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:
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 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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