What is Therapeutic Drug Monitoring (TDM)

Therapeutic drug monitoring is regular blood testing to make sure the drug (medicine) that you are taking is helping you and not harming you. Medicines work differently for each person depending on age and other health conditions. TDM helps your health care provider:

• Make sure that you have the right amount of medicine in your body.
• Make sure that there is a safe amount of medicine in your body as your body changes, like if you become pregnant or ill.
• Find out if the medicine you take is causing harm.
• Make sure that you are taking the medicine as prescribed.
• Find out if he or she needs to:
  • Adjust your dosage.
  • Switch you to a different medicine.

You may need TDM if you are taking medicines for a long time or for the rest of your life. Medicines that often require TDM include:

• Heart medicine.
• Some antibiotic medicines.
• Anti-seizure medicines (anti-epileptics).
• Medicine to treat certain lung conditions (bronchodilators).
• Medicine to reduce the activity of the body’s disease-fighting system (immunosuppressants).
• Anticancer medicine.
• Psychiatric medicine.

Each medicine that requires TDM has its own testing schedule. You may need to begin TDM soon after you start taking a medicine. After that, you might need to have follow-up tests at regular intervals, such as every few months.

What is being tested?

TDM measures the amount of medicine in your blood to make sure that it is within your therapeutic range. The therapeutic range is the amount of a medicine that is enough to help you but not harm you. All medicines have a therapeutic range.

What kind of sample is taken?

A blood sample is required for this test. It is usually collected by inserting a needle into a blood vessel.

How do I prepare for this test?

• Follow instructions from your health care provider about:
  • Getting your blood sample taken on a specific day or time. The timing is very important because the amount of the medicine in your blood changes over time.
  • When to take your medicine before the test. This allows the test to measure the amount of medicine in your blood at the right time.
• If you forget or cannot take your medicine as instructed, contact your health care provider. You may need to reschedule the test.

Tell a health care provider about:

• Any allergies you have.
• All medicines you are taking, including vitamins, herbs, eye drops, creams, and over-the-counter medicines.
• Any blood disorders you have.
• Any surgeries you have had.
• Any medical conditions you have.
• Whether you are pregnant or may be pregnant.

How are the results reported?

Your test results will be reported as a value that indicates the amount of medicine in your blood. Your health care provider will compare your results to the therapeutic range for your medicine. Therapeutic ranges vary among medicines.

What do the results mean?

If your results are within your therapeutic range, this means that there is a safe amount of medicine in your body.

If your results are above or below your therapeutic range, there is too much or too little medicine in your body. Your health care provider may change your dosage and have you come back for repeat TDM testing.

Talk with your health care provider about what your results mean and how often you need TDM tests done.

Questions to ask your health care provider

Ask your health care provider, or the department that is doing the test:

• How often do I need TDM?
• When will my results be ready?
• How will I get my results?
• What are my treatment options?
• What other tests do I need?

Summary

• Therapeutic drug monitoring (TDM) is regular blood testing to make sure the drug (medicine) that you are taking is helping you and not harming you.
• The timing of your blood test is very important because the amount of the medicine in your blood changes over time.
• If you forget or cannot take your medicine as instructed, contact your health care provider. You may need to reschedule the test.