Phenytoin, Free and Total
Ordering Recommendation

Preferred test for therapeutic drug management in patients with renal failure or conditions that may alter albumin concentrations.

Quantitative Enzyme Multiplied Immunoassay Technique
1-4 days
New York DOH Approval Status
This test is New York DOH approved.
Submit With Order
ARUP Consult®
Disease Topics
Specimen Required
Patient Preparation
Timing of specimen collection: Pre-dose (trough) draw - At steady state concentration. 
Plain Red. 
Specimen Preparation
Separate from cells ASAP or within 2 hours of collection. Transfer 2 mL serum to an ARUP Standard Transport Tube. (Min: 1 mL) 
Storage/Transport Temperature
Unacceptable Conditions
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 
Reference Interval
Effective May 16, 2016
Available Separately
Therapeutic Range
NoPhenytoin - TotalTherapeutic: 10.0-20.0 µg/mL
Toxic: > 30.0 µg/mL
NoPhenytoin - Free LevelTherapeutic: 1.0-2.5 µg/mL
Toxic: > 2.5 µg/mL
NoPhenytoin - Percent Free8.0-14.0%

Interpretive Data
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.

Compliance Statement B: For laboratory developed tests not using a RUO kit, and for FDA approved, cleared or 510(k) exempt assays with alterations. This test was developed and its performance characteristics determined by ARUP Laboratories. The U. S. Food and Drug Administration has not approved or cleared this test; however, FDA clearance or approval is not currently required for clinical use. The results are not intended to be used as the sole means for clinical diagnosis or patient management decisions.

Hotline History
View Hotline History
Component Test Code*Component Chart NameLOINC
0090125Phenytoin - Free Level3969-3
0090220Phenytoin - Percent Free10548-6
0090280Phenytoin - Total3968-5
* Component test codes cannot be used to order tests. The information provided here is not sufficient for interface builds; for a complete test mix, please click the sidebar link to access the Interface Map.
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