Escape from blockade of interfering heterophile antibodies in a two-site immunoradiometric assay for thyrotropin.

Sandwich-type immunoassays in which mouse monoclonal antibodies are used are subject to positive interference by heterophile antibodies present in human serum. Manufacturers now customarily add nonspecific mouse immunoglobulins to absorb the heterophile antibodies and eliminate such interference. We describe the case of a patient who had spurious increases in thyrotropin concentration in serum despite use of the mouse immunoglobulins included in the assay kit. This resulted in a puzzling clinical picture, a workup, and treatment. We demonstrate that the observed increases in thyrotropin were markedly reduced by including additional amounts of mouse immunoglobulins of the appropriate class, subclass, and fragment type in the assay mixture.