Medication Monitoring

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Medication Monitoring

Test Overview

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 hard, because the dose needed to produce a therapeutic level in the blood varies from person to person. Things 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 remove or break down (metabolize) medicines well 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.
  • Medicines to treat heart problems.
  • Medicines to treat seizure disorders (epilepsy).
  • Medicines to treat bipolar disorder.
  • High doses of ASA or similar medicines (salicylates).
  • Medicines used to prevent the body from rejecting an organ transplant.

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. Ask your doctor for instructions about the timing of the test.

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

How It Is Done

The health professional drawing blood will:

  • Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
  • Clean the needle site with alcohol.
  • Put the needle into the vein. More than one needle stick may be needed.
  • Attach a tube to the needle to fill it with blood.
  • Remove the band from your arm when enough blood is collected.
  • Apply a gauze pad or cotton ball over the needle site as the needle is removed.
  • Apply pressure to the site and then a bandage.

How It Feels

You may feel nothing at all from the needle puncture, or you may feel a brief sting or pinch as the needle goes through the skin. Some people feel a stinging pain while the needle is in the vein. But many people do not feel any pain (or have only minor discomfort) once the needle is positioned in the vein. The amount of pain you feel depends on the skill of the health professional drawing the blood, the condition of your veins, and your sensitivity to pain.


Blood test

There is very little risk of complications from having blood drawn from a vein.

  • You may develop a small bruise at the puncture site. You can reduce the risk of bruising by keeping pressure on the site for several minutes after the needle is withdrawn.
  • In rare cases, the vein may become inflamed after the blood sample is taken. This condition is called phlebitis and is usually treated with a warm compress applied several times daily.
  • Continued bleeding can be a problem for people with bleeding disorders. ASA, warfarin (Coumadin), and other blood-thinning medicines can also make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your health professional before your blood is drawn.


Medication monitoring measures the level of a medicine in the blood to make sure the dosage of medicine is correct. The effective blood level for each medicine is different. Your doctor will determine whether the dosage of the medicine needs to be adjusted based on your blood test result.

What Affects the Test

Reasons you may not be able to have the test or why the results may not be helpful include:

  • The time between when the medicine was first taken and the blood test. For example, if the test should be done when the medicine level is lowest, taking a dose of the medicine just before the blood sample is taken can cause a falsely high result.
  • If you take medicines other than the one(s) being monitored. This can include other prescription and non-prescription medicines or drugs such as alcohol, marijuana, and cocaine.

What To Think About

  • Medicine blood levels can be measured in a person who may have taken an overdose.
  • The dose of a medicine may need to be adjusted until the right blood levels are achieved. Dosage of a medicine may also need to be changed if a person's lifestyle changes (such as becoming more active) or if the person begins taking another medicine that can affect the medicine being monitored.
  • A prothrombin time (PT) test is a different type of monitoring test. It is used to monitor the effects of the blood-thinning medicine warfarin (Coumadin). A PT test may also be called international normalized ratio (INR). For more information, see the topic Prothrombin Time.
  • Many medicines do not have established therapeutic levels or methods to measure them. The therapeutic level is determined either by how a person's symptoms are responding to the medicine or by the onset of an adverse reaction.


Other Works Consulted

  • Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
  • Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.


By Healthwise Staff
Primary Medical Reviewer Kathleen Romito, MD - Family Medicine
Primary Medical Reviewer Donald Sproule, MD, CM, CCFP, FCFP - Family Medicine
Specialist Medical Reviewer Theresa O'Young, PharmD - Clinical Pharmacy
Last Revised January 25, 2011

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