Homogeneous nonisotopic assay for phenytoin evaluated

We evaluated a new homogeneous immunoprecipitation assay for phenytoin in human serum. No sample dilution or pretreatment is required. The new method is based on spectrophotometry of the inhibition by free phenytoin of the precipitating reaction between anti-phenytoin antibody and a phenytoin-human serum albumin conjugate. A serum test sample is simultaneously mixed with the phenytoin-albumin conjugate and rabbit antiserum to phenytoin in a centrifugal analyzer, and the subsequent reaction is monitored at 3 min. Within-run and between-run coefficients of variation were well below 7%. The relation between results for patients' test sample as determined by immunoprecipitation assay (y) and an enzyme immunoassay (x) can be expressed as y = 1.10x + 1.1 (r = 0.966, n = 66).