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UW Health SMPH
American Family Children's Hospital

UW Hospital and Clinics Lab Test Directory

Test Name: Phenytoin by HPLC

Test Code(s): LCPTN / HCLCPTN

CPT Code(s): 80185

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 Instructions: Draw specimen 18-24 hours after completion of PO loading dose, or 30 minutes prior to next dose.

With IV administration, draw specimen 2-4 hours after loading dose.

Collection Container: Preferred: Red top

Collection Volume: Preferred: 3 mL
Pediatrics: 1 mL

Sample Analyzed: Serum

Volume Required: Preferred: 1 mL
Pediatrics: 0.5 mL

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

Specimen Transport: Transport specimen to UWHC Core Laboratory (B4/220). 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:


Generally effective therapeutic concentration:

18 years and up: 10.0-20.0 mcg/mL



18 years and up: >25.0 mcg/mL

Critical Calls: View

Additional Information:

Result of BELOW indicates that level is below method detection limit of 0.5 mcg/mL. Peak serum concentrations can be seen 2-4 hours after ingestion. The half-life in an adult is 8-60 hours, and 7-29 hours in a child. Phenytoin clearance is generally considered non-linear (concentration dependent) within the therapeutic range and thus, half-life will vary between individuals and with concentration. If enzyme inducers are given simultaneously, the half-life can be reduced. Phenytoin is an enzyme inducer. Valproic acid may cause an increase in free (unbound) phenytoin concentration.


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.

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