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The page below is a sample from the LabCE course Pharmacology in the Clinical Lab: Therapeutic Drug Monitoring and Pharmacogenomics. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Unexpected Concentrations

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.
NoncompliancePatients 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.
MalabsorptionThe TDM result will reveal if the drug cannot be absorbed well through the gut and an alternative route of administration will be needed.
Drug interactionsMany drugs interfere with the absorption or metabolism of other drugs. These interactions will be revealed by TDM.
Kidney or liver diseaseAny pathology that affects elimination will cause an elevation in a drug level that will be unmasked by TDM.
Altered protein bindingChanges 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.