IgG rheumatoid factor in purified IgG fractions and whole sera from patients with rheumatoid arthritis

There are inherent technical difficulties in measuring IgG rheumatoid factor (IgG-RF) in the serum of patients with rheumatoid arthritis (RA). These arise from measuring a reaction between two IgG molecules and the interference of IgM-RF in the reaction. We compared the prevalence of IgG-RF in whole sera and purified IgG fractions from 58 RA patients (43 of whom were latex or sheep cell agglutination positive). Methods of purification were: ammonium sulphate precipitation and DEAE cellulose or protein A-Sepharose chromatography. IgG-RF was measured by two methods: (1) radioimmunoassay and ELISA with a monoclonal myeloma IgG (IgG4,K) as the antigen and radiolabelled rabbit anti-human IgG (previously absorbed on a column with IgG4,K) as the second antibody; (2) ELISA using rabbit IgG as the antigen and a peroxidase conjugated goat anti-human IgG as the second antibody. When whole sera were assayed, 18 (31%) contained IgG-RF. In contrast, only three of the IgG fractions (5%) were positive for IgG-RF by all methods, while the remainder were uniformly negative. These results suggest that IgG-RF determination in whole sera does not accurately reflect IgG-RF activity.