INVALIDITY FROM NONPARALLELISM IN A RADIOIMMUNOASSAY FOR ERYTHROPOIETIN ACCOUNTED FOR BY HUMAN SERUM ANTIBODIES TO RABBIT IGG

An immunologic crossreactant of erythropoietin seemed to develop and p ersist in serum samples from a patient during treatment and remission of idiopathic aplastic anemia. It had a steeper slope to radioimmunoas say log-dose response lines and a larger molecular size than erythropo ietin. On fractionation of serum, the apparent crossreactant was bound by staphylococcal Protein A at pH 7.5 and recovered by elution from i t at pH 3.0. Adsorption of serum from the patient, and from one of two similarly affected children, with rabbit IgG linked to agarose appear ed to remove completely the apparent crossreactant. These treated sera gave radioimmunoassay log-dose response lines essentially parallel to that given by the International Reference Preparation (IRP) for eryth ropoietin and estimates of immunoreactive erythropoietin appropriate t o the normal hemoglobin concentrations. The apparent crossreactant of erythropoietin is thus accounted for by heterophilic antibodies to rab bit IgG. These developed in the patient following treatment with rabbi t antilymphocyte globulin but seem to have arisen spontaneously in the children. Thus iatrogenic and idiopathic antibodies to rabbit IgG int erfered in a radioimmunoassay for erythropoietin in serum through thei r ability to react with the radioimmunoassay anti-erythropoietin antis erum raised in rabbits.