Therapeutic Drug Monitoring

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What is therapeutic drug monitoring?

Gloved hand holding vial of blood — Therapeutic drug monitoring (TDM) uses lab tests to check the level of medicine in your child’s body.

Therapeutic drug monitoring (TDM) uses lab tests to check the amount of medicine in your child’s body. This helps your care team know if the medicine is at a safe level, if it is working to treat the disease (within the therapeutic range), and if the medicine dose, timing, or type needs to be adjusted.

These lab tests are based on pharmacokinetics (also called PK), the study of how medicines move through the body. Pharmacokinetics uses blood samples to find out:

How the body takes up (absorbs) medicine
Where the medicine goes in the body
How the body breaks down (metabolizes) medicine
How the body gets rid of (clears) the medicine
How the medicine acts in the body

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 level is too low, the medicine will not work as well. If the level is too high, the medicine might be harmful.

Therapeutic drug levels depend on several factors. These include:

Medicine factors

Type of medicine
Dose and frequency
How the medicine is given
When the medicine level is checked after a dose is given

Patient factors

Age
Body size
How the body breaks down (metabolizes) medicines
Kidney and liver function
Other medicines the patient takes
Genes that affect the response to the medicine (also called pharmacogenomics)
Medical conditions and other health factors

How therapeutic drug monitoring works

Your care team will take a blood sample for testing in the lab.

Follow your health care provider’s instructions before the test. Make sure that your child:

Gets their test on the date and time scheduled
Takes their medicine as instructed before and after the test

The timing of the test depends on the kind of medicine. Many medicines must be checked before the medicine is given, also known as a trough or pre-dose level. But some levels are checked after your child takes their medicine, for example several hours after a dose is given. Some medicines have levels checked several times.

Your child’s test results will show the amount of medicine in their blood. Your care team will let you know what the results mean.

Therapeutic range is the safe and effective level for a medicine to work properly. This range is where the medicine works best for most patients without causing harm.
Peak level is when a medicine is at its highest level in the body. This is usually shortly after your child takes their medicine.
Trough level is when a medicine is at its lowest level in the body. This is usually just before it is time for the next dose.

Types of medicines that might be monitored

Medical centers vary on how and when they monitor therapeutic drug levels. Many medicines do not need drug levels checked because it is safe in a wide range of doses.

A drug may need monitoring if it:

Has a small therapeutic range where it works best
Must be a certain dose to treat the patient
Has more severe side effects
May interact with other medicines the patient takes

Examples of medicines that may be monitored

Type of Medicine	Examples
Antibiotics, antifungals, antivirals	Amikacin, gentamicin, tobramycin, posaconazole, voriconazole, vancomycin
Anticoagulant (stops blood clotting)	Enoxaparin, heparin, warfarin, rivaroxaban, apixaban
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)	Cyclosporine, mycophenolate, sirolimus, tacrolimus
Psychiatric (Brain)	Lithium

This list does not include all medicines that may be monitored. Always follow your care team’s instructions for therapeutic drug monitoring of any medicines your child takes.

How test results are used

Therapeutic drug monitoring helps your care team know that your child has the correct amount of medicine in their body. Depending on the results, your care provider may change the amount of medicine your child takes.

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 therapeutic drug level testing.
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.
If your doctor needs a trough (pre-dose) level of medicine, do not give the medicine before the drug level test. Usually, your child can take their medicine after the blood sample is taken and before you know the drug level result. But your care team may tell you to wait until the results come back.
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.

Questions to ask your care team

Does my child need therapeutic drug monitoring for this medicine?
How often does my child need these monitoring tests?
How will you know the medicine is working?
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

Therapeutic drug monitoring (TDM) checks the level of medicine in your child’s body.
The level of the drug can depend on your child’s size, their disease, other medicines they may take, and the dose and schedule of the medicine.
Drug levels show how much medicine is in your child’s body and how well they clear it.
Monitoring levels helps make sure the drug works well and is safe for your child.
Depending on the test results, your care team may change the dose of your child’s medicine.
Follow your care team’s instructions before the test. This helps make sure the results are accurate.