Phenytoin, Total and Free, Serum
Use
Phenytoin is used for the treatment and prevention of tonic-clonic and psychomotor seizures. Monitoring the concentration of both free and total phenytoin is crucial as it guides therapy adjustment due to its unique zero-order pharmacokinetics. Free phenytoin levels provide the best indication of therapeutic adequacy, especially in patients with renal failure where protein binding is affected, and both the total and free concentrations increase. Free concentrations rise more quickly, necessitating dosage adjustments to avoid toxicity.
Special Instructions
If not ordering electronically, complete, print, and send a Therapeutics Test Request form (T831) with the specimen.
Limitations
Due to the zero-order kinetics of phenytoin, small dose increases can lead to disproportionate increases in blood concentration, posing a risk of toxicity. Factors like uremia and coadministration with drugs such as phenobarbital or valproic acid can significantly alter phenytoin kinetics, affecting both protein binding and metabolism rates. Monitoring free phenytoin levels is crucial for accurate assessment to avoid toxicity symptoms such as nystagmus, ataxia, and tremor.
FDA ApprovedNew York Approved
Methodology
Immunoassay (KIMS)
Biomarkers
LOINC Codes
- 34540-5
- 3968-5
- 3969-3
Result Turnaround Time
1 day
Related Documents
For more information, please review the documents below
Specimen
Serum
Volume
2 mL
Minimum Volume
1 mL
Container
Red top; Submission in plastic vial
Collection Instructions
Centrifuge and aliquot serum into a plastic vial within 2 hours of collection.
Causes for Rejection
Gross hemolysis
Stability Requirements
| Temperature | Period |
|---|---|
| Room Temperature | 7 days |
| Refrigerated | 7 days |
| Frozen | 14 days |