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Phenytoin, Free

Source Blood
Mnemonic FPTN
Specimen Requirements

1 x 3 mL light green (mint) top PST tube. Gold top SST tube also acceptable.

Minimum 1.5 mL serum required.

Minimum Whole Blood Volumes for Microcollections

Collect specimen 0-60 minutes prior to next oral dose or greater than 2 hours post IV dose.

Specimen Handling CLS Staff: Send .FPTN label with specimen.
Additional Information

If Free Phenytoin is ordered, both Free and Total Phenytoin are performed at FMC.

Add on Free Phenytoin WILL NOT be performed if Phenytoin is already reported. Recollection for Free Phenytoin is required.
Add on Free Phenytoin WILL be performed if Phenytoin has not been reported and if minimum sample volume is available for testing. Order as FPTN and send to FMC Chemistry.

Free Phenytoin is not performed when the total phenytoin concentration is <10.0 or >80.0 umol/L.

Free Phenytoin is not performed if a previous result is available within six days of current request.

For additional information, see Half-life and Time to Steady State and Free Phenytoin Testing at CLS.

Testing Location Chemistry-FMC
Testing Frequency Daily
Alternate Name(s) Dilantin, Free; Free Dilantin; Free Phenytoin
Reference Interval

Therapeutic range: 4.0 - 8.0 umol/L

Click here for critical values.