Inclusion Criteria:  Admitted to the neurosurgical intensive care unit or surgical intensive care unit following severe traumatic brain injury (post-resuscitation GCS 3-8 with or without CT abnormalities) Receiving intravenous levetiracetam for seizure prophylaxis at a dose of 1000 mg every 12 hours or 1000 mg every 8 hours at time of enrollment  Exclusion Criteria:  Known history of epilepsy or seizure disorder Taking antiseizure medication prior to admission Taking medication with known effect on levetiracetam pharmacokinetics including carbamazepine, phenytoin, oxcarbazepine, mefloquine, methotrexate, mianserin, or orlistat Weight < 50 kg Anticipated survival <72 hours from injury, as deemed by the primary neurosurgical provider Acute Kidney Injury (Scr rise > 0.3 mg/dL from baseline) or creatinine clearance <50 mL/min at time of enrollment Prisoners Pregnant