Phenytoin, Total, Serum

Casandra

Casandra Test Code MC96764Version 1 (DRAFT)

Performing Lab

Lab Mayo Clinic LabLab Test ID PNYACPT Code 80185

Phenytoin, Total, Serum

Clinical Use

Use

Phenytoin is used to treat and prevent tonic-clonic and psychomotor seizures. It is important for monitoring therapeutic concentration, compliance, and toxicity. Therapeutic phenytoin levels can help in adjusting dosages to achieve appropriate control of seizures, particularly when the drug is coadministered with phenobarbital. Monitoring is essential due to phenytoin's zero-order kinetics and its potential for significant interactions with other drugs, such as valproic acid. Free phenytoin levels are crucial indicators, especially in patients with renal failure as it offers a more accurate measure of therapeutic efficacy and toxicity risk.

Special Instructions

Make sure to centrifuge and aliquot serum into a plastic vial within 2 hours of collection. Ensure to submit the forms as applicable, such as the Neurology Specialty Testing Client Test Request or the Therapeutics Test Request form.

Limitations

Phenytoin exhibits zero-order kinetics, meaning small dose increases can cause large increases in blood concentration, potentially leading to toxicity. Drug interactions, particularly with phenobarbital and valproic acid, can significantly affect phenytoin metabolism and effectiveness. Patients with renal failure may show altered free phenytoin levels, necessitating careful monitoring and dosage adjustments to avoid toxicity. Certain side effects like gingival hyperplasia and skin rash may occur even within therapeutic ranges, and phenytoin's metabolism can be altered by various conditions and treatments such as uremia.

Test Details

FDA Approved