Specimen Collection and Handling

Due to phenytoin’s long elimination half-life, the actual sampling time is unimportant. However, when making comparative measurements, it is important that the sampling time be consistent.

When using FOSPHENYTOIN, it is recommended that PHENYTOIN
concentrations not be monitored until conversion of FOSPHENYTOIN to PHENYTOIN is essentially complete.  This occurs within approximately 2 hours after the end of IV infusion and 4 hours after IM injection. Serum PHENYTOIN concentrations measured before this conversion is complete will not be accurate.
 Total phenytoin levels should be drawn as a trough (2 hours prior to the next scheduled dose) after 3-4 days of therapy to ensure steady state is achieved, unless there is concern for toxicity in which case levels can be checked anytime.

To Process: Spin specimen, separate from clot and send refrigerated.