Phenytoin, Free and Total
Also known as: FDIL
Use
This test is the preferred method for therapeutic drug management in patients with renal failure or conditions that may alter albumin concentrations. Free phenytoin monitoring is important for patients with altered or unpredictable protein binding capacity due to its high binding rate (over 90%) at therapeutic concentrations. It is also subject to drug-drug interactions because of displacement of protein binding and extensive metabolism. Calculating percent free attempts to minimize cross-reactivity differences and aids in dose optimization.
Special Instructions
Timing of specimen collection is crucial: a pre-dose (trough) draw at steady state concentration is recommended. Specimen storage temperatures and conditions must be strictly adhered to for accurate results.
Limitations
Cross-reactivity with metabolites may lead to differences in phenytoin concentrations among analytical methods. The test requires serum separated from cells as whole blood, citrated plasma, and serum separator tubes (SST) are unacceptable.
FDA ApprovedNew York Approved
Methodology
Immunoassay (Other)
Biomarkers
LOINC Codes
- 3969-3
- 10548-6
- 3968-5
Result Turnaround Time
1-4 days
Related Documents
For more information, please review the documents below
Specimen
Serum
Volume
2 mL
Minimum Volume
1 mL
Container
ARUP Standard Transport Tube
Collection Instructions
Separate from cells ASAP or within 2 hours of collection. Transfer to transport tube.
Patient Preparation
Pre-dose (trough) draw at steady state concentration.
Storage Instructions
Refrigerated.
Causes for Rejection
Whole blood, citrated plasma, SST, tubes containing liquid anticoagulant.
Stability Requirements
| Temperature | Period |
|---|---|
| Room Temperature | 4 days |
| Refrigerated | 4 days |
| Frozen | 1 month |