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.
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Specimen Required
Patient Preparation
Timing of specimen collection: Pre-dose (trough) draw - At steady state concentration.  
Plain red.  
Specimen Preparation
Separate serum from cells 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.  
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Please indicate in the supplied fields:
1.  Dose - List drug amount and include the units of measure
2.  Route - List the route of administration (IV, oral, etc.)
3.  Dose Frequency - Indicate how often the dose is administered (per day, per week, as needed, etc.)
4.  Type of Draw - Indicate the type of blood draw (Peak, Trough, Random, etc.)  
After separation from cells: Ambient: 4 days; Refrigerated: 4 days; Frozen: 1 month  
Reference Interval
Effective November 16, 2015
Available Separately Components Therapeutic Range
No Phenytoin -​ Total Therapeutic: 10.0-​20.0
Toxic: > 20.0
No Phenytoin -​ Free Level Therapeutic: 1.0-​2.0
Toxic: > 2.0
No Phenytoin -​ Percent Free 8.0-​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: 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.
CPT Code(s)
80185; 80186
Component Test Code*Component Chart NameLOINC
0090125Phenytoin - Free Level3969-3
0090220Phenytoin - Percent Free10548-6
0090280Phenytoin - Total3968-5
2011597Phenytoin Dose4225-9
2011598Phenytoin Dose Frequency52810-9
2011599Phenytoin Route45373-8
2011600Phenytoin Type of Draw49049-0
* 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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