Comparison of standard and modified enzyme immunoassay of phenytoin.

A modified method for the enzyme immunoassay (EMIT, Syva Company) of phenytoin is presented and compared with the standard method. Serum samples from 14 patients were analyzed for phenytoin content using both methods. All assays were performed by the same individual. Within-day and between-day variations of the modified method were determined. A carry-over study was done to determine if a sample with a high phenytoin concentration might contaminate subsequent samples with lower concentrations or whether samples with low concentrations could dilute subsequent samples with higher concentrations. Within-day and between-day variations of the modified method were 7.5% and 9.9%, respectively. These values are less than the 10% coefficient of variation limit claimed by the manufacturer of the standard method. The carry-over study revealed no significant carry-over with the modified method. An excellent correlation was observed between the values obtained from the two methods. The modified method can reduce assay costs by up to 40%. The modified method was found to provide accuracy and precision equivalent to the standard EMIT method at a substantial cost savings.