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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 Micro-Collections using Microtainers 

Collect specimen 0-60 minutes prior to next oral dose or greater than 2 hours post IV dose.
Specimen Handling CLS Staff: send labelled empty aliquot tube to DSC along with collected PST tube (bind tubes together with an elastic band - DO NOT pour sample into the empty aliquot tube designated for FPTN).
Additional Information If Free Phenytoin is ordered, both Free and Total Phenytoin are performed at DSC.

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

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

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

Testing not performed stat. Approved turn around time is 4-10 hours including Phenytoin (PTN) result. If PTN is required stat, do not order as Free Phenytoin. Order PTN and FPTN on separate collections (note: if PTN and FPTN are ordered on same collection, PTN will be cancelled as duplicate request and will not be performed stat).


For additional information, see Half-life and Time to Steady State and Free Phenytoin Testing at CLS.
Testing Location Chemistry-DSC
Testing Frequency Daily
Alternate Name(s) Dilantin, Free; Free Dilantin; Free Phenytoin
Reference Interval Therapeutic range: 4.0-8.0 mmol/L

Click here for critical values.