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HelpOrder Code 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, SSpecimen Type
Serum RedSpecimen 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 |
|---|---|---|
| Serum Red | Refrigerated (preferred) | 7 days |
| Frozen | 14 days | |
| Ambient | 7 days |
Reject Due To
| Gross hemolysis | Reject |
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
Method Description
Free phenytoin is isolated from serum by ultrafiltration. The phenytoin assay is based on 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. Fresh serum subjected to ultra-filtration to generate a protein-free filtrate, which is then analyzed for free phenytoin by KIMS. (Package insert: Roche Phenytoin reagent, Roche Diagnostic Corp, Indianapolis, IN)
Day(s) Performed
Monday through Sunday
Performing Laboratory
Mayo Clinic Laboratories in Rochester
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.