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

Most Likely Diagnosis: Tuberculous Meningitis

The most likely diagnosis is tuberculous (TB) meningitis (Option B), based on the critically low CSF/plasma glucose ratio of <0.4 combined with lymphocytic predominance in an IV drug user. 1

Key Diagnostic Reasoning

The Critical CSF/Plasma Glucose Ratio

• The CSF/plasma glucose ratio of <0.4 is the decisive finding here. The Infectious Diseases Society of America emphasizes that this ratio is more diagnostically useful than absolute CSF glucose values, particularly when serum glucose is abnormal 1
• In TB meningitis, the CSF/plasma glucose ratio is typically <0.5, which is very low compared to viral meningitis where the ratio remains >0.36 1
• The Centers for Disease Control and Prevention states that if the CSF/plasma glucose ratio is <0.5, TB meningitis is highly likely 1
• While the absolute CSF glucose of 250 mg/dL appears normal, this is misleading because the patient has severe hypoglycemia (blood glucose <0.4), making the ratio the critical parameter 1

Lymphocytic Predominance Pattern

• Lymphocytic predominance is characteristic of TB meningitis, though neutrophils may predominate early in the disease course 1
• TB meningitis, fungal infections, and Listeria monocytogenes should be considered in patients with subacute presentation or immunocompromised state, as they can produce lymphocytic pleocytosis 2
• Approximately 10% of bacterial meningitis cases present with lymphocytic predominance, but these typically do not have such a profoundly low CSF/plasma glucose ratio 3

Risk Factor Consideration

• IV drug users are at significantly increased risk for TB meningitis due to higher rates of HIV infection, malnutrition, and exposure to TB in high-risk environments 2
• This population is also at risk for immunocompromised states that predispose to TB reactivation 2

Why Not the Other Options?

Viral Meningitis (Option A) - Excluded

• Viral meningitis typically presents with normal or only slightly decreased CSF glucose 4
• The CSF/plasma glucose ratio in viral meningitis remains normal or slightly low but stays above 0.36 4
• A ratio of <0.4 essentially excludes viral meningitis 1, 4
• While lymphocytic predominance fits viral meningitis, the profoundly low glucose ratio does not 4

Bacterial Meningitis (Option C) - Less Likely

• Bacterial meningitis typically shows neutrophil predominance (80-95%), not lymphocytic predominance 3, 1
• While 10% of bacterial meningitis cases can have lymphocytic predominance, the CSF/plasma glucose ratio would need to be <0.36 to favor bacterial over TB meningitis 1
• The ratio of <0.4 falls in the TB meningitis range rather than typical bacterial meningitis 1

Fungal Meningitis (Option D) - Possible but Less Likely

• Fungal meningitis can present with lymphocytic pleocytosis and low CSF/plasma glucose ratio similar to TB meningitis 2
• However, TB meningitis is more common than fungal meningitis in IV drug users, even in immunocompromised patients 2
• The European Society of Clinical Microbiology and Infectious Diseases notes that low CSF glucose with lymphocytic predominance suggests tuberculosis or fungal infection, but TB is the more prevalent diagnosis 1

Critical Management Implications

• If bacterial meningitis cannot be definitively excluded, start empiric antibiotics immediately, including ceftriaxone + vancomycin + ampicillin if >50 years or immunocompromised 2
• Treatment should begin within one hour of presentation when bacterial meningitis is suspected 2
• Brain MRI with contrast should be performed to evaluate for basal meningeal enhancement, which is characteristic of TB meningitis 5
• CSF should be sent for AFB smear, TB culture, and TB PCR testing 5

Common Pitfall to Avoid

The absolute CSF glucose value of 250 mg/dL can be misleading when serum glucose is abnormal. Always calculate the CSF/plasma glucose ratio rather than relying on absolute CSF glucose values alone 1. In this case, the seemingly "normal" CSF glucose is actually inappropriately elevated relative to the severely low serum glucose, indicating impaired glucose transport across the blood-brain barrier—a hallmark of TB meningitis 1.