Ordering Recommendation

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

New York DOH Approval Status

This test is New York state approved.

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


Quantitative Enzyme Multiplied Immunoassay Technique




1-4 days

Reference Interval

Effective May 16, 2016

Available Separately
Therapeutic Range
No Phenytoin - Total Therapeutic: 10.0-20.0 µg/mL
Toxic: > 30.0 µg/mL
No Phenytoin - Free Level Therapeutic: 1.0-2.5 µg/mL
Toxic: > 2.5 µg/mL
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 Category



Hotline History


CPT Codes

80185; 80186


Component Test Code* Component Chart Name LOINC
0090125 Phenytoin - Free Level 3969-3
0090220 Phenytoin - Percent Free 10548-6
0090280 Phenytoin - Total 3968-5
* 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.


  • Dilantin, Free
  • Dilantin, Total and Free
  • Diphenylan
  • Diphenylhydantoin
  • Fosphenytoin
  • Free Dilantin
  • Free Phenytoin
  • Phenytek
  • pheyntoin blood concentration
  • Protein Free Phenytoin
Phenytoin, Free and Total