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
Specimen Required
Patient Preparation
Timing of specimen collection: Pre-dose (trough) draw - At steady state concentration.  
Plain red. Also acceptable (avoid if possible): Serum separator tube (follow preparation instructions below).  
Specimen Preparation
Separate serum from cells within 2 hours of collection. Transfer 2 mL serum to an ARUP Standard Transport Tube. (Min: 1 mL)
: Serum in a gel separator tube stored at room temperature is acceptable if separated from the gel within 2 hours. Serum in a gel separator tube stored refrigerated is acceptable if separated from the gel within 1 hour.  
Storage/Transport Temperature
Unacceptable Conditions
Citrated plasma. Tubes that contain liquid anticoagulant.  
After separation from cells: Ambient: 4 days; Refrigerated: 4 days; Frozen: 1 month  
Reference Interval
Effective February 19, 2013
Available Separately Components Therapeutic Range
No Phenytoin -​ Total Therapeutic: 10.0-​20.0 µg/mL
Toxic: > 30.0 µg/mL
No Phenytoin -​ Free Level Therapeutic: 1.0-​2.0 µg/mL
Toxic: > 3.0 µg/mL
No Phenytoin -​ Percent Free 8.0-​14.0%
Interpretive Data
Free phenytoin may be important to monitor in patients with altered or unpredictable protein binding capacity because phenytoin is highly bound (greater than 90%) 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.

See Compliance Statement B:  
CPT Code(s)
80185; 80186
Component Test Code*Component Chart Name
0090125Phenytoin - Free Level
0090220Phenytoin - Percent Free
0090280Phenytoin - Total
* 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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