Drug level monitoring in paediatric practice.

A knowledge of the concentration of a drug in blood or other tissue fluid is now considered an important and sometimes essential part of the therapeutic management of various conditions. Its importance has increased as information showing wide interindividual variation in drug response has become recognised. Such variation arising from differences in absorption, distribution, and elimination appears to be greater during development, and yet many drugs are still prescribed for children in doses calculated by scaling down adult doses according to weight or surface area.1 This conventional dose approach is satisfactory only for drugs with a large therapeutic index (the difference between therapeutic and toxic drug levels is wide). Doses of most drugs need to be individually tailored according to the child's particular requirements. Titration of dose to the therapeutic response is the best means of achieving this. However, this approach is unsuitable for most of the commonly prescribed drugs in children (antimicrobials, anticonvulsants, anti-inflammatory agents, cardiac glycosides) as the response to these is not readily clinically evaluated, and there may be a subtherapeutic or toxic response. As a result, drugs may be discarded as useless or too toxic. For many drugs, there is a close relationship between plasma level and therapeutic effect, and the value of knowing the plasma level has been proved for a number of drugs (Table 1). Although fewer data are available for these drugs in children, in the case of most drugs the plasma level-therapeutic response relationship will be better than that between dosage and clinical effectiveness. However, it should be remembered that the clinical effects of some drugs cannot be adequately monitored by