What is the most likely diagnosis for an intravenous (IV) drug user presenting with symptoms of meningitis, normal cerebrospinal fluid (CSF) glucose levels, hypoglycemia, and lymphocytic predominance in CSF analysis?

Tuberculous Meningitis

The most likely diagnosis is tuberculous meningitis (Option B), given the combination of lymphocytic predominance, very low CSF/plasma glucose ratio (<0.4), and the patient's IV drug use status placing them at higher risk for TB.

Diagnostic Reasoning

CSF Glucose Analysis is Critical

The key to this diagnosis lies in interpreting the CSF glucose correctly:

• The CSF/plasma glucose ratio is <0.4, which is critically low and essentially excludes viral meningitis 1, 2
• In viral meningitis, the CSF/plasma glucose ratio remains >0.36 and is typically normal or only slightly decreased 1, 3
• A CSF/plasma glucose ratio <0.5 makes tuberculous meningitis highly likely, despite the lymphocytic predominance 2, 4
• The absolute CSF glucose of 250 mg/dL appears normal, but this is misleading when the blood glucose is so low—the ratio is what matters diagnostically 4

Lymphocytic Predominance Supports TB Meningitis

• Lymphocytic predominance is characteristic of TB meningitis, though neutrophils may predominate early in the disease course 4, 5
• TB meningitis classically presents with lymphocytic pleocytosis (typically 5-500 cells/μL), markedly elevated protein (>1 g/L), and very low CSF glucose 1, 4
• While viral meningitis also shows lymphocytic predominance, it maintains a normal or only slightly low CSF/plasma glucose ratio 3

IV Drug Use as a Risk Factor

• IV drug users have increased risk for TB meningitis due to higher rates of HIV infection and immunosuppression 4
• HIV testing should be performed immediately in this patient, as HIV-positive individuals have higher incidence and mortality from TB meningitis 4

Why Other Options Are Less Likely

Viral Meningitis (Option A) - Excluded

• The **CSF/plasma glucose ratio of <0.4 essentially excludes viral meningitis**, which maintains ratios >0.36 1, 2, 3
• Viral meningitis shows normal or only slightly low CSF glucose, not the very low ratio seen here 3

Bacterial Meningitis (Option C) - Less Likely

• Bacterial meningitis typically shows neutrophil predominance (80-95%), not lymphocytic predominance 1, 4
• While the low CSF/plasma glucose ratio (<0.36) would support bacterial meningitis, the lymphocytic predominance makes this unlikely unless this represents partially treated bacterial meningitis 1
• However, there is no mention of prior antibiotic use in this case 1

Fungal Meningitis (Option D) - Possible but Less Likely

• Fungal meningitis does present with lymphocytic predominance and low CSF glucose 1
• However, TB meningitis is more common than fungal meningitis in IV drug users, particularly those with HIV 4
• The very low CSF/plasma glucose ratio (<0.5) is more characteristic of TB than fungal meningitis 4

Critical Clinical Pitfalls

• Do not rely on absolute CSF glucose values when serum glucose is abnormal—always calculate the CSF/plasma glucose ratio 4
• TB meningitis can have atypical presentations with normal protein, normal glucose, or even neutrophil predominance early in disease 6
• Empiric anti-tuberculous therapy should be initiated immediately based on clinical suspicion supported by CSF findings, without waiting for microbiological confirmation 4, 5
• A four-drug regimen (isoniazid, rifampin, pyrazinamide, and ethambutol or streptomycin) should be started for 2 months, followed by continuation therapy for 10 months (total 12 months) 5