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 - Serum Tube(s), NO GEL - Full Tube Collection  - Gel tubes are not acceptable.
Specimen Required:
Serum: 1.5 mL
Pediatric Serum: 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.
Special Processing:  Allow serum specimen to clot completely at room temperature to ensure complete clot formation has taken place prior to centrifugation. Centrifuge and aliquot within 2 hours of collection.
Stability: 4 days refrigerated, 2 months frozen

Shipping & Storage: Send refrigerated or frozen serum to HSC-MS5 for testing.
Attach original requisition
Address to HSC Auto Chem
Testing Laboratory (MB): HSC (Health Sciences Centre) – Biochemistry. All testing performed at HSC Auto Chem.
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
See Also:
More Information:
Interpretation & Assay Interferences: 
Half-life: 8-60 hours (dose dependent)
Time to steady state: 2-13 days