Therapeutic drug monitoring--the need for audit?

The experience of therapeutic drug monitoring (TDM) of seven drugs as part of the normal biochemistry services over the period April 1982-83 is reported. Reagent costs alone for these assays exceeded pounds 15,000. Of 1,841 digoxin estimations, 56 per cent fell within the therapeutic range whereas 25 per cent were potentially toxic. Fifty per cent of 968 lithium measurements were lower than 4 mg L-1, placing many of these patients at risk of treatment failure. Only 29 per cent and 32 per cent of 369 theophylline and 440 phenytoin concentrations respectively were within their well-established ranges. In only around 20 per cent of requests for theophylline and phenytoin assay was sufficient clinical information supplied to the laboratory. In the majority of patients with theophylline or phenytoin concentrations outwith the therapeutic range, further analysis was not requested and so optimisation of dosage cannot be assumed to have occurred. The wide therapeutic ranges of carbamazepine and phenobarbitone ensured that most patients attained acceptable concentrations. Sodium valproate analysis was requested on 160 occasions despite the poor correlation between the concentration of this drug and its therapeutic and toxic effects. TDM can be an expensive exercise which must be subjected to rigorous cost-benefit analysis. Requests should be made on a customised drug assay form and interpretative advice on individual patient problems made available. Recommendations for the organisation, daily running and clinical supervision of TDM are made.