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Test Code PTN Phenytoin (Dilantin) LAB31

Performing Laboratory

UnityPoint The Finley Hospital

Test Method

Turbidimetric Inhibition Immunoassay; Roche cobas Pro

Performance

Testing is performed Monday through Sunday.

Routine: Typically completed within 4 hours after the specimen arrives at the testing site.

STAT: Typically completed within 30 minutes after the specimen arrives at the testing site.

Specimen Requirement

  1. Lithium heparin plasma (green) is the specimen of choice.
  2. Serum (red) and EDTA plasma (purple) are also an acceptable sample types.
  3. Gel tubes are not acceptable. 
  4. It is recommended that the serum be separated from contact with cells within 2 hours from time of collection.
  5. Phenytoin specimens are stable for 4 days at 25°C, 4 days at 2-8°C, and 1-2 months at -20°C.
  6. Draw Times:

Suspected toxicity: Draw specimen when symptoms appear.

Peak levels: For “Dilantin” brand phenytoin, draw specimen at least 6 hours after last dose. For other brands of phenytoin, draw specimen 1-2 hours after last dose.

Trough levels: Draw specimen just prior to next dose.

Useful For

Phenytoin (diphenylhydantoin) has been used extensively for seizure control in patients having both grand mal epilepsy (major motor), cortical focal seizures, and temporal lobe epilepsy. Serum level monitoring of the drug is essential in order to achieve maximal seizure control while maintaining minimal blood levels. Because of individual variation in absorption and metabolism, optimum levels may vary.

Reference Values

10.0-20.0 mcg/mL

  • Toxic Level:  > 25 mcg/mL

CPT Code Information

80185 Therapeutic Drug Levels: Phenytoin

LOINC Code Information

3968-5 Phenytoin [Mass/Volume] In Serum Or Plasma