Rheuma factor Screen ELISA is a solid phase enzyme immunoassay with highly purified Fc fragment of human immunoglobuline (IgG) for the combined quantitative detection of IgG, IgM and IgA rheumatoid factors (RF) in human serum. The assay is an aid in the diagnosis of rheumatoid arthritis (RA). Rheumatoid factors (RF), first described in 1940 as antibodies reacting with gamma globulins, are autoantibodies directed against the C-terminal part of the constant region of the IgG heavy chain, the IgG Fc. Although named after the disease they were initially associated with, RFs are found both in the healthy population and several diseases. The diseases commonly associated with high RF concentrations are rheumatoid arthritis (RA; 50-90%) and Sjögren`s syndrome (75-95%). They are also found in systemic lupus erythematosus (SLE; 15-35%), systemic sclerosis (20-30%) polymyositis / dermatomyosisits (5-10%), cryoglobulinemia (40-100%) and mixed connective tissue diseases (MCTD; 50-60%). Although the presence of IgM RF in the serum has been regarded as the most important serological indicator for RA, thus included in the ACR list of criteria for the diagnosis of this disease, RF of IgG and IgA subclass are important for diagnosis as well. Determination of these isotypes provides additional information with regard to diagnosis, differential diagnosis and follow-up of RA in comparison to conventional techniques such as latex agglutination test and nephelometry. Whilst RF of subclass IgM are most sensitive for diagnosis of RA, thus most suitable for screening, RF of subclass IgG are most specific for RA and like subclass IgA correlate with clinical parameters and disease activity. The presence of all three subclasses together is 100 % specific for RA. RF in SLE are associated with sicca syndrome, hypergammaglobulinemia, high titer of anti-nuclear antibodies, anemia and usually SS-A and SS-B antibodies appearance. All three subclasses are found in SLE. Especially the subclass IgA defines a subgroup of SLE patients characterized by distinct autoimmune phenomena and high disease activity in the absence of nephritis.