CSF analysis

A 35-year-old man, recently arrived from a country with a high burden of tuberculosis, presents to the emergency department with a 2-week history of worsening headache, low-grade fever, and general malaise. Over the past 48 hours, he has become increasingly drowsy and confused. He denies cough, weight loss, or night sweats. His past medical history is unremarkable, and he is not on any regular medications. On examination, he is febrile (38.5°C), drowsy but rousable, with a Glasgow Coma Scale (GCS) of 13/15 (E3 V4 M6). His neck is stiff, and Kernig's sign is positive. Fundoscopy is normal. Chest auscultation is clear. Neurological examination reveals no focal deficits. Initial blood tests show a mild lymphocytosis, normal electrolytes, and normal renal and liver function. A non-contrast CT scan of the brain shows subtle basal meningeal enhancement. A lumbar puncture is performed. CSF Results: * Appearance: Slightly turbid * Opening pressure: 25 cm H2O (elevated) * White cell count: 150 cells/µL (normal <5), 80% lymphocytes * Red cell count: 5 cells/µL * Protein: 1.5 g/L (normal 0.15-0.45) * Glucose: 1.8 mmol/L (normal 2.5-4.5), simultaneous blood glucose 5.5 mmol/L Given the clinical presentation and investigation findings, what is the most appropriate immediate next step in management?