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

Robin E. Levitski DVM1,2 and Lauren A. Trepanier DVM, PhD, DACVIM, DACVCP3
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  • 1 Department of Medical Sciences, School of Veterinary Medicine, University of Wisconsin, Madison, WI 53706.
  • | 2 Veterinary Specialty Hospital, PO Box 9727, Rancho Santa Fe, CA 92067.
  • | 3 Department of Medical Sciences, School of Veterinary Medicine, University of Wisconsin, Madison, WI 53706.

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 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-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 concentrations 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. (J Am Vet Med Assoc 2000;217:200–204)

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 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-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 concentrations 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. (J Am Vet Med Assoc 2000;217:200–204)