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Laboratory:

Clinical Biochemistry (HSC ONLY - AUTO)

Test Name:

PHENYTOIN, TOTAL AND FREE (Alternate Name(s): Dilantin, Diphenylhydantoin) - (S)

Test Code: FPYN

Clinical Information:

Phenytoin has been used extensively for seizure control. About 90% of the total phenytoin in the blood is bound to proteins, primarily albumin. Only the unbound ("free") drug is pharmacologically active. The presence of other drugs and several clinical conditions (e.g. hypoalbuminemia, uremia, pregnancy, burns and liver failure) can affect the extent of protein binding, thus altering the free drug concentration without changing the total drug concentration.

Free phenytoin may be important to monitor in patients with altered or unpredictable protein binding capacity. Phenytoin is also subject to drug-drug interactions due to displacement of protein binding and extensive metabolism.

Collection Devices:

Preferred Device:	1 - Li Heparin, NO GEL 4.0 mL Tube(s) - Full Tube Collection Li Heparin, NO GEL 4.0 mL Tube(s) - Full Tube Collection Manufacturer: BD (Becton Dickinson) Catalogue #: 367884 - Gel tubes are not acceptable.
Alternate Device:	1 - Serum Tube(s), NO GEL - Full Tube Collection Serum Tube(s), NO GEL - Full Tube Collection Manufacturer: Becton Dickinson Catalogue #: 366430

Specimen Required:

Plasma: 1.5 mL

Pediatric Plasma: 1.5 mL

Collection Information: For oral therapy, collection time should be consistent for a given patient.
For IV therapy, collection must be at least 2 hours after end of dose.
Do not draw blood from an IV line through which the drug has been administered as it can produce erroneous results and impact patient care.
Centrifuge within 2 hours of collection.
Stability: 4 days refrigerated, 2 months frozen

Referral:

Shipping & Storage: Send refrigerated or frozen plasma to HSC-MS5 for testing.
IMPORTANT:
Attach original requisition
Address to HSC Auto Chem
Testing Laboratory (MB): HSC (Health Sciences Centre) – Biochemistry. All testing performed at HSC Auto Chem.

Requisition:

_SH Biochemistry Hematology Requisition (R250-10-92)

_SH Hospital Biochemistry Hematology Non Winnipeg Brandon (R250-10-91)

_SH Winnipeg Brandon Biochemistry Hematology Requisition (R250-10-94)

_SH Winnipeg Brandon Hospital Biochemistry Hematology Requisition (R250-10-93)

Reference Values:

Total Phenytoin
Therapeutic: 10-20 mg/L (40-80 µmol/L)
Toxic: >20 mg/L (>80 µmol/L)

Free Phenytoin
Therapeutic: 1.0-2.0 mg/L (4.0-8.0 µmol/L)
Toxic: ≥ 2.5 mg/L (>12 µmol/L)

Method of Analysis: Ultrafiltration & Kinetic Interaction Microparticles in Solution (KIMS) assay on Roche

Availability:

Daily