Guide 18 → Blood Monitoring for Pet Seizure Medications
Blood monitoring for seizure medications — what the tests mean
Your vet keeps asking for bloodwork. Here is exactly what they are checking, why it matters, and what the results tell you about your pet's treatment.
Why monitoring matters
Anti-seizure medications are prescribed at specific doses — but how much of that dose is actually in your pet's bloodstream varies between individual animals due to differences in metabolism, body composition, and other medications. A dose that produces adequate blood levels in one dog may be too low or too high in another. Regular blood monitoring allows your veterinarian to verify that medication levels are within the therapeutic range — high enough to control seizures but not so high as to cause toxicity.
Phenobarbital monitoring
Phenobarbital is the most widely used anti-seizure medication in dogs and requires the most systematic monitoring. Two types of blood tests are essential:
Serum phenobarbital level
This test measures the concentration of phenobarbital in the blood. The goal is to achieve a level within the established therapeutic range — generally considered to be 20–40 µg/mL in dogs, though your veterinarian may target a specific range based on your pet's individual situation.
- Low level — May explain poor seizure control; your vet may consider a dose increase
- High level — Associated with toxicity risk; your vet may consider a dose reduction
- When to test — An initial level is typically checked 2–4 weeks after starting or changing a dose. Thereafter, monitoring every 6 months is standard in stable patients.
- Timing matters — Phenobarbital level samples should be drawn at a consistent time relative to dosing — typically before the morning dose (trough level) or as directed by your vet
Liver function tests
Phenobarbital is metabolized by the liver and, at sustained high levels, can cause hepatotoxicity (liver damage). Regular liver function monitoring is essential for all pets on long-term phenobarbital. Tests typically include:
- ALT (alanine aminotransferase) — Enzyme released by damaged liver cells; elevated values suggest liver stress
- ALP (alkaline phosphatase) — Often elevated in dogs on phenobarbital even without liver damage (enzyme induction); requires interpretation in context
- Albumin — A protein produced by the liver; low albumin indicates reduced liver synthetic function
- Bile acids (pre- and post-prandial) — The most sensitive test for liver function; often recommended in addition to standard enzyme panels, particularly if other values are abnormal
Phenobarbital induces production of ALP — a liver enzyme — through a normal pharmacological mechanism, not liver damage. Many dogs on phenobarbital have elevated ALP without any evidence of liver dysfunction. Your vet will look at the full panel, including albumin and bile acids, to assess whether elevated ALP represents true liver stress. Do not assume elevated ALP means your pet's liver is failing — context is everything.
Potassium bromide monitoring
Potassium bromide (KBr) is used as an add-on to phenobarbital in some dogs. It reaches stable blood levels slowly — over several months — making monitoring timing particularly important.
- Serum bromide levels should ideally be checked after a minimum of 3 months on a stable dose
- Therapeutic range is generally 1000–2000 mg/L in dogs on combination therapy; 2000–3000 mg/L in dogs on bromide alone
- Bromide is excreted by the kidneys — kidney function should be monitored, particularly in older patients
- Salt intake affects bromide levels — a significant change in dietary salt (switching foods, starting salty treats) can alter bromide blood levels and potentially affect seizure control
Levetiracetam (Keppra) monitoring
Levetiracetam does not require routine blood level monitoring for toxicity in the same way as phenobarbital. It is not hepatotoxic and does not have a narrow therapeutic window that requires regular adjustment. However, clinical monitoring — tracking seizure frequency and watching for behavioral side effects — remains important.
Complete blood count (CBC)
Phenobarbital can, in rare cases, cause bone marrow suppression — a reduction in the production of blood cells. A complete blood count (CBC) checks red blood cells, white blood cells, and platelets. Significant abnormalities in these values while on phenobarbital should prompt discussion with your vet about whether the medication is causing this effect.
How to prepare for monitoring visits
- Know the exact time your pet received their last dose of medication
- Bring your seizure diary — monitoring appointments are an opportunity to review seizure frequency data
- Note any changes in your pet's behavior, appetite, water intake, or activity since the last visit
- Ask your vet specifically what time of day the blood sample should be drawn relative to medication dosing
- If your pet has been on a different diet or had unusual salt intake, mention this
- Blood monitoring verifies that anti-seizure medication levels are in the therapeutic range for your individual pet
- Phenobarbital requires monitoring of both drug levels and liver function — typically every 6 months in stable patients
- ALP elevation on phenobarbital is common and does not automatically indicate liver damage — ask your vet to explain the full picture
- Potassium bromide levels take months to stabilize; dietary salt changes can significantly alter bromide levels
- Levetiracetam does not require routine blood level monitoring but clinical monitoring is still essential
- Bring your seizure diary and medication timing information to every monitoring appointment
- Podell M, et al. 2015 ACVIM small animal consensus statement on seizure management in dogs. Journal of Veterinary Internal Medicine. 2016;30(2):477–490. doi.org/10.1111/jvim.13841
- Bhatti SFM, et al. International veterinary epilepsy task force consensus proposal: medical treatment of canine epilepsy in Europe. BMC Veterinary Research. 2015;11:176. doi.org/10.1186/s12917-015-0464-z
- Dayrell-Hart B, Steinberg SA, VanWinkle TJ, Farnbach GC. Hepatotoxicity of phenobarbital in dogs: 18 cases (1985–1989). Journal of the American Veterinary Medical Association. 1991;199(8):1060–1066.
- Müller PB, et al. Effects of long-term phenobarbital treatment on the liver in dogs. Journal of Veterinary Internal Medicine. 2000;14(2):165–171. doi.org/10.1111/j.1939-1676.2000.tb01528.x
Walk in prepared
Bring the record your monitoring visit needs.
This guide recommends arriving at each monitoring appointment with your seizure diary and exact medication timing. The Pet Seizure Log gives you one organized place to track seizure frequency, dose times, and the changes your vet asks about — so bloodwork results can be read against a clear picture of how your pet is actually doing.
Explore the Pet Seizure Log →Help advance pet seizure research
Your tracking data could contribute to better outcomes for every pet with epilepsy.