Fig. 1.

Interference by rheumatoid factor in the ELISA and concentrations of IgG, IgA and IgM proteins in sera from patients with systemic lupus erythematosus (SLE), primary Sjögrens’ syndrome (SS), rheumatoid arthritis (RA), systemic sclerosis (SSc), polymyositis/dermatomyositis (PM/DM) or mixed connective tissue disease (MCTD). (a, b) Rheumatoid factor was added to nine rheumatoid factor-negative sera in the concentration range of 125–2000 U/ml. Each of the nine sera are represented by individual symbols. (a) OD₄₅₀ readings in ELISA for IgG antibodies to EBNA-2(N). (b) OD₄₅₀ readings in ELISA for IgG antibodies to EBNA-1(C).