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Test ID PNTFT Phenytoin, Total and Free, Serum

Useful For

Monitoring for appropriate therapeutic concentration of both free and total phenytoin: free phenytoin level is the best indicator of adequate therapy in renal failure

Profile Information

Test ID Reporting Name Available Separately Always Performed
PNYF Phenytoin, Free, S Yes Yes
PNYA Phenytoin, Total, S Yes Yes

Method Name

PNYF: Membrane Separation/KIMS

PNYA: Kinetic interaction of Microparticles in a Solution (KIMS)

Reporting Name

Phenytoin, Total and Free, S

Specimen Type

Serum Red

Specimen Required

Container/Tube: Red top

Specimen Volume: 2 mL

Collection Instructions: Red-top tubes should be centrifuged and aliquoted within 2 hours of collection.

Specimen Minimum Volume

1 mL

Specimen Stability Information

Specimen Type Temperature Time Special Container
Serum Red Refrigerated (preferred) 7 days
  Frozen  14 days
  Ambient  7 days

Reference Values

Phenytoin, Total

Therapeutic: 10.0-20.0 mcg/mL

Critical value: ≥30.0 mcg/mL


Phenytoin, Free

Therapeutic: 1.0-2.0 mcg/mL

Critical value: ≥2.5 mcg/mL

Day(s) Performed

Monday through Sunday

Report Available

Same day/1 day

Performing Laboratory

Mayo Clinic Laboratories in Rochester

LOINC Code Information

Test ID Test Order Name Order LOINC Value
PNTFT Phenytoin, Total and Free, S 34540-5


Result ID Test Result Name Result LOINC Value
PNYF Phenytoin, Free, S 3969-3
PNYA Phenytoin, Total, S 3968-5

NY State Approved


CPT Code Information

Phenytoin, total-80185

Phenytoin, free-80186


If not ordering electronically, complete, print, and send a Therapeutics Test Request (T831) with the specimen.

Test Classification

This test has been cleared, approved, or is exempt by the US Food and Drug Administration and is used per manufacturer's instructions. Performance characteristics were verified by Mayo Clinic in a manner consistent with CLIA requirements.