Effect of timing of blood collection on serum phenobarbital concentrations in dogs with epilepsy

Objective—To determine whether there are therapeutically relevant changes in serum phenobarbital concentrations throughout a daily dosing interval in epileptic dogs receiving phenobarbital for ≥ 3 weeks. Design—Prospective study. Animals—33 epileptic dogs receiving phenobarbital. Procedure—Serum phenobarbital concentrations were measured at 0 hour (trough), 3 hours, and 6 hours after oral administration of phenobarbital in epileptic dogs that had received phenobarbital twice daily for a minimum of 3 weeks. For each dog, trough, 3-hour, and 6-hour serum phenobarbital concentrations were evaluated to determine whether they were within the same therapeutic category (lower, middle, or upper end of the therapeutic range of 15 to 45 µg/ml), or whether there was a > 30% change in serum concentrations throughout the day. Results—Ninety-one percent (30/33) of dogs had trough, 3-hour, and 6-hour serum phenobarbital concentrations in the same therapeutic category. Only 9% (3/33) of dogs had trough, 3-hour, and 6-hou...