Phenytoin, Total and Phenobarbital Group, Serum

Casandra

Casandra Test Code MC36762Version 1 (DRAFT)

Performing Lab

Lab Mayo Clinic LabLab Test ID PNYGCPT Codes 80184, 80185

Phenytoin, Total and Phenobarbital Group, Serum

Clinical Use

Use

This test is useful for monitoring the therapeutic concentration of phenytoin and phenobarbital in the serum to ensure appropriate medication dosage, assess compliance, and detect potential drug toxicity. Phenytoin is commonly prescribed to treat tonic-clonic and psychomotor seizures, while phenobarbital acts as a CNS suppressant that aids in managing generalized and partial seizures. Both drugs are frequently coadministered to control complex seizure disorders. Monitoring these levels helps prevent toxicity associated with small increases in dosage, which can significantly affect blood concentrations.

Special Instructions

If not ordering electronically, a completed ‘Therapeutics Test Request’ form should accompany the specimen. This test is available for New York state clients and requires specific specimen handling such as immediate centrifugation and aliquoting of serum into plastic vials.

Limitations

The pharmacokinetics of phenytoin are affected by coadministered drugs such as phenobarbital and valproic acid, which can alter absorption, protein binding, and clearance rate. Phenytoin does not follow classical half-life kinetics as its clearance rate varies with drug concentration. Phenobarbital's effects also vary due to its enzyme-induction properties, necessitating careful dosage management to maintain therapeutic levels while avoiding toxicity. Usage may induce acute intermittent porphyria.

Test Details

FDA Approved

New York Approved

Methodology

Immunoassay (Kinetic Interaction of Microparticles in a Solution (KIMS))

Biomarkers

2 components

Phenytoin, Total, S • Kinetic Interaction of Microparticles in a Solution (KIMS)Phenobarbital, S • Kinetic Interaction of Microparticles in a Solution (KIMS)