Potassium bromide blood level in dogs and cats

What it shows

Bromide leaves the body very slowly and is not broken down by the liver: it is excreted unchanged in the urine. The half-life in dogs is long — references give about 15 days, about 21 days, or a range of 25–46 days, and the difference between individual animals is large. Because of that, the level reaches a stable value (the steady state) only after about 2.5–4 months.

One described schedule is a test about 1 month and about 4 months after the start of the regular regimen, then every 6–12 months in a stable dog. Some vets check an early sample after 2–3 weeks to estimate where the level is heading, and a sample right after a loading course shows only a temporary value, not the steady state. A new check is also usual before a major change of the preparation.

Because the level changes so little during the day, the timing of the sample matters less than for other antiepileptic medicines; labs still often ask for a sample just before the next tablet, and the same timing each time makes results easier to compare.

Higher than reference — what it may be a sign of

• Above the target window. Sleepiness, wobbliness, more thirst, more urination and a bigger appetite are among the described side effects; at a high level signs such as lethargy, disorientation, agitation and loss of coordination are described.
• Less salt (chloride) in the diet than before. Chloride intake affects how fast bromide is cleared: more chloride clears it faster, so a drop in chloride intake can let the level rise.
• Reduced excretion. Bromide is excreted unchanged in the urine, so the vet reads the level together with kidney values.

A high result is not a signal to change anything at home: the vet decides what to do with it.

Lower than reference

A level below the window may mean a higher risk of seizures coming back. Practical reasons include a sample taken before the steady state was reached, missed tablets, a switch of formulation, and a change of diet: a diet richer in chloride speeds up bromide excretion and lowers the level. This is why the vet asks to keep the diet and the amount of salt stable.

Look at together with

• Chloride — many analyzers cannot tell bromide from chloride, so chloride on the biochemistry form can come out falsely high in an animal on bromide. This is an artefact of the method, not a real excess of chloride.
• Phenobarbital — bromide is often combined with it, and the target window for bromide is lower in that case.
• Creatinine and the rest of biochemistry — bromide leaves the body with the urine.

Units and conversion

In the app this row is kept in mg/dL. Veterinary references and some labs write bromide in mg/mL; 1 mg/mL equals 100 mg/dL, so 2 mg/mL is 200 mg/dL. If the form uses another unit, the row keeps the unit printed on the form.

Reference range

The target range is on your lab’s form. For bromide it is not a range of healthy animals but a window in which the medicine usually works, and the target for a particular dog is set by the vet. The Merck Veterinary Manual gives about 1–2 mg/mL (100–200 mg/dL) when bromide is used together with phenobarbital; for bromide alone one of its chapters gives 2–3 mg/mL (200–300 mg/dL), another 1–3 mg/mL (100–300 mg/dL).

Cats and dogs

Sources

1. Charalambous M. Epilepsy in Small Animals. Merck Veterinary Manual, professional version, full review October 2025.
2. Castel A. Anticonvulsants in animals (pharmacology of the nervous system). Merck Veterinary Manual, professional version, full review April 2025.
3. Auburn University Clinical Pharmacology Laboratory. Bromide — therapeutic drug monitoring notes.
4. Auburn University College of Veterinary Medicine. Therapeutic Drug Monitoring (TDM): pricing and sample collection protocols (2023).
5. Texas A&M Veterinary Medical Diagnostic Laboratory. Therapeutic drug monitoring in dogs and cats.