Phenobarbitone has an elimination half-life between 37 and 89 hours making it relatively easy to maintain therapeutic levels. There is no significant fluctuation in serum phenobarbitone levels throughout a daily dosing interval in most epileptic dogs and it has been shown that phenobarbitone levels do not show significant troughs and peaks throughout a day with twice daily therapy. Therefore timing of the serum collection for determining the phenobarbitone levels is not critical.
(JAVMA: 217; July 2000 p200-204)
To check the therapeutic level, collect blood between 8 and 10 hours after the previous dose. Measurement of serum concentrations is recommended at 7 to 10 days after initiation of therapy (or dosage change).
(Nelson & Couto 2003)
To determine the therapeutic levels, collect blood any time during dosing interval. Due to the prolonged half-life of potassium bromide, between 60 and 120 days will be required before testing the therapeutic levels unless loading dose regimens have been utilised initially.
Therapeutic success of thyroid supplementation should be judged first on clinical grounds and if necessary, corroborated with measurements of serum T4 following achievement of steady state concentrations (generally within 1 - 2 weeks). Although either peak (4 - 6 hours post-dosage) or trough (immediately prior to dosage) levels may be measured, there are more defined guidelines for peak samples.