Test Name: Phenytoin
Health Link Test Code: DIL
LIS Test Code: PTN
CPT Code(s): 80185
Methodology: Enzyme Immunoassay
Days Performed: Daily, 24 hours.
In-Lab Turnaround Time: 4 hours.
Stat In-Lab Turnaround Time: 1 hour.
Collection Container: Red top
Also Acceptable: Green top (with lithium heparin anticoagulant)
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 Volume: 1 mL
Pediatric Collection Volume: 0.4 mL
Sample Analyzed: Serum or plasma
Testing Volume: 0.5 mL
Pediatric Testing Volume: 0.2 mL
Centrifuge. Transfer cell-free serum or plasma to plastic vial. Refrigerate.
Transport specimen to the laboratory. Transport with coolant pack if coming from clinic location.
Specimens collected in red cap with yellow ring (SST) or green cap with yellow ring (PST) tubes are not acceptable.
Generally effective therapeutic concentration: 10.0-20.0 ug/mL
Critical Calls: yes
Result of "BELOW" indicates that level is below instrument detection limit of 1.8 ug/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.
Phenytoin results for patients with significant renal dysfunction may be falsely elevated due to interference from increased fractions of HPPH, a major phenytoin metabolite, in the patient sample.