Phenobarbital

Service Description

For routine therapeutic monitoring, a 'trough' level (collected immediately before next dose) is recommended. A 'peak"sample (4 to 5 hours after last dose) can be used to evaluate for possible toxic levels. Toxicity can be seen in a small proportion of patients with levels in the therapeutic range. Ideal therapeutic range to minimize toxicity while maintaining efficacy may be between 20-30 ug/mL. There may be significant variability in the half-life of phenobarbital. If breakthrough seizures are occurring, and inadequate dosing or short half-life is suspected, both peak and trough samples should be analyzed. Therapeutic monitoring should be performed after two to four weeks of consistent dosage following initiation of treatment or dosage change to allow most patients to achieve a relatively steady state. Patients on lower doses (mg/kg) may take longer to come to steady state. Consistent timing of sampling remains important for comparison across time as there may still remain some fluctuation throughout the day, especially for patients receiving higher doses (mg/kg). Monitoring should then be repeated at a minimum of every six months thereafter, depending on clinical response.