Linked to EAHP Statements:
Section 4: Clinical pharmacy services

ACPE UAN: 0475-0000-16-020-L04-P. A knowledge based activity.

Abstract

Concentrations of drugs can be used as surrogates for drug effects when there is no obvious relationship between dose and effect. At a given dose, drug concentration is influenced by bioavailability, volume of distribution, hepatic and renal clearance. Patient factors such as pharmacogenetic difference in bioavailability, systemic metabolism and kidney function greatly influence drug concentration and therefore therapeutic drug monitoring (TDM) could assist with dose individualisation. TDM involves the measurement and interpretation of drug concentrations in order to individualise therapy. When interpreting concentration measurements, factors that need to be considered include the sampling time in relation to last dose, duration of treatment with the current dose, dosing schedule, other drug therapy, relevant disease state, desired clinical targets. Clinical pharmacists use pharmacokinetic principles to assess these interpretations. Usually pharmacists advise that it is better to wait until a steady state has been reached before a sample is taken, but if pharmacokinetic software equipped with population pharmacokinetic models is used, then there is no need to wait for a steady state to be reached. However, the sampling time is important for correct modelling as the concentration of drug changes during the dosing interval.

Several methods have been developed for prediction of individual dose requirements. These are based on calculation of clearance and volume of distribution from one or a few drug concentrations. Bayesian techniques have been, also, successfully used to estimate the pharmacokinetic parameters of a drug and hence used to predict appropriate dosage regimens to achieve the optimal response.

Teaching goals:

• To present the indications for TDM;
• To present the genetic variance in bioavailability and clearance;
• To explain the importance of appropriate sampling time for TDM;
• To provide the examples of drugs suitable for TDM.