Heteroantibody: Phantom of the Immunoassay The heterophilic antibody

or heteroantibody, has been haunting immunoassays for many years. Interference by heteroantibodies in human sera can either increase or decrease the results of an immunoassay. For instance, by binding to a site other than the analyte-binding site, the heteroantibodies are capable of cross-linking the signal antibody with the capture antibody, thereby generating a false assay response. Such analytical errors may lead to mistakes in diagnosis and even to unnecessary surgical procedures (1). Estimates of the prevalence of heteroantibody interfer-ence vary considerably, although as many as 40 % of indi-viduals may have one or more heteroantibodies that could interfere with some immunoassays (2). Many of the ever-expanding number of sensitive immunoassays containin