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In many cases it is relatively easy to evaluate the pharmacological effects of a drug by clinical observation, and initial dosage regimens can be modified to increase the therapeutic effect or to eliminate unwanted effects. Measurement of drug concentrations in blood can be performed to help with diagnosis or to optimise therapy for those drugs where response (therapeutic or toxic effects) cannot be readily evaluated from clinical observation alone. Examples of drugs where monitoring can usefully aid clinical judgement, together with target ranges, are shown in Table 1.3.

As a result of pharmacokinetic and pharmacodynamic variability, the following factors should be considered when interpreting drug concentration measurements:

1 Is the patient responding to therapy or showing symptoms of toxicity?

2 Was the sample taken at steady state?

3 Was the sampling time appropriate for the drug?

4 Where is the concentration relative to the ‘target’ range (Table 1.3)?

5 If the patient is not responding or has toxicity, how should the dose be modified?

Unexpectedly low concentrations may indicate poor adherence or an absorption problem (e.g. secondary to vomiting).