Levitski RE & Trepanier LA
Effect of timing of blood collection on serum phenobarbital concentrations in dogs with epilepsy.

J Am Vet Med Assoc, 217(2): 200-204, 2000
ISSN: 0003-1488 Journal of the American Veterinary Medical Association (PubMed)

Abstract
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 45mug/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-hour serum concentrations in different therapeutic categories with a > 30% change in concentrations throughout the day. Significant differences were not detected among mean serum phenobarbital concert when comparing the trough, 3-hour, and 6 hour samples for all dogs. Conclusions and Clinical Relevance - There is no therapeutically relevant change in serum phenobarbital concentrations throughout a daily dosing interval in most epileptic dogs. Therefore, timing is not important when collecting blood samples to measure serum phenobarbital concentrations in most epileptic dogs treated long-term with phenobarbital.