Therapeutic Drug Monitoring (TDM) is the measurement of drug concentrations in-patients who are prescribed a drug for a specific condition. For example, the levels of Digoxin in a person taking Digoxin for a heart condition. These are not tests for illegal or nonprescribed drugs!

Why do TDM?

Sometimes it is important to know the concentration of the drug you are taking. Each drug has a minimum and maximum level for best performance. If it is under the minimum level it will not work and if it is over the maximum you may experience side effects. For this reason drug concentration levels must be monitored and maintained within a specific range.

For some drugs and in some situations your doctor cannot estimate this range without TDM. Your size, sensitivity to drugs, speed of metabolism, and the amount of drug your intestine absorbs, etc. all vary from patient to patient and affect the drug concentration in you.

TDM blood collections must be done at specific times because drug levels are always changing. This is because when a drug is taken there is a time delay while the intestine absorbs the drug. Usually about an hour after the drug is taken it reaches a very high level and then the level falls rapidly while the drug spreads throughout the whole body. The drug level then declines further as the body metabolizes the drug. For consistency, the blood sample must be taken at a specified time. In general, the best time is directly before the drug is usually taken. Therefore in most cases the best time to go to the laboratory is first thing in the morning. This may mean delaying taking the drug until the blood sample is taken. For most people this is not a problem, if it is for you, your doctor will make other arrangements.