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Phenytoin

Chemical Pathology

Notes

Phenytoin is a widely used anticonvulsant drug, being particularly useful due to its long half-life when once daily dosing is appropriate. This drug is metabolised by the cytochrome P450 system in the liver to an inactive metabolite para-hydroxyphenyl phenylhydantoin (HPPH) which is conjugated with glucuronic acid and excreted in the urine.

Phenytoin has a narrow therapeutic index and small changes in dose or in metabolic capacity e.g. due to intercurrent illness or hepatic impairment may cause large increases in plasma concentrations with acute toxic side-effects.

The main indications for monitoring are:

  • On initiating therapy
  • During intravenous therapy in status epilepticus
  • Unexpected deterioration in seizure control
  • As an adjunct in diagnosing toxicity
  • When interacting drugs are added or withdrawn
  • In pregnancy

Sample requirements

  • For oral therapy, due to the long elimination half-life of phenytoin the actual sampling time is unimportant. However, when making comparative measurements it is important that the sampling time is consistent e.g. pre-dose each time.
  • Time to steady-state varies but samples should usually be collected 7 - 10 days after each small dose change to assess trend until new steady state achieved.
  • For intravenous administration, samples should be collected 2-4 hours post dose

For adults, blood taken into a 5mL gold top gel tube (or rust top for the Acute Unit)

5ml gold tube

For children, blood taken into a 2mL lithuim heparin tube

2ml lithium heparin BD paediatric tube

For infants, blood taken into a 1mL plain minicollect tube (or 0.8mL minicollect lithium heparin tube)

1ml serum tube

Storage/transport

Send at ambient temperature to the laboratory on the same day the sample is collected.

Required information

Relevant clinical details including dose and time of sample relative to time of last dose.

Turnaround times

The assays are run throughout the day and night.

The in-lab turnaround time is less than 24 hours.

The test can be ordered as an urgent request.

Therapeutic range

5 - 20 mg/L

(Phenytoin is 90% protein bound principally to albumin, and hence in certain situations assay of a free phenytoin level may be more appropriate. Please contact the duty biochemist to discuss if required as this test is not locally available).

Further information

To learn more about phenytoin visit Lab Tests Online

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