False-positive Waaler-Rose test due to anti-rabbit IgM antibodies in IgA deficiency

The presence of antibodies to rabbit IgM in an IgA deficient patient with degenerative arthropathy is described. The anti-IgM antibodies caused a false-positive Waaler-Rose test. Antibodies to rabbit IgM in titres greater than or equal to 1/8 were found in 20 of 35 IgA deficient individuals. No relationship between the patients' symptoms and the presence of these antibodies could be discerned and their biological significance is unknown. Two of 100 normal blood donors, none of 62 rheumatoid arthritis patients and 9 of 50 systemic lupus erythematosus patients showed anti-rabbit IgM antibodies in titres greater than or equal to 1/8. Unless an IgG fraction of antibodies is used for sensitization of erythrocytes in the Waaler-Rose test, the results may be misleading.