirds) 3
• Bacterial meningitis shows ratio <0.36 3, 1
• TB meningitis shows ratio <0.5, often much lower 1

Immediate Management Implications

Treatment for TB meningitis should be initiated immediately once clinical suspicion is supported by CSF findings: 4, 6

• Start at least four first-line drugs: isoniazid, rifampin, pyrazinamide, and streptomycin or ethambutol
• Adjunctive corticosteroids improve mortality in TB meningitis 4
• Do not wait for culture confirmation, as AFB smear and culture have low sensitivity 4, 7