Seizure Medicine Levels in Blood

Test Overview

A medicine blood level test measures how much of your medicine is in your blood. Your doctor checks it to make sure that you are taking a safe and effective dose. This testing is also called therapeutic drug monitoring.

Why It Is Done

Seizure medicines are usually monitored because:

  • At a low blood level, they don't control seizures as well as they should.
  • At a high blood level, they may cause problems.

Your doctor may use your test results to adjust your dose.

You may need testing when you take a seizure medicine such as:

  • Carbamazepine.
  • Clozapine.
  • Levetiracetam.
  • Phenobarbital.
  • Phenytoin.
  • Valproic acid.

When these medicines are used to treat bipolar disorder, nerve pain, or migraine headaches, testing for low levels isn't as common as for seizures.

How To Prepare

Depending on which medicine you take, your doctor may schedule your test at a certain time of day. For example, your test may be done right before you take a dose of medicine.

Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will mean. To help you understand the importance of this test, fill out the medical test information form .

How It Is Done

The health professional who takes a sample of your blood will:

  • Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
  • Clean the needle site with alcohol.
  • Put the needle into the vein. More than one needle stick may be needed.
  • Attach a tube to the needle to fill it with blood.
  • Remove the band from your arm when enough blood is collected.
  • Put a gauze pad or cotton ball over the needle site as the needle is removed.
  • Put pressure on the site and then put on a bandage.

How It Feels

The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.


There is very little chance of a problem from having a blood sample taken from a vein.

  • You may get a small bruise at the site. You can lower the chance of bruising by keeping pressure on the site for several minutes.
  • In rare cases, the vein may become swollen after the blood sample is taken. This problem is called phlebitis. A warm compress can be used several times a day to treat this.


A medicine blood level test measures how much of your medicine is in your blood. Your doctor checks it to make sure that you're taking a safe and effective dose. This testing is also called therapeutic drug monitoring.

Your doctor or test results may mention a "therapeutic range" for your medicine. The range for "normal" varies from lab to lab. Your lab report should show what range your lab uses for "normal." Also, your doctor will evaluate your results based on your health and other factors. So a number that is outside the normal range may still be normal for you.

What Affects the Test

Your test results can be affected by:

  • The time of day and when your last dose was. Your peak level is soon after you've taken a dose. Your "trough" or low level is just before your next dose.
  • How well or poorly your body processes this medicine.
  • Other medicines, herbs, or supplements you take.

What To Think About

  • Keeping the right amount of medicine in your blood is important. So take your medicine exactly as prescribed.
  • Be sure to talk to your doctor if you have any questions about your medicine.


Current as of: December 9, 2019

Author: Healthwise Staff
Medical Review: Kathleen Romito, MD - Family Medicine
Adam Husney, MD - Family Medicine
Martin J. Gabica, MD - Family Medicine
E. Gregory Thompson, MD - Internal Medicine

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