Ligand displacement immunoassay--demonstration of its use for the measurement of serum phenobarbital and phenytoin.

We describe a clinically useful ligand displacement immunoassay for the measurement of serum or plasma phenobarbital and phenytoin. A conjugate of drug-specific antiserum covalently bound to micro-crystalline cellulose and a beta-galactosidase-labelled drug derivative is prepared as a lyophilized reagent. Sample is added to the conjugate and, after 5-min incubation at room temperature, the bound and displaced enzyme-ligand are separated by brief centrifugation. The enzyme activity of the displaced enzyme-ligand is measured on a spectrophotometer or centrifugal analyser. Linear calibration curves are obtained with appropriate sample dilution, allowing generation of the standard curve with a saline zero reference and a single calibrator. The performance of the assay compared favourably with other immunoassays; comparison studies with clinical specimens gave a correlation with EMIT of 0.96 (n = 122) for phenobarbital and 0.95 (n = 96) for phenytoin.