Medication Monitoring

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

Medication monitoring is important for people whose bodies do not break down (metabolize) medications effectively because of poor function in the liver or kidneys. Also, some medications 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 medication, 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 medications that may interfere with each other.

Medication 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 medication. The lowest, or trough, level occurs right before taking the next dose.

Monitoring can be done for nearly any type of medication, 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.
• Medications to treat heart problems. These include digoxin (Lanoxin), quinidine (Cardioquin, Quinidex, Quinaglute, Quin-Release), or procainamide.
• Medications to treat seizure disorders (epilepsy). These include phenytoin (such as Dilantin), carbamazepine (such as Tegretol), and valproic acid (Depacon, Depakote, Depakene).
• Asthma medications, such as theophylline.
• Barbiturates, such as phenobarbital.
• Medications to treat bipolar disorder, such as lithium (Eskalith).
• High doses of aspirin or similar medications (salicylates).
• Medications 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 medication warfarin (Coumadin).

Medication monitoring may be done to:

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

It is important to know exactly when you took the last dose of the medication 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 medications you are taking and any drugs (such as alcohol, marijuana, or cocaine) you are using. Sometimes these can interfere with testing.

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.

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. However, 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.
• Rarely, 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. Aspirin, warfarin (Coumadin), and other blood-thinning medications can also make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medication, tell your health professional before your blood is drawn.

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

Factors that can interfere with your test or the accuracy of the results include:

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

• Medication blood levels can be measured in a person who may have taken an overdose.
• The dose of a medication may need to be adjusted until the right blood levels are achieved. Dosage of a medication may also need to be changed if a person's lifestyle changes (such as becoming more active) or if the person begins taking another medication that can affect the medication 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 medication warfarin (Coumadin). A PT test may also be called international normalized ratio (INR). For more information, see the medical test Prothrombin Time.
• Many medications 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 medication or by the onset of an adverse reaction.

Other Works Consulted

• Fischbach FT, Dunning MB III, eds. (2004). Manual of Laboratory and Diagnostic Tests, 7th ed. Philadelphia: Lippincott Williams and Wilkins.

• Pagana KD, Pagana TJ (2006). Mosby’s Manual of Diagnostic and Laboratory Tests, 3rd ed. St. Louis: Mosby.