Pharmacokinetics and therapeutic drug level monitoring

Pharmacokinetics and therapeutic drug level monitoring

Pharmacokinetics addresses the absorption, distribution, metabolism and excretion of drugs in patients. A sound knowledge of the pharmacokinetic profiles of different drugs enables the pharmacist to assess the dosing requirements for certain drugs in patients in extremes of age and in the presence of impairment of kidney and liver function. Clinically important drug interactions and adverse reactions can sometimes be predicted. Dosing calculations of aminoglycoside antibiotics are usually made by employing pharmacokinetic principles.

A number of medicines in common use have a narrow therapeutic index; that is, the difference between the lowest effective dose and a potentially toxic dose can be quite small. In many cases it is necessary or desirable to undertake therapeutic drug level monitoring (TDM) to ensure that patients can be treated safely. TDM services include the measurement of drug levels in the patient’s blood and the application of clinical pharmacokinetics to optimise drug therapy. There is a wide range of medicines that fall into this category, but TDM services typically include aminoglycoside antibiotics, anticonvulsants, immune suppressants, digoxin, lithium and theophylline. Monitoring drug levels in patients can also provide an important indicator as to whether they are taking their medicine. Clinical pharmacy input into TDM services can range from the provision of simple advice to other clinicians on when to take samples and how to interpret results, to fully fledged services that may include collection and laboratory analysis of the blood sample.