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

Robin E. Levitski Department of Medical Sciences, School of Veterinary Medicine, University of Wisconsin, Madison, WI 53706.
Veterinary Specialty Hospital, PO Box 9727, Rancho Santa Fe, CA 92067.

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Lauren A. Trepanier Department of Medical Sciences, School of Veterinary Medicine, University of Wisconsin, Madison, WI 53706.

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 DVM, PhD, DACVIM, DACVCP

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)

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