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Medicines Monitoring

Phenytoin monitoring

Published Last updated
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. Required
    2. Consider
  3. Ongoing once stable
    1. Consider
  5. Abnormal results
    1. Leucopenia
    2. Hepatic function
    3. Vitamin D deficiency
  7. Notes
    1. Advice to patients
    2. Brand prescribing
    3. Bone fracture
  9. Bibliography
  10. Update history

Before starting

Required

  • Baseline
    • HLAB* 1502 alleleif Han Chinese or Thai origin patient

Han Chinese or Thai or other Asian patients

Screen for HLA-B*1502 allele genotype for individuals with Han Chinese, or Thai ancestry. These patients may be at increased risk of Stevens-Johnson syndrome when treated with phenytoin. Consider screening for other at-risk Asian populations such as Philippines, Malaysia and Vietnam.

Consider

  • Baseline
    • Full blood count
    • Liver function tests
    • Urea and electrolytes
    • Vitamin D

Ongoing once stable

Consider

  • Periodically
    • Urea and electrolytes
    • Vitamin D
    • Liver function testsparticularly in elderly or black patients, or patients with a history of liver disease.

Phenytoin plasma concentration

Routine monitoring of plasma phenytoin concentrations is not recommended. Consider monitoring when investigating:

  • adherence
  • unexplained loss of seizure control
  • suspected toxicity
  • a recent dose adjustment
  • a pharmacokinetic interaction
  • onset of specific clinical conditions (such as 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 to 20 mg/L (40 to 80 micromol/L).

There are some people who may be controlled with lower serum levels. For example, in pregnancy, the elderly and certain disease states where protein binding may be reduced, which may increase unbound phenytoin levels. It may be more appropriate to measure free plasma-phenytoin concentration for these people.

Abnormal results

Leucopenia

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

Hepatic function

Withdraw phenytoin immediately in cases of acute hepatotoxicity.

Effect of hepatic impairment on toxicity

Patients with hepatic impairment may be more susceptible to toxicity. Phenytoin is highly protein bound and when protein binding is reduced such as 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

Advice to patients

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

Blood or skin disorders

Seek immediate medical attention if symptoms develop such as:

  • fever
  • rash
  • mouth ulcers
  • bruising, or bleeding

Symptoms of phenytoin toxicity

Seek immediate medical attention if symptoms develop such as:

  • nystagmus
  • diplopia
  • slurred speech
  • ataxia
  • confusion
  • hyperglycaemia

Suicidal ideation and behaviours

Seek medical advice if symptoms develop such as:

  • mood changes
  • distressing or suicidal thoughts

Brand prescribing

Phenytoin is classified by the MHRA as a category 1 medicine. People treated for epilepsy should be maintained on a specific manufacturer’s preparation of phenytoin.

Bone fracture

Decreasing bone mineral density, osteopenia, osteoporosis and fractures may occur in patients on long-term therapy with phenytoin. Consider vitamin D supplementation for people with risk factors for these conditions who are on long-term phenytoin. Risk factors include:

  • immobility for long periods
  • inadequate sun exposure
  • inadequate dietary calcium intake

Bibliography

Update history

  1. Full review and update. HLAB* 1502 allele moved to required from consider if Han Chinese or Thai origin at baseline. Minor changes to structure.
  1. Published