What is the most likely diagnosis for an IV (intravenous) drug user presenting with symptoms of meningitis, including fever and headache for 3 weeks, worsening headache, low cerebrospinal fluid (CSF) glucose, elevated protein, and lymphocytic predominance?

Tuberculous Meningitis

The most likely diagnosis is B. Tuberculous meningitis, based on the subacute 3-week presentation, lymphocytic predominance, markedly low CSF glucose, elevated protein, and IV drug use risk profile. 1

Key Diagnostic Features Supporting TB Meningitis

The CSF profile is classic for tuberculous meningitis:

• Lymphocytic predominance is characteristic of TB meningitis, though neutrophils may predominate early in the disease course 1, 2
• CSF glucose <0.4 mmol/L (assuming this is the CSF value) with a CSF/plasma glucose ratio <0.5 is highly specific for TB meningitis (96% specificity) 1
• Markedly elevated protein (3 g/L) is typical of TB meningitis, which characteristically shows protein >1 g/L 1
• Subacute presentation over 3 weeks strongly favors TB meningitis; a clinical history >5 days is independently predictive with 93% sensitivity 1, 3

Why Other Diagnoses Are Excluded

Bacterial meningitis (Option C) is ruled out because:

• Bacterial meningitis typically shows neutrophil predominance (80-95%), not lymphocytic predominance 4, 1
• The 3-week subacute course is incompatible with acute bacterial meningitis, which progresses rapidly 4
• While bacterial meningitis can show low glucose, the CSF/plasma ratio would typically be <0.36, and the lymphocytic predominance makes this diagnosis unlikely 1

Viral meningitis (Option A) is excluded because:

• Viral meningitis typically presents with normal or only slightly low CSF glucose, with CSF/plasma ratio remaining >0.36 1, 5
• The markedly elevated protein (3 g/L) far exceeds the mild elevation seen in viral meningitis 5
• The 3-week progressive course is atypical for viral meningitis 5

Fungal meningitis (Option D) is less likely because:

• While fungal meningitis can present similarly with lymphocytic predominance and low glucose, TB meningitis is far more common in IV drug users, particularly those with HIV risk 1
• The specific CSF profile with protein of 3 g/L and the clinical timeline are more characteristic of TB meningitis 2, 6

Critical Clinical Context

IV drug use is a major risk factor for TB meningitis due to:

• Higher rates of HIV infection in this population, which dramatically increases TB meningitis risk 1
• HIV testing must be performed immediately in this patient 1

Diagnostic Algorithm Applied

When six features are present, TB meningitis has 93% sensitivity and 77% specificity 3:

1. ✓ Clinical history >5 days (3 weeks)
2. ✓ Headache present
3. ✓ CSF WBC likely <1000/mm³ (lymphocytic predominance suggests moderate elevation)
4. ✓ Lymphocyte proportion >30%
5. ✓ Protein >100 mg/dL (3 g/L = 300 mg/dL)
6. ✓ Low CSF glucose

This patient meets all criteria, confirming TB meningitis as the diagnosis. 3

Critical Management Pitfall

Treatment must be initiated immediately without waiting for culture confirmation, as CSF AFB smear has low sensitivity (positive in only 30% of cases) and culture takes weeks 2, 6. Empiric four-drug therapy (isoniazid, rifampin, pyrazinamide, and ethambutol or streptomycin) plus adjunctive corticosteroids should be started now 2.