False positive immunometric assays caused by anti-immunoglobulin antibodies: a case report

A serological phenomenon causing aberrant results with monoclonal immunoenzymetric assays (IEMA's) is reported. Two different commercial IEMA kits detected low levels of choriogonadotropin (hCG) in the serum of a non-pregnant woman. These assays detected between 32 and 55 IU/l of serum hCG over a 3-wk period; however, an RIA for beta-subunit and two monoclonal immunoradiometric assays (IRMA's) detected no hCG (less than 5 IU/l). An IEMA measurement of creatine kinase MB isozyme was also elevated. Antisera to either human immunoglobulin or specifically to human IgM, added to the serum prior to assay, substantially decreased these IEMA reactions. Addition of either mouse serum or purified mouse IgG totally abolished them. It is concluded that these spurious reactions were most likely caused by an IgM antibody which binds to native and enzyme-labelled mouse IgG, but not to iodinated IgG.