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Test Code PNTFT Phenytoin, Total and Free, Serum

Important Note

  • Collect in Plain Red Tube
  • Send tubes to Specimen Processing STAT or Spin down with-in 2 hours of draw
  • Enter last dose date/time in computer order notes

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


Collection Container/Tube: Red top

Submission Container/Tube: Plastic vial

Specimen Volume: 2 mL

Collection Instructions: Centrifuge and aliquot serum into a plastic vial 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

Interpretation

Dose should be adjusted to achieve steady-state concentrations of total phenytoin between 10.0 and 20.0 mcg/mL, and free phenytoin between 1.0 and 2.0 mcg/mL. The range for percent free phenytoin is 8% to 14%. However, response and side effects will be individual.

 

In patients with renal failure, total phenytoin is likely to be less than the therapeutic range of 10.0 to 20.0 mcg/mL. Severe toxicity occurs when the total blood concentration exceeds 30.0 mcg/mL.

Clinical Reference

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

2. Moyer TP: Therapeutic drug monitoring. In Tietz Textbook of Clinical Chemistry. Fourth edition. Edited by CA Burtis, ER Ashwood. Philadelphia, WB Saunders Company, 2005, pp 1237-1285

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

CPT Code Information

Phenytoin, total-80185

Phenytoin, free-80186

Forms

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.