Evaluation of the provision of rapid drug plasma assays in an outpatient anticonvulsant clinic.

The benefits of providing a rapid assay service for phenytoin and carbamazepine were assessed during a 3-month study in an outpatient anticonvulsant clinic. Fifty-three patients were tested using the EMIT assay system, and a questionnaire was used to compare the physicians' choice of drug dosage and appointment interval before and after each patient result was available. Results were in the therapeutic range for 10 (24%) of the 42 patients on phenytoin and 20 (74%) of the 27 on carbamazepine. The management of 29% of the patients on phenytoin and 22% on carbamazepine was affected, with approximately equal numbers of changes made to dosage (phenytoin 19%, carbamazepine 15%) and appointments (phenytoin 19%, carbamazepine 11%). Of 34 patients with subtherapeutic plasma concentrations, 19 (56%) did not have their doses altered because their seizures were well controlled. Five patients had phenytoin concentrations above 80 mumol/l, but only 1 had toxic symptoms. The dose was lowered in 2 cases, and 2 more were reduced at the next visit when symptoms of toxicity had developed. It was concluded that both clinical evidence and drug plasma concentrations were considered when making decisions about patient management. The rapid assay service was useful for detecting noncompliers, confirming suspected toxicity, and aiding decision making in doubtful cases.