Laboratory: Clinical Biochemistry
Test Name:
Test Code: MISC
Clinical Information:
This test is used for monitoring both total and free phenytoin in patients with renal failure.
In uremia, by-products of metabolism accumulate and bind to albumin, displacing phenytoin, which causes an increase in the free (active) fraction. In renal failure, the ability to clear the free phenytoin fraction is also significantly reduced. Both the total and free concentration of phenytoin increase, with the free fraction rising faster than the total. Dosage must be adjusted to avoid toxicity. The free phenytoin level is therefore the best indicator of adequate therapy in renal failure.
Must be approved by a Clinical Chemist prior to sendout.
Click here for the Approval For Test Referral Biochemistry (F260-11-02)  
Send referral request form and a test requisition specifying Phenytoin, Total and Free to be sent to Mayo Laboratories. Do not use the phenytoin check box on the requisition.
Collection Devices:
Specimen Required:
Serum: 2.0 mL
Pediatric Serum: 1.0 mL

Centrifuge and aliquot within 2 hours of collection
Stability 7 days refrigerated, 14 days frozen
Serum: 2.0 mL

Store and ship frozen
Reference Values:
Phenytoin, Total
Therapeutic: 10 - 20 mg/L
Toxic:             > 20
Phenytoin, Free
Therapeutic:   1.0 - 2.0 mg/L
Toxic:              > 2.0
Within 1 Week
See Also:
More Information:
Store and ship serum refrigerated (preferred) or frozen to Mayo Medical Laboratories.
Serum Tube(s), NO GEL - Full Tube Collection
Manufacturer: Becton Dickinson

Catalogue #: 366430