Therapeutic drug monitoring (TDM) involves measuring drug concentrations in blood plasma or serum to assess whether they fall within the therapeutic range or require dose adjustments. It helps identify adverse drug reactions and therapeutic resistance caused by excessively high or low drug levels. Several internal and external factors can influence these levels. Notably, the activity of drug-metabolising enzymes and the efficiency of transport molecules play crucial roles in a drug’s bioavailability. Low enzyme activity typically results in elevated drug levels, increasing the risk of side effects and toxicity, whereas high enzyme activity may reduce serum levels, leading to treatment failure. Prodrugs are an exception to this, where enzymes must convert the drug into its active form. In such cases, low enzyme activity can lead to therapy resistance, as seen with tramadol, clopidogrel, and tamoxifen.

Interactions with drugs, phytopharmaceuticals, and food, along with a patient’s genetic predisposition, significantly affect enzyme metabolism and transport molecule function, potentially altering drug levels. By measuring these levels, clinicians can adjust dosages or select alternative medications. If drug levels remain outside the therapeutic window despite ruling out interactions and ensuring patient compliance, genetic testing may help identify underlying causes. TDM is commonly performed using or . Immunoassays are also used for some active substances.

Indications for determining the drug level:

• Reduced or no therapeutic success
• Worsening of symptoms during therapy
• Questions about patient compliance
• Adverse drug reactions
• Possible intoxication
• Polypharmacy
• Suspected drug interactions
• Suspected pharmacogenetic disposition