IgG rheumatoid factor: analysis of various species of IgG for detection by radioimmunoassay.

IgM RF is known to react with many species of IgG. This study examined nine species of IgG to determine which was the most sensitive and specific to detect IgG RF by radioimmunoassay. For 19 IgM RF-positive sera, horse IgG was significantly more sensitive than all IgG preparations tested except rabbit IgG. The IgG used include sheep, swine, goat, bovine, and chicken as well as human and baboon Fc. For 22 IgM RF-negative sera from patients with rheumatoid arthritis, only one was positive for IgG RF with rabbit IgG, whereas five were positive with horse IgG. Analysis after preparative ultracentrifugation of a variety of sera positive and negative for IgM RF indicated that horse IgG was more sensitive, whereas rabbit IgG was more specific for the detection of IgG RF by our radioimmunoassay. IgM RF, on the other hand, was more sensitively detected by human Fc than by any preparation other than whole human IgG.