Effect of empiric dosing on blood levels of theophylline and phenytoin

To evaluate empiric dosing guidelines for aminophylline and phenytoin in the management of acute exacerbations of obstructive lung disease and seizure disorders, we utilized an emergency department (ED)-based EMIT system to measure stat plasma theophylline and phenytoin levels in patients intended to receive these drugs. Plasma drug level results were available prior to initiation of therapy. Of the patients evaluated, 45 of 163 (27.6%) aminophylline patients and 21 of 73 (28.7%) phenytoin patients were projected to have plasma concentrations below the recommended therapeutic range if empiric dosing schedules were employed. In addition, 39 of 163 (23.9%) aminophylline and 10 of 73 (13.7%) phenytoin patients, had they received these drugs empirically, would have had plasma theophylline and phenytoin levels in excess of the recommended therapeutic range using empiric dosing. We conclude that the use of empiric dosing guidelines for aminophylline and phenytoin in the ED do not reliably produce therapeutic plasma concentrations. The development of a stat drug analysis laboratory in the hospital or within the ED would improve the effectiveness and safety of these acutely used drugs with narrow therapeutic ranges.