Phenytoin, Free and Total
Dilantin, Free (Unbound) Phenytoin, Serum
Test Codes
EPIC: LAB176, SOFT: PHENF, Antrim #17243
Department
Toxicology
Specimen Collection Criteria
Collect: One plain Red-top tube. (Minimum Whole Blood: 2.0 mL)
Do not use Serum Separator Tubes.
Time of Collection: Trough, just prior to the next dose.
Record the exact time of specimen collection on the tube or in the computer system.
Physician Office/Draw Specimen Preparation
Let specimens clot 30-60 minutes. Refrigerate (2-8°C or 36-46°F) the collection tube within two hours of collection. (Minimum: 2.0 mL blood)
Preparation for Courier Transport
Transport: Collection tube, refrigerated (2-8°C or 36-46°F). (Minimum: 2.0 mL blood)
Rejection Criteria
- Serum Separator (SST) tubes.
- Severely lipemic or hemolyzed specimens.
In-Lab Processing
Let specimens clot 30-60 minutes. Centrifuge specimen to separate serum from cells. Deliver an aliquot of serum or the collection tube immediately to the appropriate testing station.
Storage
Specimen Stability for Testing:
Room Temperature (20-26°C or 68-78.8°F): 24 hours
Refrigerated (2-8°C or 36-46°F): 7 days
Frozen (-20°C/-4°F or below): 3 months
Specimen Storage in Department Prior to Disposal:
Frozen (-20°C/-4°F or below): 2 months
Laboratory
Royal Oak Toxicology Laboratory
Performed
Sunday – Saturday, 24 hours a day.
STAT results are available within 2 hours of receipt in the Laboratory.
Routine results available within 24 hours.
Reference Range
Therapeutic Range: 1.0-2.0 mcg/mL.
Critical Range: Greater than 3.0 mcg/mL.
Test Methodology
Total: Enzyme Immunoassay.
Free: Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS) after Ultra-Filtration.
This test was developed, and its performance characteristics determined by Beaumont Health. It has not been cleared or approved by the FDA. The Laboratory is regulated under CLIA as qualified to perform high-complexity testing. This test is used for clinical purposes and should not be regarded as investigational or for research.
Interpretation
This assay includes both Free and Total Phenytoin Levels.
Uremia, diminished serum albumin, and concurrent drugs that are highly bound to albumin will increase the free fraction of phenytoin, increase clearance and decrease total phenytoin levels. Unbound phenytoin concentrations will be unchanged and are the concentrations most highly correlated with therapeutic effect and toxicity.
Clinical Utility
Phenytoin is an anticonvulsant drug. Measurement of free drug concentrations can be a critical factor in assessing a patient's drug status. Free drug concentrations are considered most closely related to therapeutic effect. There are a variety of instances, such as renal failure, concentration-dependent nonlinearities, drug-drug interactions, and liver disease where measurement of total drug concentrations can be misleading and only the free drug concentration permits accurate assessment and dose adjustment.
CPT Codes
80186, 80185.
LOINC: 3969-3, 3968-5
Contacts
Toxicology Laboratory – RO
248-551-8058
Name: Toxicology Laboratory – RO
Location:
Phone: 248-551-8058
Last Updated
11/11/2024
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