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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.
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.
Most drug levels are checked with a blood test.
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.
Many medicines don’t need drug levels checked. Some of the reasons include:
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.
The goal drug level for a medicine depends on a several factors such as:
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.
Reviewed: August 2018