Phenytoin, Free, Serum
Use
The test is useful for monitoring the appropriate therapeutic concentration of free phenytoin, which is considered the best indicator of adequate therapy in patients with renal failure. It also helps in assessing patient compliance and detecting toxicity. Free phenytoin, being the active form of the drug, crosses biological membranes to bind to receptors, producing an enhanced pharmacologic effect. This test aids in providing accurate assessment of phenytoin activity, especially in patients co-administered with valproic acid.
Special Instructions
If not ordering electronically, complete and send a Therapeutics Test Request (T831) with the specimen.
Limitations
Phenytoin is highly protein-bound, and only the free, unbound form is active. Variations in free phenytoin levels can be caused by concurrent medication use, such as valproic acid, which may alter phenytoin levels via displacement from protein-binding sites and inhibition of metabolism. Small increases in dose could significantly elevate blood concentration, leading to toxicity. Therefore, it's critical to measure free phenytoin levels for therapy assessment, particularly in renal failure or when there's potential drug interaction.
FDA ApprovedNew York Approved
Methodology
Immunoassay (Kinetic Interaction Microparticles in Solution (KIMS))
Biomarkers
LOINC Codes
- 3969-3
- 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 tube; submit in plastic vial
Collection Instructions
Centrifuge and aliquot serum into 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 |