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Test Code PNYF Phenytoin, Free, Serum

Reporting Name

Phenytoin, Free, S

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

Performing Laboratory

Mayo Clinic Laboratories in Rochester

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

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.

CPT Code Information


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

Clinical Reference

Richens A: Clinical pharmacokinetics of phenytoin. Clin Pharmacokinet 1979;4:153-169 

Method Description

Fresh serum subjected to ultra-filtration to generate a protein-free filtrate, which is then analyzed for free phenytoin by the kinetic interaction of microparticles in a solution (KIMS). Phenytoin antibody is covalently coupled to microparticles and the drug derivative is linked to a macromolecule. The kinetic interaction of microparticles in solutions is induced by binding of drug-conjugate to the antibody on the microparticles and is inhibited by the presence of phenytoin in the sample. A competitive reaction takes place between the drug conjugate and phenytoin in the serum sample for binding to the phenytoin antibody on the microparticles. The resulting kinetic interaction of microparticles is indirectly proportional to the amount of drug present in the sample.(Package insert: Roche Phenytoin reagent, Roche Diagnostic Corp, Indianapolis, IN)

Report Available

Same day/1 day

Reject Due To

Gross hemolysis Reject

Method Name

Membrane Separation/Kinetic Interaction Microparticles in Solution (KIMS)


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