05.01.3 Therapeutic Drug Monitoring

It is obvious that it will be expensive to monitor the levels of drugs by taking blood samples, and detecting the levels of drugs in them. Therapeutic drug monitoring is usually not needed if the toxic levels are above the therapeutic concentrations (Figure 5.2). However therapeutic drug monitoring may be needed if toxic levels are similar or overlap therapeutic concentrations (Figure 5.2).

Figure 5.2 Therapeutic drug monitoring (Copyright QUT, Sheila Doggrell)

In practice, therapeutic drug monitoring is required for a number of drugs including the anti-epileptic, phenytoin. Therapeutic drug monitoring is used early in treatment with phenytoin to find dose that gives therapeutic effect. Phenytoin is being given to prevent seizures. If a seizure occurs, it may indicate the concentration of phenytoin in the plasma is below the therapeutic dose. Thus, therapeutic drug monitoring of phenytoin commonly occurs after a seizure. Finally, the levels of phenytoin are measured after a change of dose.

Another use of therapeutic drug monitoring is in poisoning to determine the treatment required, and this occurs with paracetamol overdose. If levels are above a certain standard, treatment of the overdosage is needed to prevent liver toxicity. This treatment is discussed in eChapter 6.