Medication monitoring measures the level of a medicine in the blood to make sure the dosage of medicine is correct. Sometimes the amount of medicine that helps (therapeutic level) is very close to the amount that can cause harm (toxic level). Determining the correct dosage of a medicine for each person can be difficult, because the dose needed to produce a therapeutic level in the blood varies from person to person. Factors that affect medicine levels in the blood include age, weight, activity level, the speed at which the body breaks down the medicine, how the medicine was taken (such as by mouth, patch, or shot), and other medicines a person is using.

Medication monitoring is important for people whose bodies do not break down (metabolize) medicines effectively because of poor function in the liver or kidneys. Also, some medicines can damage an organ (such as the liver or kidneys) or cause other problems at the dose used for treatment. If you are taking such a medicine, its amount in your blood may need to be checked regularly to ensure your dosage is at a safe and effective level.

Medication monitoring can also be useful for infants, older adults, people with heart failure or changes in heartbeat (arrhythmia), or people taking two or more medicines that may interfere with each other.

Medicine blood levels often depend on when the last dose was taken. The highest, or peak, level usually occurs 30 to 60 minutes after taking the medicine. The lowest, or trough, level occurs right before taking the next dose.

Monitoring can be done for nearly any type of medicine, but it is most often done for:

• Certain antibiotics, such as amikacin (Amikacin), gentamicin, or tobramycin (Nebcin). These antibiotics may cause hearing loss or kidney damage if their level in the blood gets too high.
• Medicines to treat heart problems. These include digoxin (Lanoxin), quinidine (Cardioquin, Quinidex, Quinaglute, Quin-Release), or procainamide.
• Medicines to treat seizure disorders (epilepsy). These include phenytoin (such as Dilantin), carbamazepine (such as Tegretol), barbiturates (such as phenobarbital), and valproic acid (Depacon, Depakote, Depakene).
• Asthma medicines, such as theophylline.
• Medicines to treat bipolar disorder, such as lithium (Eskalith).
• High doses of aspirin or similar medicines (salicylates).
• Medicines used to prevent the body from rejecting an organ transplant, such as cyclosporine (Neoral, Sandimmune, SangCya).

A prothrombin time (PT) test is used to monitor the effects of the blood-thinning medicine warfarin (Coumadin).

Why It Is Done

Medication monitoring may be done to:

• Confirm you are receiving an effective dose of medicine.
• Determine whether you are receiving too much medicine.
• Ensure that you are taking a medicine properly.

How To Prepare

It is important to know exactly when you took the last dose of the medicine for which you are being tested. Testing is usually done right before you take another dose. Ask your health professional for instructions about the timing of the test.

Tell your health professional about all the prescription and nonprescription medicines you are taking and any drugs (such as alcohol, marijuana, or cocaine) you are using. Sometimes these can interfere with testing.