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Test ID PNYF Phenytoin, Free, Serum

Useful For

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


Assessing compliance and toxicity

Method Name

Membrane Separation/Kinetic Interaction Microparticles in Solution (KIMS)

Reporting Name

Phenytoin, 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

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

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.

LOINC Code Information

Test ID Test Order Name Order LOINC Value
PNYF Phenytoin, Free, S 3969-3


Result ID Test Result Name Result LOINC Value
PNYF Phenytoin, Free, S 3969-3

NY State Approved


CPT Code Information



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