Phenytoin blood level
Quick answer: This test measures the amount of phenytoin, a medicine for epilepsy, in your blood. It helps doctors check the dose, and look into side effects such as unsteadiness, or seizures that are not controlled.
- SampleBlood
- TakesAbout 5 minutes
- Before the testYour doctor will tell you when to give the sample; it is often taken just before your next dose.
- In fertility careIf you take this medicine and are planning a pregnancy, talk to your doctor before trying, as it can affect pregnancy planning.
How the test is done
- 1You keep taking phenytoin as prescribed before the test
- 2Blood is drawn, usually just before the next dose
- 3You tell the staff the time of your last dose and other medicines
- 4The lab measures total, and sometimes free, phenytoin
What your results can mean
Reading your report
Results are read with your dose, symptoms and blood albumin; do not change your dose without talking to your doctor.
If the result is outside the usual range
A high level can cause unsteadiness, slurred speech, drowsiness and jerky eye movements. A low level can be seen with missed doses or interacting medicines and can make seizures more likely.
Who usually has this test
People taking phenytoin, especially after dose changes, with side effects or seizures, in pregnancy, or with low blood albumin.
Risks and aftercare
Bruising is possible; continue your medicine as prescribed.
Names and options
Also called: Dilantin level, Eptoin level, phenytoin test
Listed by the hospital as
- PHENYTOIN, SERUM
Questions about this test
What is a free phenytoin level?
Most phenytoin in the blood is bound to protein, and the free part is the active part. It is measured when albumin is low, such as in pregnancy or kidney or liver disease.
Why do small dose changes affect phenytoin levels so much?
The body can break down phenytoin only at a limited rate. Once this limit is reached, a small dose rise can cause a large rise in the level.
Does phenytoin affect pregnancy planning?
Phenytoin can carry some risks in pregnancy, and its levels often change. Talk to your neurologist before trying to conceive, and do not stop it on your own.
Related tests
Tamara Hospital & IVF Centre, Rajajinagar
34/3, 10th Cross Road, opp. Vardhaka Sardar Patel School, 1st N Block, 2nd Stage, Rajajinagar, Bengaluru 560010
The hospital is open 24 hours. Call before you come to check the timing for your scan or sample collection.
Get directions ↗IVF Pulse fertility care at Tamara Hospital →
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This page explains each test in general terms so you know what to expect. It is not medical advice: your doctor decides which tests you need and what your results mean. Reference ranges differ between laboratories. Updated 4 November 2026.