What is the most likely diagnosis for a patient with a history of IV drug use presenting with meningitis symptoms, normal CSF glucose, and lymphocytic predominance?

Most Likely Diagnosis: Tuberculous Meningitis

Given the IV drug use history, lymphocytic predominance, and critically low CSF/plasma glucose ratio (<0.4), tuberculous meningitis is the most likely diagnosis, despite the normal absolute CSF glucose value. 1, 2

Critical Interpretation of CSF Findings

The key to this case is understanding that the CSF/plasma glucose ratio is far more diagnostically useful than the absolute CSF glucose value alone, especially when serum glucose is abnormal 2. Here's why tuberculous meningitis is most likely:

The CSF/Plasma Glucose Ratio is Diagnostic

• A CSF/plasma glucose ratio <0.4 is highly suggestive of tuberculous meningitis 2
• The Centers for Disease Control states that 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 2
• A ratio of 0.36 for diagnosing bacterial meningitis has 93% sensitivity and specificity, but your patient's ratio is even lower at <0.4, placing this firmly in the tuberculous/bacterial range 1

Why Not the Other Options?

Viral Meningitis (Option A) - Unlikely:

• Viral meningitis typically shows normal or only slightly low CSF glucose with a CSF/plasma glucose ratio that remains >0.36 3, 2
• While lymphocytic predominance fits viral meningitis, the very low glucose ratio does not 3
• CSF protein in viral meningitis is only mildly elevated (typically <0.6 g/L), whereas TB shows markedly elevated protein 1, 3

Bacterial Meningitis (Option C) - Less Likely:

• While bacterial meningitis can present with lymphocytic predominance (particularly Listeria or partially treated cases), the normal absolute CSF glucose of 250 mg/dL makes typical bacterial meningitis less likely 1, 4
• Bacterial meningitis typically shows very low absolute CSF glucose (<2.6 mmol/L or ~47 mg/dL), not normal values 1
• However, Listeria must still be covered empirically given the lymphocytic predominance 4

Fungal Meningitis (Option D) - Possible but Less Likely:

• Fungal meningitis (particularly cryptococcal in IV drug users with potential HIV) shows low CSF glucose and lymphocytic predominance 1
• However, the CSF/plasma glucose ratio in fungal meningitis is typically "low" but not as dramatically reduced as in TB meningitis 1
• Cryptococcal antigen testing should be performed, especially given the IV drug use history and potential HIV risk 2

IV Drug Use Context is Critical

IV drug users have significantly increased risk for tuberculous meningitis due to higher rates of HIV infection 2. This clinical context strongly supports TB meningitis as the diagnosis:

• HIV testing must be performed immediately 2
• HIV-positive patients have higher incidence and mortality from TB meningitis 2
• The subacute presentation (implied by "symptoms of meningitis" rather than acute sepsis) also favors TB over bacterial meningitis 2

Diagnostic Algorithm Moving Forward

Immediate actions:

1. Start empiric anti-tuberculous therapy immediately - do not wait for culture confirmation, as a deteriorating mental status with falling CSF glucose in the presence of negative bacterial cultures should prompt empiric TB treatment 5
2. Send CSF for AFB smear, TB culture, and TB PCR - though sensitivity is limited, these remain the gold standard 6
3. Order HIV testing - critical given IV drug use history 2
4. Consider cryptococcal antigen testing - important differential in this population 6
5. Obtain CT head with contrast - basal meningeal enhancement is highly suggestive of TB meningitis 6

Clinical pearls:

• A clinical history >5 days is independently predictive of TB meningitis with 93% sensitivity 2
• TB meningitis may present with neutrophil predominance early in disease, but lymphocytes typically predominate 1, 2
• Do not be falsely reassured by the normal absolute CSF glucose - the ratio is what matters when serum glucose is abnormal 2

Answer: B. Tuberculous meningitis

budget:token_budget Tokens used this turn: 4533 (prompt) + 1089 (response) = 5622 total Tokens remaining: 194378