Specimen Requirements

Container/Tube:

Preferred: Plain red-top tube, serum physically removed from cells as soon as possible

Acceptable: Gold-top serum gel tube or Green-top (heparin) tube, must be removed within 2 hours from gel separator, EDTA plasma, and Na-Heparin
Specimen Volume: A minimum volume of 1.2 µL plus a dead volume is required for testing.

Stability: Specimens should not be repeatedly frozen and thawed. Do not freeze Na- and Li-heparin plasma. Specimens are stable for 4 days at 2-8 ⁰C and 25 ⁰C or 1-2 month at -15 to -25 ⁰C.

Collection Instructions: Peak level should be drawn 2 to 6 hours after oral dosing or 3 to 9 hours post dose with sustained release formulations.

Note: 

1. Phenytoin time to steady state is after 4 to 24 days of chronic oral dosing.

2. If fosphenytoin is drug being given, specimen cannot be drawn until at least 2 hours after an intravenous dose or 4 hours after an intramuscular dose to allow enough time for fosphenytoin to be converted to phenytoin.

3. Levels may be ordered at any time toxicity is suspected.