TDM assumes that there is a predictable relationship between drug dose and blood or plasma drug concentration, and between drug concentration and therapeutic outcome.
TDM is not applied to all drugs, but where relevant, it can guide clinicians to find the ‘right’ dose for drugs that might be more difficult to dose, owing for instance to individual differences in drug pharmacokinetic profiles, i.e., the manner in which a drug is absorbed, distributed, metabolised, and excreted.
TDM is also used for monitoring drugs with narrow therapeutic ranges, drugs known to cause therapeutic and adverse effects, and drugs for which target concentrations are difficult to monitor.
Some of the main drug types that are subject to TDM include:
Immunosuppressive drugs: Solid organ transplant recipients depend upon a combination of lifelong immunosuppressive drugs to maintain the function of their transplant by avoiding immune rejection. While prospective and randomised studies to evaluate the clinical benefits of TDM are few, several studies have shown correlations between low immunosuppressant exposure and risk of acute organ rejection, and high immunosuppressant exposure and toxicity (1).
Antibiotics used in intensive care units: TDM helps to guide antimicrobial treatment in intensive care settings, where multi-drug resistant organisms may emerge, and where sepsis is a common and complex complication that may impact the pharmacokinetic profiles of certain drugs.
Antibiotics, anti-fungals, and anti-epileptic drugs used to treat premature babies: Premature babies treated in neonatal units often receive a range of medications to mature their organs, combat infections, and manage seizures. Since organs and metabolic pathways continuously develop in the early stages of life, TDM is important to guide ongoing treatment effectiveness and safety.
Treatments for chronic tuberculosis (TB): Here, TDM may be used in cases of disease relapse, when patients don’t respond as expected to TB treatment, or when aberrant TB drug concentrations are suspected.
Medications given to extremely obese individuals: Impaired organ function, altered gastric emptying and altered absorption due to the presence of excess subcutaneous fat in individuals who are morbidly obese can impact drug pharmacokinetics and render standard dosing ineffective.
Medications used to treat psychiatric disorders, e.g., lithium: Here, TDM may be used to detect issues with treatment compliance, and/or in patients who are taking several other medications, to ensure that blood or plasma drug concentrations are within a safe and effective range.
Anti-seizure drugs: For several drugs in this category, a clear relationship has been established between blood drug concentration and therapeutic effect. TDM can aid in the individualisation of therapy in special groups such as pregnant women, and to adjust for individual variations in pharmacokinetics.