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UW Health SMPH
American Family Children's Hospital

UW Hospital and Clinics Lab Test Directory

Test Name: Phenytoin, Free by HPLC


CPT Code(s): 80185, 80186

Test Description: Test Component:

Total phenytoin and free phenytoin

Methodology: Liquid Chromatography

Clinical Significance: Anticonvulsant

Days Performed: Mon-Fri, dayshift.

Turnaround Time: Stat: Not available stat.
Routine: 24 hours.

Specimen Requirements

Specimen: Blood

Collection Container: Preferred: Red top

Collection Volume: Preferred: 9 mL
Pediatrics: 3 mL

Sample Analyzed: Serum

Volume Required: Preferred: 2 mL
Pediatrics: 1 mL

Specimen Processing: Centrifuge. Transfer cell-free serum to plastic vial. Refrigerate.

Specimen Transport: Transport specimen to laboratory. Transport with coolant pack if coming from outreach location.

Unacceptable Criteria: Specimens collected in red cap with yellow ring (SST) tubes are not acceptable.

Stability: Ambient: 1 day
Refrigerated: 3 days
Frozen: 7 days


Interpretive Guidelines:

The free level of phenytoin is expected to be approximately 10% of the total level. Free levels greater than 3 mcg/mL usually produce toxicity.


Generally effective therapeutic concentration for total phenytoin:

18 years and up: 10 - 20 mcg/mL


Generally effective therapeutic concentration for free phenytoin:

18 years and up:  1 - 2 mcg/mL

Critical Calls: View

Additional Information:

For patients with significant renal dysfunction the UWHC Clinical Toxicology Laboratory recommends that the HPLC assay for phenytoin be ordered. Contact the UWHC Toxicology Laboratory at (608) 263-7029 for specific information regarding these assays.


Result of "BELOW" indicates that result is below the method detection limit of 0.5 mcg/mL. The total concentration of a drug in the blood includes the protein-bound fraction. Only the free drug is considered to be pharmacologically active. Due to the fact that phenytoin is highly bound to plasma proteins (89-95%), any alteration in the protein binding of phenytoin due to uremia, hypoalbuminemia, ingestion of other drugs, or age can result in a significantly different clinical response to a total phenytoin concentration. When a patient's clinical response does not agree with the total phenytoin concentration, or the protein binding of a patient is believed to be abnormal, the free phenytoin level may correlate more accurately than the total level with the clinical effectiveness or toxicity of the drug. In such cases, the determination of the free level of phenytoin is essential. Total phenytoin, free phenytoin, and % free phenytoin will be reported.


This test was developed and its performance characteristics determined by the UWHC Clinical Laboratories.  This test meets the standards for clinical testing.  This test should not be regarded as investigational or for research use.  The US Food and Drug Administration (FDA) has not approved or cleared this test; however, the FDA has determined that such clearance or approval is not necessary.

Free Phenytoin Dilantin

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