Lab Test

Phenytoin Level

Total Phenytoin, Dilantin, Phenytoin Level, Serum

Test Codes

Antrim #17143, EPIC: LAB5410, SOFT: PHENY

Specimen Collection Criteria

Collect: One plain Red-top tube. (Minimum Whole Blood: 1.0 mL)

Do not use Serum Separator Tubes.
See Minimum Pediatric Specimen Requirements for Microtainer® collection.
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 and refrigerate (2-8°C or 36-46°F) the collection tube within two hours of collection. (Minimum: 1.0 mL blood)

Preparation for Courier Transport

Transport: Collection tube, refrigerated (2-8°C or 36-46°F). (Minimum: 1.0 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:

Collection Tubes
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): Unacceptable

Serum Specimens (Pour-Overs)
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:

Refrigerated (2-8°C or 36-46°F): 7 days


Farmington Hills Chemistry Laboratory
Grosse Pointe Chemistry Laboratory
Royal Oak Toxicology Laboratory
Troy Chemistry Laboratory


Sunday – Saturday, 24 hours a day.
STAT results available within 1 hour of receipt in the Laboratory.
Routine results available within 24 hours.

Reference Range

Therapeutic Range
Patients 0-3 Years: 6-15 mcg/mL.
Patients Older than 3 Years:
10-20 mcg/mL.

Test Methodology

Enzyme Immunoassay.


Uremic specimens (serum creatinine greater than 12.0 mg/dL) may falsely elevate Phenytoin levels due to the presence of Phenytoin metabolites.

Clinical Utility

Phenytoin is an anticonvulsant drug. It is occasionally used as an antiarrhythmic. In the treatment of epilepsy, phenytoin is indicated for grand mal epilepsy, cortical focal seizures and temporal lobe epilepsy. Phenytoin has a narrow therapeutic index and a wide interindividual variability in the rate of metabolism and clearance necessitating the determination of blood levels for patients receiving therapy. This assay is used to monitor the therapeutic drug level and evaluate the toxicity of phenytoin.

CPT Codes



Last Updated


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This directory currently reflects information only for specimens collected and/or processed at the
Farmington Hills, Grosse Pointe, Royal Oak, and Troy campuses.