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HelpTest Code LAB32 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)
Specimen 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 | Special Container |
|---|---|---|---|
| Serum Red | Refrigerated (preferred) | 7 days | |
| Frozen | 14 days | ||
| Ambient | 7 days |
Reject Due To
| Gross hemolysis | Reject |
Reference Values
Therapeutic: 1.0-2.0 mcg/mL Critical value: ≥2.5 mcg/mLInterpretation
Dose should be adjusted to achieve steady-state blood concentration of free phenytoin between 1.0 and 2.0 mcg/mL. The range for percent free phenytoin is 8% to 14%.
Severe toxicity occurs when the free phenytoin concentration is ≥2.5 mcg/mL. However, response and side effects will be individual.
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)
Day(s) Performed
Monday through SundayReport Available
Same day/1 dayPerforming Laboratory
Mayo Clinic Laboratories in Rochester
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 |
CPT Code Information
80186
Forms
If not ordering electronically, complete, print, and send a Therapeutics Test Request (T831) with the specimen.