Clinical Use: Recommended sampling time:

  • 3.5 hours post dose (oral)
  • 2-4 hours post dose (IV)
Background: Acting on the motor cortex to block seizures and inhibit the spread of the seizure, phenytoin is the anti-epileptic drug of choice for the treatment of tonic-clonic (grand mal) and psychomotor seizures. Since the enzyme which hydroxylates phenytoin in the liver is saturatable then any small increase in dose can lead to toxicity if the enzyme is close to saturation. For this reason, and wide differences in absorption and protein binding between patients, it is important to measure serum phenytoin levels as part of a TDM programme. Measurements are also used in the diagnosis and treatment of phenytoin overdose.
Reference Ranges: Therapeutic levels: 10 – 20mg/L
Associated Diseases:
Patient Preparation:
Specimen Requirements: Sample type:

  • Serum and plasma (Lithium Heparin)

Sample identification:

  • Three patient identifiers from
    • Name
    • D.O.B.
    • Address
    • N.H.S. number
    • Unit Number

    should match on the specimen and request form. This check may be performed prior to centrifugation by the central sort department.

Turnaround Time: 2 hours
Additional Information: 3hrs post dose (oral)
2 – 4hrs post dose (IV)
Referred Test:
Location: