A 10-day-old baby boy is brought to the physician with fever, irritability and poor feeding for the past 48 hours. On examination, he is agitated and has a fever of 38.4°C and a bulging fontanelle. His heart rate is 140 beats/min, respiratory rate is 40 breaths/min, and oxygen saturation is 96% on room air. A lumbar puncture reveals cloudy cerebrospinal fluid (CSF) with a WBC count of 35 cells/uL (0-22 cells/uL), protein of 100 mg/dl (19-140), and glucose of 40 mg/dl (17-162). Gram stain of CSF is negative for bacteria. In addition to starting empirical antibacterial therapy, which of the following is the most appropriate next step in management?