Therapeutic drugs can significantly improve the quality of life in animals with painful or long-term illnesses. However, it’s critical to monitor the level of medication in an animal’s body to ensure the dosage is safe and effective. The bodily response to therapeutic drugs is particularly important to monitor in pets with chronic conditions, such as epilepsy. Too high of a dose could cause drug toxicity and a quantity that is too low may raise the risk of seizure recurrences. Veterinarians must communicate with dog and cat owners to find an optimal patient dose. A valuable technique to achieve an effective dosage is called Therapeutic Drug Monitoring (TDM.)
TDM in veterinary practice
TDM is a method used to locate a precise drug quantity that will optimize efficacy and minimize toxicity. This is accomplished by measuring drug levels in a patient’s blood at designated intervals. Drug concentrations are manipulated to remain within a particular therapeutic window which helps veterinarians target a suitable treatment for the animal.
Travis Mays, MS, Ph.D., is the section head for the Texas A&M Veterinary Medical Diagnostic Laboratory’s, TVMDL, analytical chemistry section. His section is charged with performing the tests that monitor drug levels.
“[TDM] is crucial for medications with narrow therapeutic indices where the difference between an effective dose and a toxic dose is small,” Mays said. “By monitoring these levels, we can adjust dosages to optimize efficacy and minimize toxicity.”
Observing changes in patient health status
Drugs are absorbed, processed and eliminated differently. These natures can change based upon age, general state of health, concurrent medications, liver function and genetic makeup.
Catherine Campbell, DVM, is TVMDL’s small animal veterinary diagnostician and regularly consults with small animal clients on test offerings and result interpretation.
“It is recommended to monitor levels when the patient has a change in clinical signs,” Campbell said. “For example, sedation or continued seizure activity, dosage adjustment, diet change or even the addition of other drugs that may affect the availability of that seizure medication to the animal.”
Attention to changes in the manufacturer of medications is also important.
Sample submission of common medications
Proper sample timing and submission is crucial for accurate test results. Mays notes timing as a recurring culprit for misinterpretation of drug levels. Common drugs monitored in veterinary practice include phenobarbital, bromide, Keppra and zonisamide. Each of these are primarily used to manage epilepsy.
For phenobarbital, levels are measured after 2-3 weeks of initiating therapy or changing doses.
Upon completion of this protocol, measurements are taken every 6-12 months if the patient is stable.
TVMDL recommends collecting blood samples two hours post-dose for peak measurement of the drug. A second sample should be collected shortly before the next dose, which is usually 12 hours after the first.
Because of bromide’s long half-life, levels are typically checked after 3 months. The timing of bromide collection is less critical, but consistency can help with interpretation of the test results.
For zonisamide, the levels might be checked after about two weeks of starting treatment and then periodically based on clinical response. When sampling zonisamide, TVMDL suggests recording a trough level.
As for Keppra, measurements are usually taken within one week since it is a drug that does not accumulate. Because Keppra is short lived, collecting a sample two hours post pill will locate the peak quantity in dogs. Regarding cats, samples are collected four hours after the first dose and shortly before the next.
Keep TDM in your toolbox
TDM is a tool used to provide insight into the dosage and rate at which treatment is administered to optimize patient health with minimal side effects. Overall, consistent communication with pet owners, timely follow-ups and attention to detail can drive substantial results in providing a better quality of life in furry companions with long-term or chronic health conditions. For more information on TVMDL’s test offerings, visit tvmdl.tamu.edu or call the College Station laboratory at 1.888.646.5623.
