Immunofixation improves the detection of monoclonal gammopathy of undetermined significance (M.G.U.S.) in patients with rheumatoid arthritis.

Monoclonal gammopathy of undetermined significance (MGUS) was detected by immunofixation (IFX) in 6% of patients with rheumatoid arthritis (RA), whereas it was detected by immunoelectrophoresis in only 1% of these patients. Most of the patients with both RA and MGUS had polyclonal B cell activation, together with their monoclonal band, extra-articular features and long-lasting disease, which suggests a direct relationship between MGUS and RA status.