Rheumatoid Factors (RF) IgM

Rheumatoid factors are autoantibodies directed against the Fc portion (tail) of immunoglobulin G (IgG). The most useful class of rheumatoid factor is IgM, found in around 50% of patients with rheumatoid arthritis (RA). Rheumatoid factors are present in mixed cryoglobulinaemia (Type II & III) and Sjogren's syndrome and to a lesser degree in other systemic autoimmune rheumatic diseases, such as SLE, systemic sclerosis, juvenile rheumatoid arthritis and polymyositis. RF are also found in 5-10% of healthy adults and in those with current infections. An RF result can neither confirm nor exclude rheumatoid arthritis.

Both RF and CCP have moderate sensitivity for RA (50-60%) and therefore a negative test does not exclude disease. Weakly positive results may have little significance and the likelihood ratio for RA increases as CCP and RF levels increase (https://doi:10.1136/rmdopen-2021-002099). CCP antibodies are more specific for both early and established RA compared to rheumatoid factor (RF) antibodies. Both antibodies are included in the ACR/EULAR 2010 classification criteria for RA (Aletaha D, et al. Arthritis Rheum. 2010 Sep;62(9):2569-81.).

This assay is performed by chemiluminescent microparticle immunoassay.

Differential diagnosis of inflammatory arthritis
Connective tissue disease
Cryoglobulinaemia

<30 IU/ml Negative
30-50 IU/ml Weak Positive

Range verified in house