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Therapeutic Drug Monitoring

What is therapeutic drug monitoring?

Therapeutic drug monitoring uses lab tests to check the amount of medicine in your child’s body. These tests help your child’s care team know if:

  • Their medicine is at a safe level
  • Their dose needs changing
  • The drug is working to fight their disease

Pharmacokinetics is an area of medicine that studies how drugs work in the body. Pharmacokinetic studies use patient samples to find out:

  • How the body takes up medicine
  • How the medicine acts
  • Where the medicine goes
  • How the body gets rid of (clears) the medicine

Therapeutic drug levels

Each medicine works best at a certain level in the body. This is called the therapeutic drug level or therapeutic range. If the medicine level is too low, it will not work well. If the medicine level is too high, it might be harmful.

The right level of medicine depends on several factors, such as:

  • Kind of medication
  • Disease or condition treated
  • Time the medicine level is checked

The correct dose of a medicine for a patient depends on factors such as:

  • Age
  • Size
  • How the patient’s body breaks down (metabolizes) medicines
  • Disease treated
  • The health of patient’s organs (kidneys and liver in particular)
  • Other medicines the patient takes
  • How a person’s genes might affect their response to the medicine

Speak to your care team about the therapeutic range for your child’s medicine.

How therapeutic drug monitoring works

Your care team will tell you if medicine levels need monitoring. Follow their instructions about how and when your child should take their medicine and what to do when it is time for their drug level test.

Your child’s care team will take a blood sample from your child. The drug level is measured in the lab. The peak level is when the medicine is at its highest level in the body. This is usually shortly after your child takes their medicine. The lowest level, or trough, is usually right before the time for their next dose.

The time for testing depends on the kind of medicine. Many medicines have a trough (pre-dose) level checked before your child takes their next dose. But some levels are checked 4 to 6 hours after your child takes their medicine. Still other medicines have levels checked several times to make sure the body is removing the drug well.

When getting a drug level test, it is important to follow your health care provider’s instructions. Make sure that your child:

  • Gets their test on the date and time instructed
  • Takes their medicine as instructed

Your child’s test results will come back as a drug level. This level shows the amount of medicine in their blood. The level needs to be within a therapeutic range for their type of medicine. This range is where the medicine has the best chance of working without causing harm in most patients. Depending on the results, your care provider may change the amount of medicine your child takes.

Types of medicines that might be monitored

Medical centers vary on how and when they monitor drug levels. Monitoring levels makes sure the drug stays at an acceptable level in the body, works well, and is safe. This amount may change, depending on a person and their health.

A drug may need monitoring if it:

  • Has a small therapeutic range where it works best
  • Needs to have a specific effect
  • Must be a certain dose to fit the patient
  • Has more severe side effects
  • May interact with other drugs the patient takes

Examples of monitored medicines

Type of Medicine Examples
Antibiotics, antifungals, antivirals Amikacin, gentamicin, meropenem, posaconazole, tobramycin, vancomycin, voriconazol
Anticoagulant (stops blood clotting)  Enoxaparin, heparin, warfarin 
Anti-seizure medicines  Carbamazepine, lacosamide, lamotrigine, levetiracetam, phenobarbital, phenytoin, topiramate, valproic acid, zonisamide
Anti-cancer medicines  Methotrexate, mitotane, topotecan
Heart medicines  Digoxin, digitoxin, lidocaine, procainamide
Bronchodilator (breathing)  Theophylline
Immunosuppressants (reduce immune response for autoimmune disease, transplant)  Azathioprine, cyclosporine, mycophenolate, sirolimus, tacrolimus 
Psychiatric (Brain)  Lithium

This list does not include all medicines that are monitored. Always follow the specific instructions of your health care provider for therapeutic drug monitoring of any medicines your child takes.

Many medicines do not need drug levels checked because:

  • The medicine works well.
  • It is safe in a wide range of doses.
  • The medicine is monitored in other ways.

How test results are used

Therapeutic drug monitoring helps your care team know that your child has the correct amount. When your child takes more medicine, or if they take the same amount more often, the drug level goes up. When they take less medicine, or take the same amount less often, the drug level goes down. Levels can also vary with how a patient’s body metabolizes and clears the drug, their health at that time, and other medicines they are taking.

Based on the test results, your health care provider might need to change your child’s dose.

Always follow your care team’s instructions on how to give and monitor medicines.

Tips for patients and families

  • Follow your care team’s instructions about how and when to take the medicine. This is very important on the day of drug level testing.
  • If your doctor needs a trough (pre-dose) level of medicine, do not let your child take their medicine before their drug level test. Usually, your child can take their medicine after their blood draw and before you know the drug level result. But your care team may tell you to wait until after your child’s test is done, and they know the result.
  • Give your child their medicine as close to their regular time as possible. Stay on schedule.
  • Write down the time(s) your child took their medicine before their drug level test.
  • Tell your care team about missed doses, including those taken by mouth that may have been thrown up (vomited).
  • Tell your care team about other medicines, vitamins, or supplements your child takes. These can affect how a drug works or is handled by the body.
  • Waiting for test results can be stressful. This is a common reaction. Try to do things that help you and your child not think about the test results, like getting enough physical activity or talking about your fears.

Questions to ask your care team

  • Does my child need drug monitoring while taking this medicine?
  • What monitoring tests does my child need?
  • How often does my child need these monitoring tests?
  • What instructions must I follow before my child has their levels of this medicine tested?
  • Are there any foods or medicines my child should avoid that affect the levels of this medicine?
  • Do my child’s other medicines or supplements affect this medicine’s levels or how it works?
  • When will I get the test results?

Key points about therapeutic drug monitoring

  • Some medicines must be monitored.
  • Therapeutic drug monitoring checks the level of medicine in your child’s body.
  • Drug levels show how much medicine is in your child’s body and how well they clear it.
  • The level of the drug depends on the patient’s body, the disease, and other things they may take.
  • Follow a regular medication schedule and your doctor’s instructions, especially before testing.

Reviewed: December 2023