When should I consider adjusting the dose of a drug based on therapeutic drug monitoring results?

You should consider adjusting the dose of a medication based on therapeutic drug monitoring (TDM) results when the measured drug concentration falls outside the established therapeutic range, when there is a lack of clinical response, or when adverse effects suggest toxicity despite standard dosing. This includes situations where the patient’s clinical status changes, such as altered renal or hepatic function, drug interactions, or adherence concerns, which may affect drug levels and efficacy.

Specifically, TDM is most valuable for drugs with narrow therapeutic windows, significant pharmacokinetic variability, or where clinical endpoints are difficult to assess directly. Dose adjustment should be guided by correlating drug concentrations with clinical response and side effects rather than relying solely on numerical values.

For example, in antiepileptic drugs, dose adjustment is recommended when plasma concentrations are subtherapeutic and seizures persist, or when concentrations are supratherapeutic and toxicity occurs, always considering individual patient factors (Patsalos et al., 2008). UK guidelines similarly emphasize dose adjustment in response to TDM results in conditions like bipolar disorder and ADHD, where clinical monitoring and drug levels together inform management ¹.

In summary, dose adjustment based on TDM should be considered when:

• Drug levels are outside the therapeutic range.
• There is poor clinical response or breakthrough symptoms.
• Signs of toxicity or adverse effects are present.
• Patient factors alter drug metabolism or clearance.
• Potential drug interactions affect drug levels.

This integrated approach ensures safe and effective medication use tailored to the individual patient’s needs ¹ (Patsalos et al., 2008).