TDM provides a quantitative measure of the circulating concentration of a drug. The physician determines if the dosage of the drug needs to be adjusted based on this information.
If a drug concentration is determined to be outside the therapeutic range, it may be for one of the reasons listed in the table below.
| Reason | Discussion |
| Noncompliance | Patients may (intentionally or unintentionally) not take the drug as prescribed. TDM can thus help monitor compliance. |
| Dosing errors | The dose may have been erroneous or inappropriate, given the patient's condition. |
| Malabsorption | The TDM result will reveal if the drug cannot be absorbed well through the gut and an alternative route of administration will be needed. |
| Drug interactions | Many drugs interfere with the absorption or metabolism of other drugs. These interactions will be revealed by TDM. |
| Kidney or liver disease | Any pathology that affects elimination will cause an elevation in a drug level that will be unmasked by TDM. |
| Altered protein binding | Changes in serum proteins can lead to big changes in the amount of free drug in serum. |
Variations in the genetics of drug-metabolizing enzymes can also affect drug concentrations in the body. This is the field of pharmacogenomics that will be discussed later in the course.
