Effect of timing of blood collection on serum phenobarbital concentrationsin dogs with epilepsy

Objective-To determine whether there are-therapeutically relevant changes i n serum phenobarbital concentrations throughout a daily dosing interval in epileptic dogs receiving phenobarbital for greater than or equal to 3 weeks . Design-Prospective study. Animals-33 epileptic dogs receiving phenobarbital. Procedure-Serum phenobarbital concentrations were measured at 0 hour (troug h), 3 hours, and 6 hours after oral administration of phenobarbital in epil eptic dogs that had received phenobarbital twice daily for a minimum of 3 w eeks. For each dog, trough, 3-hour, and 6-hour serum phenobarbital concentr ations were evaluated to determine whether they were within the same therap eutic category (lower, middle, or upper end of the therapeutic range of 15 to 45 mu g/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 s erum phenobarbital concentrations in the same therapeutic category. Only 9% (3/33) of dogs had trough, 5-hour, and 6-hour serum concentrations in diff erent therapeutic categories with a > 30% change in concentrations througho ut the day. Significant differences were not detected among mean serum phen obarbital concentrations when comparing the trough, 3-hour, and 6-hour samp les for all dogs. Conclusions and Clinical Relevance-There is no therapeutically relevant cha nge in serum phenobarbital concentrations throughout a daily dosing interva l in most epileptic dogs. Therefore, timing is not important when collectin g blood samples to measure serum phenobarbital concentrations in most epile ptic dogs treated long-term with phenobarbital.