Diagnostic decision. The measurement of anticonvulsant agent levels.

Monitoring levels of antiepileptic drugs is useful in formulating a therapeutic plan but not in making a diagnosis. Differences in assay methods are not important. Differences in therapeutic ranges do not reflect procedural differences but merely the source that is quoted. Therapeutic ranges are not absolute criteria for therapy, but guides to a clinical balance between dose and side effects. Because therapeutic ranges reflect trough (early morning) serum levels, clinical determinations should be done at this time. Serum drug determinations should be used to establish the successful drug level for a patient; assess compliance; identify the relative contributions of each drug to efficacy and to side effects in complex programs; identify bioavailability problems; and characterize idiosyncrasies in the rate of drug metabolism.