Rheumatoid factors are autoantibodies of IgM, IgG, IgA or even IgE class which recognise antigenic determinants on the Fc region of IgG. Remarkably, the exact nature of the antigenic determinants is still not recognised.Since the rheumatoid factor is detected in the presence of a vast excess of IgG in the serum, the antigen which is detected is often referred to as altered IgG. An alternative explanation is that the anti-IgG antibodies are of low affinity and are only detected on aggregated antigen (latex particles in our assay). Rheumatoid factor (usually IgM) is present in approximately 70% of patients with RA. Higher levels and the presence of IgG and IgA RFs correlate with more severe disease.The assay that we use is supposed to detect all classes of RF but probably detects mainly IgM. The presence of RF is not essential for the diagnosis of RA (so-called seronegative arthritis). RF also occurs in other autoimmune diseases (SLE, Scleroderma, Sjögrens) and in chronic infections (septicaemia, bacterial endocarditis)
Rheumatoid factor is thus a rather non-specific assay. Anti-CCP is much more sensitive and specific for rheumatoid arthritis.
Diagnosis of rheumatoid arthritis, differential diagnosis of rheumatic disease. However, the presence of rheumatoid factor does not mean rheumatoid arthritis. Patients with rheumatoid arthritis having high levels of rheumatoid factor generally have poorer prognosis, more severe progressive disease with greater joint and bone destruction and higher incidence of nodules and multisystem involvement. Rheumatoid factor levels are generally fairly constant in individual patients although during a long period of remission some reduction can occur.
The presence of rheumatoid factor does NOT mean rheumatoid arthritis . RFs are found in 80-90% patients with rheumatoid arthritis but also patients with
SLE (15-30%); Sjögrens syndrome (80-100%); MCTD (50-60%) and
systemic sclerosis (20-30%) amongst others.Patients with rheumatoid arthritis and high levels of rheumatoid factor generally have poorer prognosis, more severe progressive disease with greater joint and bone destruction and higher incidence of nodules and multisystem involvement. Rheumatoid factor levels are generally fairly constant in individual patients although during a long period of remission some reduction can occur. The levels do not in general correlate with disease activity.Low levels of rheumatoid factor are common in a wide range of autoimmune or infectious diseases especially those associated with hypergammaglobulinaemia including viral hepatitis, chronic liver disease, syphilis, sarcoidosis, leprosy, pulmonary fibrosis. High levels (> 400 IU/ml) are most common in Rheumatoid Arthritis and/or SjÃ¶grens syndrome especially associated with vasculitis or cryoglobulinaemia. High levels can also be found in bacterial endocarditis where they fall after effective treatment.
Serum Separator Tube (SST)
Increased levels of lipids (lipaemia), haemoglobin (haemiolysis), or the prescence of icterus in the assay sample may affect the assay result.
Assay range notes
5 – 7 days
Immunology The James Cook University Hospital