When should the blood sample be collected for therapeutic monitoring of phenobarbitone?
In a recent study reported at the ACVIM forum in June, researchers from the University of Wisconsin reported that in most dogs, there are no significant difference serum phenobarbitone concentrations in samples obtained prior to dosing, or at 3 or 6 hours postpill. The exception was in a dog receiving a high dose (10 mg/kg/day) of phenobarbitone. Importantly, the dogs in the study were receiving the same dose of phenobarbitone for at least 7 days, and were not receiving any other drugs that may have an effect on serum phenobarbitone concentrations (P450 enzyme inhibitors, such as cimetidine). Thus, when you are monitoring therapeutic serum phenobarbitone concentrations in dogs, you can collect a sample at any time of the day as long as: 1) the dose has been constant for at least 7 days; 2) the dog is not receiving any drugs that would alter the P450 enzymes; and 3) the dog is receiving < 8 mg/kg/day of phenobarbitone.
