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Medicines Monitoring

Phenytoin monitoring

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Topics: MonitoringPhenytoin
Using this page · Individualise medicines monitoring

This medicines monitoring page has been written using publications and expert opinion. It is designed to save clinician time, but not replace professional responsibility. When using this page you should: ensure an individualised monitoring plan is developed in partnership with the patient and take account of any locally agreed advice and guidance.

Contents
  1. Before starting
    1. Consider
  2. Ongoing once stable
    1. Required
    2. Consider
  3. Abnormal results
    1. Leucopenia
    2. Effect of hepatic impairment on toxicity
    3. Vitamin D deficiency
  4. Notes
    1. Phenytoin plasma concentration monitoring
    2. Advice to patients
    3. Maintaining patients on appropriate preparations
    4. Effects of hepatic impairment on toxicity
  5. Bibliography

Before starting

Consider

  • Baseline
    • Full blood count
    • HLAB* 1502 allelein Han Chinese or Thai origin patients
    • Liver function tests
    • Urea and electrolytes
    • Vitamin D

Han Chinese or Thai patients

Avoid phenytoin unless essential in Han Chinese or Thai patients who possess the HLAB* 1502 allele, since these patients may be at an increased risk of Stevens-Johnson syndrome.

Ongoing once stable

Required

  • Every 2-5 years
    • Bone metabolism e.g. calcium or ALP
    • Full blood count
    • Liver function tests
    • Urea and electrolytes
    • Vitamin D

Consider

  • 6 monthly
    • Bone metabolism e.g. calcium or ALP
  • Periodically
    • Plasma phenytoin concentration

Abnormal results

Leucopenia

Withdraw phenytoin where severe, progressive, or associated with clinical symptoms.
Use suitable alternative if necessary.

Effect of hepatic impairment on toxicity

Patients with hepatic impairment may be more susceptible to toxicity. This is because phenytoin is highly protein bound and when protein binding is reduced, e.g. in hypoalbuminemia, there will be an increase in unbound phenytoin levels.

Vitamin D deficiency

Phenytoin is thought to affect bone mineral metabolism which may lead to vitamin D deficiency, hypocalcaemia, and hypophosphatemia in chronically treated epileptic patients.

Notes

Phenytoin plasma concentration monitoring

When to consider

Drug level monitoring in patients with epilepsy is not normally necessary.
However, it may be necessary when assessing or investigating:

  • adherence
  • unexplained loss of seizure control
  • suspected toxicity
  • a recent dose adjustment
  • a pharmacokinetic interaction
  • onset of specific clinical conditions (e.g. pregnancy, organ failure, status epilepticus)

Ranges and interpretation

Where drug level monitoring is felt to be necessary, dosage should be adjusted according to serum levels where assay facilities exist.
Generally, the therapeutic phenytoin serum level is 10–20µg/ml (or 40–80 micromol/L). However, there are some cases which may be controlled with lower serum levels e.g.:

  • some tonic clonic seizures
  • elderly patients or those with hepatic impairment may achieve therapeutic control with drug levels below the normal range. (This is because phenytoin is highly protein bound and when protein binding is reduced, e.g. in hypoalbuminemia, there will be an increase in unbound phenytoin levels.)

Advice to patients

Advise patients and their carers to be aware of signs of:

  • Blood or skin disorders; they should seek immediate medical attention if symptoms such as fever, rash, mouth ulcers, bruising, or bleeding develop.
  • Symptoms of phenytoin toxicity; they should seek immediate medical attention if symptoms such as nystagmus, diplopia, slurred speech, ataxia, confusion, or hyperglycaemia develop.

Maintaining patients on appropriate preparations

Doctors and pharmacists should maintain patients on a specific manufacturer’s preparation of phenytoin. This ensures the MHRA advice is followed. Be aware that preparations containing phenytoin sodium are not bioequivalent to those containing phenytoin base.

Effects of hepatic impairment on toxicity

Patients with hepatic impairment may be more susceptible to toxicity. This is because phenytoin is highly protein bound, and when protein binding is reduced, e.g. in hypoalbuminaemia, there will be an increase in unbound phenytoin levels.

Bibliography

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