False-Positive Rose-Waaler Rheumatoid Factor Titre Given by Anti-Ruminant IgM Antibody

Abstract
An antibody against ruminant (goat, sheep, bovine) IgM [immunoglobulin M] is described which also cross-reacts with rabbit IgM. The antibody was found in the serum of a patient with adult celiac disease and a selective IgA deficiency. This patient gave a false high Rose-Waaler rheumatoid factor titer due to the presence of the serum antibody to rabbit IgM that was capable of agglutinating rabbit antibody-sensitized erythrocytes.