Monitoring of Therapeutic Drug Levels

Phenobarbitone

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)

Dixogin

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)

Potassium Bromide

To determine the therapeutic levels, collect blood 1 hour prior to the next dose. 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.

Thyroid Supplementation

For levothyroxine, monitoring depends on the dosing schedule used. Serum total T4 ? TSH levels should be measured: 4 to 8 weeks after initiating therapy, whenever signs of thyrotoxicosis develop, or when there has been minimal or no response to therapy.

Twice Daily Dosing Regimen:
The total T4 level +/- TSH levels should be checked 4 to 6 hours after dosing.

Once Daily Dosing Regimen:
The total T4 +/- TSH levels should be checked immediately before and 4 to 6 hours after dosing. (Feldman & Nelson 2004)

DCI-01 Drug Monitoring (V2.1, 06-04-2004)

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