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Phenytoin Test

The phenytoin test monitors blood levels of this anticonvulsant, as prescribed by your doctor. As part of comprehensive health monitoring, this test is essential due to phenytoin's narrow therapeutic index and nonlinear pharmacokinetics.

Doctor's Assessment Included

When is this value abnormal?

Decision limits per result, in mg/l
Result Value (mg/l)
Subtherapeutic < 8
Therapeutic 8–20
Above therapeutic range ≥ 20

Trough level (drawn before the next dose). Refers to total phenytoin. Toxic: > 20 mg/l.

Source: UMC Utrecht

Reference ranges may vary between laboratories. When you order a test, a BIG-registered doctor assesses your personal results in context. For treatment decisions, discuss your results with your GP.

Phenytoin: what this test measures

This test measures the blood concentration of the anticonvulsant phenytoin, as prescribed by your doctor. Due to its narrow therapeutic index, even small changes in dose or metabolism can lead to significant shifts in blood levels.

Phenytoin: why this value matters

Phenytoin has a narrow therapeutic index and nonlinear pharmacokinetics, meaning that small dose increases can cause disproportionately large rises in blood levels. Regular monitoring is essential to maintain effective seizure control while avoiding toxicity.

Phenytoin: how the blood test works

Referral
No GP referral needed. After the order we arrange the referral through ZorgDomein, within one business day; orders after 17:00 or at the weekend start on the next business day. ZorgDomein emails a barcode, and that barcode is what the draw site needs.
The draw
A blood draw from a vein in the arm, at a partner laboratory's draw site. No home kit, no finger prick.
Which test
on its own €49,-
Where
650+ draw sites run by independent laboratories across the Netherlands. Draw sites

Phenytoin: how to prepare

  1. Draw just before your next dose

    Required

    A drug level is measured just before your next dose. If you take it at a fixed time each day, have the blood drawn just before then. For lithium, at least 12 hours after the last dose.

  • Water is always fine. Drink as usual beforehand, it makes the draw easier.
  • Take your usual medication, unless your own doctor told you otherwise.

Phenytoin: when is testing worthwhile?

Testing is recommended when starting phenytoin, after dose changes, when adding or removing interacting medications, or if seizures recur or toxicity symptoms appear. Your healthcare provider will determine the appropriate monitoring schedule.

Phenytoin: symptoms of a high or low value

Low Levels

Low phenytoin levels may indicate subtherapeutic dosing, which can be associated with an increased risk of breakthrough seizures. Consult your healthcare professional for interpretation of results.

High Levels

High phenytoin levels may indicate toxicity, which can be associated with nystagmus (involuntary eye movements), ataxia (unsteady gait), slurred speech, confusion, and in severe cases, cardiac complications. Seek immediate medical attention if you experience these symptoms.

Phenytoin: lifestyle and this value

Take phenytoin exactly as prescribed by your doctor and do not adjust your dose without medical guidance. Be aware that many medications, foods, and supplements can interact with phenytoin. Attend all scheduled monitoring appointments.

Phenytoin: frequently asked questions

Why is phenytoin monitoring so important?

Phenytoin has nonlinear pharmacokinetics, meaning small dose changes can cause large, unpredictable changes in blood levels. This makes regular monitoring essential to maintain levels within the therapeutic range.

Can food or supplements affect phenytoin levels?

Yes, certain foods, enteral nutrition, calcium supplements, and antacids can interfere with phenytoin absorption. Discuss any dietary changes or new supplements with your healthcare provider.

What is the difference between total and free phenytoin?

Total phenytoin measures both protein-bound and unbound drug, while free phenytoin measures only the active unbound portion. Your doctor may order free phenytoin levels if you have low albumin or kidney disease.

Phenytoin: sources

  1. UMC Utrecht, Divisie Laboratoria en Apotheek. Bepalingenwijzer: therapeutische geneesmiddelspiegels (amitriptyline, nortriptyline, carbamazepine, valproïnezuur, fenytoïne, digoxine, lamotrigine). Geraadpleegd 14 juli 2026. (retrieved 2026) umcutrecht.nl

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Medical standards

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Medical policy
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Phenytoin

€49,-