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UW Health SMPH
American Family Children's Hospital

UW Hospital and Clinics Lab Test Directory

Test Name: Phenytoin

Test Code(s): PTN / DIL

CPT Code(s): 80185

Methodology: Particle Enhanced Turbidometric Inhibition Immunoassay (PETINIA)

Clinical Significance: Anticonvulsant

Days Performed: Daily, 24 hours.

Turnaround Time: Stat: 1 hour.
Routine: 4 hours.


Specimen Requirements

Specimen: Blood

Collection Instructions: If drug is given PO, collect specimen 18 - 24 hours after loading dose or 30 minutes prior to next dose. If drug is given IV, collect specimen within 2 - 4 hours after loading dose.

Collection Container: Preferred: Red top
Also Acceptable: Green top (with lithium heparin anticoagulant)

Collection Volume: Preferred: 1 mL
Pediatrics: 0.4 mL

Sample Analyzed: Serum or plasma

Volume Required: Preferred: 0.5 mL
Pediatrics: 0.2 mL

Specimen Processing: Centrifuge.  Transfer cell-free serum or plasma 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) or green cap with yellow ring (PST) tubes are not acceptable.

Stability: Ambient: 2 hours on cells


Interpretation

Interpretation:

 

Generally effective therapeutic concentration: 10-20 mcg/mL

Critical Calls: View

Additional Information:

Result of "BELOW" indicates that level is below instrument detection limit of 0.4 mcg/mL. Peak serum concentrations can be seen 2-4 hours after ingestion. After oral administration the half-life is about 22 hours, although 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.

Diphenylhydantoin Dilantin

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