Preferred test for therapeutic drug management in patients with renal failure or conditions that may alter albumin concentrations.
Quantitative Enzyme Multiplied Immunoassay Technique
Timing of specimen collection: Pre-dose (trough) draw - At steady state concentration.
Separate from cells ASAP or within 2 hours of collection. Transfer 2 mL serum to an ARUP Standard Transport Tube. (Min: 1 mL)
Whole blood. Citrated plasma. Serum separator tubes (SST). Tubes that contain liquid anticoagulant.
After separation from cells: Ambient: 4 days; Refrigerated: 4 days; Frozen: 1 month
Effective May 16, 2016
|No||Phenytoin - Total||Therapeutic: 10.0-20.0 µg/mL
Toxic: > 30.0 µg/mL
|No||Phenytoin - Free Level||Therapeutic: 1.0-2.5 µg/mL
Toxic: > 2.5 µg/mL
|No||Phenytoin - Percent Free||8.0-14.0%|
The therapeutic range is based on serum pre-dose (trough) draw at steady-state concentration. Free phenytoin may be important to monitor in patients with altered or unpredictable protein binding capacity because phenytoin is highly bound (greater than 90 percent) at therapeutic concentrations. Phenytoin is also subject to drug-drug interactions due to displacement of protein binding and extensive metabolism. Cross-reactivity with metabolites may account for differences in phenytoin concentrations among analytical methods. Calculating percent free attempts to minimize differences in assay cross-reactivity and may be useful in dose optimization.
|Component Test Code*||Component Chart Name||LOINC|
|0090125||Phenytoin - Free Level||3969-3|
|0090220||Phenytoin - Percent Free||10548-6|
|0090280||Phenytoin - Total||3968-5|
- Dilantin, Free
- Dilantin, Total and Free
- Free Dilantin
- Free Phenytoin
- pheyntoin blood concentration
- Protein Free Phenytoin