Lab Test

Phenytoin Level, Free (including Total)

Dilantin, Free (Unbound) Phenytoin, Serum

Test Codes

Antrim #17243, EPIC: LAB5411, SOFT: PHENF



Specimen Collection Criteria

Collect: One plain Red-top tube. (Minimum Whole Blood: 2.0 mL)

Do not use Serum Separator Tubes.
Time of Collection:
 Trough, just prior to the next dose.
Record the exact time of specimen collection on the tube or in the computer system.

Physician Office/Draw Specimen Preparation

Let specimens clot 30-60 minutes. Refrigerate (2-8°C or 36-46°F) the collection tube within two hours of collection. (Minimum: 2.0 mL blood)

Preparation for Courier Transport

Transport: Collection tube, refrigerated (2-8°C or 36-46°F). (Minimum: 2.0 mL blood)

Rejection Criteria

  • Serum Separator (SST) tubes.
  • Severely lipemic or hemolyzed specimens.

In-Lab Processing

Let specimens clot 30-60 minutes. Centrifuge specimen to separate serum from cells. Deliver an aliquot of serum or the collection tube immediately to the appropriate testing station.


Specimen Stability for Testing:

Room Temperature (20-26°C or 68-78.8°F): 24 hours
Refrigerated (2-8°C or 36-46°F): 7 days
Frozen (-20°C/-4°F or below): 3 months

Specimen Storage in Department Prior to Disposal:

Frozen (-20°C/-4°F or below): 2 months


Royal Oak Toxicology Laboratory


Sunday – Saturday, 24 hours a day.
STAT results are available within 2 hours of receipt in the Laboratory.
Routine results available within 24 hours. 

Reference Range

Therapeutic Range: 1.0-2.0 mcg/mL.
Critical Range: Greater than 3.0 mcg/mL.

Test Methodology

Total: Enzyme Immunoassay.
Free: Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS) after Ultra-Filtration. 

This test was developed and its performance characteristics determined by Beaumont Health. It has not been cleared or approved by the FDA. The Laboratory is regulated under CLIA as qualified to perform high-complexity testing. This test is used for clinical purposes and should not be regarded as investigational or for research.


This assay includes both Free and Total Phenytoin Levels.

Uremia, diminished serum albumin, and concurrent drugs that are highly bound to albumin will increase the free fraction of phenytoin, increase clearance and decrease total phenytoin levels. Unbound phenytoin concentrations will be unchanged and are the concentrations most highly correlated with therapeutic effect and toxicity.

Clinical Utility

Phenytoin is an anticonvulsant drug. Measurement of free drug concentrations can be a critical factor in assessing a patient's drug status. Free drug concentrations are considered most closely related to therapeutic effect. There are a variety of instances, such as renal failure, concentration-dependent nonlinearities, drug-drug interactions, and liver disease where measurement of total drug concentrations can be misleading and only the free drug concentration permits accurate assessment and dose adjustment.

CPT Codes

80186, 80185.
LOINC: 3969-3, 3968-5


Last Updated


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