Therapeutic Drug Monitoring

During treatment for childhood cancer, patients may have tests to check the level of a medicine in the body. This is known as therapeutic drug monitoring. These tests are important to make sure that drug levels are in the right range. Results can also help personalize the amount of medicine the patient receives.

Drug levels aren’t checked for all medicines, but some medicines that are given as part of cancer treatment will need levels checked. The medical specialty area that studies the activity of drugs in the body is called pharmacokinetics.

What is a drug level?

A drug level, also called a drug concentration, is the amount of medicine in the body. When more medicine is given or given more often, the drug level goes higher. When less medicine is given or given less often, the drug level goes lower. 

The drug level in the body is usually highest, known as the peak level, right after the medicine is given. The lowest level, or trough, is usually right before it’s time for the next dose.

How are drug levels checked?

Most drug levels are checked with a blood test.

Why is checking drug levels important?

Each medicine works best at a certain level in the body. If the drug level is too low, it will not be effective. If the drug level is too high, it might be harmful. The amount of medicine in the body is a balance between the amount that is given and the amount that the body gets rid of in a given period of time.

The correct dose of a medicine for a patient depends on different factors including age, size, genetics, and the disease being treated. Patients can also differ in how they break down, or metabolize, medicines. Health conditions and other medicines can also affect the activity of a drug in the body.

Checking drug levels help your care team make sure that the amount of medicine given is just right for each patient. Based on the test results, medicine can be adjusted if needed.

After a medicine is given, the body works to process and clear the medicine from the body. Measuring a drug level lets the care team know how much medicine is in the body at a given time and how well the body is working to clear the medicine.

What are some drugs that have levels checked?

Medical centers may differ in when and how certain drug levels are monitored. However, some medicines that may be checked include busulfan, cyclosporine, enoxaparin, methotrexate, tacrolimus, vancomycin, voriconazole, and certain anti-seizure medicines.

Why don’t all medicines need drug levels checked?

Many medicines don’t need drug levels checked. Some of the reasons include:

1. The medicine works well and is safe at a wide range of doses.
2. The medicine works on a condition that can be used to monitor the effect of the medicine (like lowering blood pressure or reducing fever).
3. The goal drug level isn’t well established.

When are drug levels checked?

The timing of drug level monitoring depends on the medication. Many medicines have a “trough” or “pre-dose” level checked before the next dose is given. But some medicines, like the blood thinner enoxaparin and some antibiotics, have levels checked 4 to 6 hours after the medicine is given. Other medicines like methotrexate have levels checked multiple times to make sure the body is getting rid of the drug well.

What does the drug level mean?

The goal drug level for a medicine depends on a several factors such as:

• the medication
• the condition being treated
• the time the level was checked

Since the goal drug level depends on the patient and situation, it’s best to ask the care team what the goal range is for the medication at that time.

Tips for patients and families

• Take your medicine as close to the scheduled time as possible every day (for example, every 8 hours, every 12 hours, or every 24 hours).
• Remember to follow your doctor or pharmacist’s instructions about how and when to take the medicine on the day the level will be drawn. If a pre-dose level for a medicine will be checked, don’t take the medicine before the level is checked. Most of the time, it is OK to take the medicine after the blood sample is drawn and before you know the drug level result. However, in rare cases, you will be told to wait until the level is known.
• Write down what time(s) the medicine was given before the level is checked.
• Tell your care team about any missed doses, including doses given by mouth that might have been thrown up. Let the team know if you have any concerns about inaccurate dosing, such as taking too much or too little of a medicine.
• Always tell your care team about any other medicines, vitamins, or supplements. These can affect how a drug works.