Antibody-mediated interference of a homogeneous immunoassay.

We hypothesized that an antibody-mediated interference could arise in a homogeneous immunoassay used to determine the presence of cocaine metabolites in urine. Urine specimens containing benzoylecgonine (BE) at concentrations near the National Institute on Drug Abuse (NIDA) threshold were assayed in replicate determinations by EMIT. Excess reagent protein (containing antibody specific for cocaine metabolites) was added to specimens to test for an antibody-mediated interference. Replicates of the BE-fortified specimens tested by EMIT that did not contain excess reagent antibody were all positive by the assay, while those that contained the excess reagent antibody were all negative. Because it may be difficult to detect excess interfering antibody by using some traditional tests for urine adulteration, we present these findings to illustrate a potential problem for some homogeneous immunoassays in forensic urine drug testing programs.