A comparison of plasma phenytoin level determinations by EMIT and gas-liquid chromatography in patients with renal insufficiency.

Monitoring the blood levels of antiepileptic drugs (AED) has proven to be of value in the clinical management of epileptic patients. We have compared the plasma concentrations of AED determined by enzyme multiplied immunoassays (EMIT) and gas-liquid chromatography (GLC) assays and identified a group of patients (n = 22) in whom the EMIT values for phenytoin were up to three times higher than those obtained by GLC. All these patients were oliguric with various degrees of azotemia, most of them undergoing weekly hemodialysis. The ratio of EMIT over GLC was 2.0 in 14 patients, 2.5 in 5 patients, and near 3.0 in 3 patients. These ratios remained relatively constant during repeated determinations over several months. The magnitude of difference between EMIT and GLC values (microgram/ml) was not related to the degree of azotemia. The discrepancy remained unchanged when various modifications of EMIT and GLC techniques were used. When phenytoin was added in vitro to plasma from nonepileptic oliguric azotemic patients, the measured phenytoin values were close to and not higher than the target values by either methodology. We feel that, when monitoring phenytoin blood levels in epileptic patients with renal insufficiency, one should be aware of a discrepancy between phenytoin values assayed by GLC and EMIT.