ABC of monitoring drug therapy. Making the most of plasma drug concentration measurements.

Criteria that a drug should satisfy for plasma concentration measurements to be useful * Difficulty in interpreting clinical evidenceIn the previous articles in this series we have outlined the principles of of therapeutic or toxic effects monitoring drug therapy by measuring the plasma drug concentration and * A good relation between the plasma drug shown how they can be applied to specific drugs (digoxin, lithium, concentration and the therapeutic or toxic theophylline, phenytoin, aminoglycoside antibiotics, and cyclosporin), for effect, or both which the criteria in the box are sufficiently fulfilled to justify monitoring.* A low toxic:therapeutic ratio * Does not metabolise to important active metabolites on 25