Estimation of IgM rheumatoid factors by fluorimetry.

Sheep red cells coated with rabbit IgG antibody may be used to detect IgM rheumatoid factors either by agglutination or microscopically by the fluorescence of antihuman IgM antibody conjugate bound to rheumatoid factors on the cell surface. By substituting for red cells the plastic surface of a filter disc coated with rabbit IgG it was possible to elute the bound conjugate and measure the fluorescence in a fluorimeter. The results of both the sheep cell agglutination test and the fluorescence test agreed quite well for the majority of sera; both tests seemed to reflect IgM rheumatoid factor concentrations in these sera. The difficulties encountered in attempting to define and measure accurately rheumatoid factor concentrations in a serum are emphasised.