Bromide (potassium or sodium salt) is characterized by a half-life of approximately 21 days in dogs and 14 days in cats; variability among animals is likely to be marked. Bromide is influenced by chloride intake (more chloride intake results in more rapid elimination). As such, during a 12 or 24 hr dosing interval, little drug is eliminated between doses and drug concentrations will accumulate until a steady-state is reached. Because of this long half-life, the sampling time for bromide can be any time during a dosing interval.
Baseline: “Steady-state” should occur in 3 to 5 drug half-lives, or 2.5 to 3 months after dosing at the same dosing regimen. Baseline steady-state concentrations can be determined only at that time. To pro-actively assess the appropriateness of your dose, (for example, patients at risk for therapeutic failure), we recommend that you check concentrations half-way to steady-state, that is, at one half-life (2 to 3 weeks). This concentration can be doubled to predict steady-state concentrations. A steady-state sample should be collected at 3 months to confirm baseline concentrations.
Loading: If the patient is loaded, in the “at risk” patient, we suggest that a sample be collected the day after the loading dose is administered to determine “what was accomplished” with loading and again 2 to 3 weeks later, to test the accuracy of the maintenance dose, and finally at 2.5 to 3 months to establish baseline. Alternatively 2 -3 months into maintenance dosing.
Rationale: While concentrations achieved once a loading dose is completed may be in the therapeutic range, the patient is NOT at steady-state and will not be until it has been dosed with the maintenance dose for 2.5 to 3 months. If the maintenance dose does not maintain what you accomplished with the loading dose, serum bromide concentrations will slowly decline or increase, depending on how incorrect the maintenance dose is. (or increase, which is often not problematic), until steady-state is reached. The patient may start seizuring again at 3 to 4 weeks post load if the maintenance dose is too low, or become groggy if it is too high. A sample collected at 2 to 3 weeks into the maintenance dose (after loading is completed) will assure that the maintenance dose is “maintaining” what you achieved with loading. If you collect a 2-3 week sample without also collecting the post-load sample, we will not be able to tell if serum bromide concentrations have changed compared to the loading concentration.
Other considerations: Consider monitoring in advance of a drastic change in the preparation being given (ie, chew tablet to solution, change in pharmacy, etc). Remember that bromide will cause serum chloride concentrations to appear deranged due to interference with the chloride assay.