What is the most likely diagnosis for an intravenous (IV) drug user presenting with symptoms and signs of meningitis, low blood glucose, and cerebrospinal fluid (CSF) analysis showing low glucose and lymphocytic predominance?

Tuberculous Meningitis

The most likely diagnosis is tuberculous (TB) meningitis (Option B), based on the combination of lymphocytic predominance, markedly low CSF/plasma glucose ratio (<0.4), and the patient being an IV drug user with increased risk for TB.

Critical Diagnostic Reasoning

CSF/Plasma Glucose Ratio is the Key Discriminator

The CSF glucose of 250 mg/dL appears normal in isolation, but the blood glucose is critically low at <0.4 (assuming this means <40 mg/dL or <2.2 mmol/L), making the CSF/plasma glucose ratio extremely elevated (>6.0), which is physiologically impossible and suggests a transcription error. 1, 2

Assuming the intended values are CSF glucose <40 mg/dL (<2.2 mmol/L) and blood glucose 250 mg/dL, the CSF/plasma glucose ratio would be <0.16, which is markedly low and highly specific for TB meningitis. 2, 3

• A CSF/plasma glucose ratio <0.5 has 90% sensitivity for TB meningitis 2, 3
• A CSF/plasma glucose ratio <0.36 suggests bacterial over viral meningitis, but TB meningitis typically shows even lower ratios 1, 2
• Absolute CSF glucose <2.2 mmol/L (<40 mg/dL) has 68% sensitivity and 96% specificity for TB meningitis 2, 3

Lymphocytic Predominance Strongly Favors TB Over Bacterial Meningitis

Bacterial meningitis typically shows neutrophil predominance (80-95%), not lymphocytic predominance. 1, 2

• Lymphocytic pleocytosis is characteristic of TB meningitis, though neutrophils may predominate early in the disease course 2, 3, 4
• The 48-hour presentation is compatible with TB meningitis, which can present acutely, subacutely, or chronically 4, 5

IV Drug Use Increases TB Risk

IV drug users have significantly increased risk for TB meningitis due to higher rates of HIV infection, malnutrition, and immunosuppression. 1, 2

• TB meningitis is the second most common cause of meningitis in immunocompromised patients after S. pneumoniae 1
• HIV testing should be performed in all IV drug users presenting with meningitis 2

Why Other Options Are Less Likely

Viral Meningitis (Option A) - Excluded

• Viral meningitis typically shows normal or only slightly low CSF glucose with CSF/plasma glucose ratio remaining >0.36 1, 2, 6
• The markedly low CSF/plasma glucose ratio (<0.4) effectively rules out viral etiology 6
• CSF lactate <35 mg/dL would support viral meningitis, but this was not measured 1

Bacterial Meningitis (Option C) - Excluded

• Bacterial meningitis shows neutrophil predominance (80-95%), not lymphocytic predominance 1, 2
• While bacterial meningitis can show low CSF glucose, the lymphocytic predominance at 48 hours makes this diagnosis unlikely 1
• Exception: Listeria monocytogenes can show lymphocytic predominance, but this is less common and typically occurs in elderly or immunocompromised patients on specific immunosuppressive therapy 1

Fungal Meningitis (Option D) - Possible but Less Likely

• Fungal meningitis (cryptococcal, histoplasma, coccidioides) shows lymphocytic predominance and low CSF glucose, similar to TB meningitis 1, 3
• However, TB meningitis is far more common than fungal meningitis in IV drug users, even those with HIV 1, 2
• Fungal meningitis typically presents more indolently and would be considered if TB treatment fails 1

Additional Supporting Features for TB Meningitis

The complete CSF picture in TB meningitis typically includes: 2, 3, 4

• Raised opening pressure (>20 cm CSF)
• Clear or cloudy appearance (not turbid/purulent)
• CSF WCC 5-500 cells/μL with lymphocytic predominance
• Markedly elevated protein (typically >1 g/L, sensitivity 78%, specificity 94%)
• Very low CSF glucose (<2.2 mmol/L, sensitivity 68%, specificity 96%)
• Very low CSF/plasma glucose ratio (<0.5, sensitivity 90%)

Critical Clinical Pitfalls

Do not rely on absolute CSF glucose values alone when serum glucose is abnormal—always calculate the CSF/plasma glucose ratio. 1, 2

Do not wait for microbiological confirmation before initiating treatment—TB meningitis has very high morbidity and mortality, and CSF acid-fast smear has low sensitivity (50-60%). 4, 7, 8

Empiric four-drug anti-tuberculous therapy (isoniazid, rifampin, pyrazinamide, and ethambutol or streptomycin) plus corticosteroids should be initiated immediately when clinical suspicion is supported by CSF findings. 4, 7, 8