A 32-year-old woman presents with a 3-day history of fever, headache, and neck stiffness. Lumbar puncture is performed. CSF analysis shows: WBC 250/μL (85% lymphocytes), protein 120 mg/dL, glucose 55 mg/dL (serum glucose 110 mg/dL), Gram stain negative, and bacterial culture negative after 24 hours. What is the most appropriate next investigation to confirm the diagnosis?