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Reaction Sequence
Phenobarbital (PHBR)
Phenobarbital is a commonly used drug used to treat seizures in a variety of species.
Phenobarbital levels should be evaluated during initial dosing and throughout treatment to ensure
that the blood levels are within the targeted therapeutic range.
Principal Reasons for Performing the Test
Phenobarbital is a controlled barbiturate medication that is used to treat veterinary patients that
have seizures. The dosage of phenobarbital needs to remain within a specific range to be effective.
If the level is <10 µg/mL, there may not be a sufficient level of phenobarbital to prevent seizures. If
the level >30 µg/mL in cats or >40 µg/mL in dogs, phenobarbital can be toxic and potentially life
threatening.
In most patients, steady state is achieved after 2–3 weeks of consistent dosing with phenobarbital.
Once steady state is achieved, timing of sample collection is not important in more than 90% of
patients. However, there can be variability of the phenobarbital half-life in a small percentage of
patients. Therefore, if toxicity is suspected, a peak sample (4–5 hours post-pill) may be helpful,
and if breakthrough seizures are occurring and inadequate dosing is suspected, a trough level
(collected immediately prior to the next dose) may be helpful.
Most Common Abnormalities Indicated by the Test
Over or under dosage of medication.
Sample Type and Precautions
Do not use separator tubes as contact with the gel may decrease levels.
Complementary Tests
CBC, full chemistry panel, urinalysis, bile acids (minimally 2 times per year)
Reaction Sequence
Potassium (K)
Potassium is the major cation of intracellular fluid, where it is the major buffer within the cell,
facilitates nerve conduction and muscle function, and helps maintain osmotic pressure. Abnormally
high or low potassium levels cause changes in muscle irritability, respiration, and myocardial
function.
Appendices
