Rheumatoid factor (RF) can be divided into five types: IgM, IgA, IgG, IgD and IgE (Note: in clinical internal medicine, it is described as type 4, without IgD; but in laboratory diagnostics, it is described as type 5), which is a class of autoantibodies against antigenic epitopes on the IgG FC fragment in the rheumatoid arthritis sera, and the IgM-RF is the main type of RF, and is also the RF that is routinely tested in the clinic. IgM-RF is the main type of RF, and is the most commonly detected RF in the clinic. rheumatoid factor-positive rheumatoid arthritis (RA) patients are more likely to have extra-articular manifestations, such as subcutaneous nodules and vasculitis, etc. The positivity rate of IgM-RF is 60%-78%. Detection method RF is found in serum of RA patients, it is an autoantibody with denatured IgG as the target antigen, mainly in serum and joint fluid of RA patients, it is an anti-denatured IgG antibody, belongs to IgM type. It can bind to IgGFc segment.RF-producing B-cell clones exist in RA patients and about 50% of healthy people, and RF can be synthesized in large quantities under the direct action of denatured IgG (or IgG bound to antigens) or EBV.The RF-producing cell clones in healthy people are fewer, and soluble factors secreted by mononuclear cells can inhibit the production of RF, so it is not easy to be measured in general. Determination of IgG, IgA, IgM RF class usually use ELISA indirect method, that is, heat agglutination of rabbit IgG coated reaction plate microtiter wells, add the test sample, and then add anti-human IgG, IgA, IgM enzyme-labeled antibody, so that the reaction and then color with the substrate. At present, many laboratories have carried out the application of turbidimetric method or ELISA method for quantitative detection of IgM-RF, the normal value range of the former is about > IgM-RF commonly used detection method is a semi-quantitative latex agglutination test, it is generally believed that the RF titer < 20 IU / ml. Clinical significance of the rheumatoid factor 1, RF is the diagnosis of rheumatoid arthritis is one of the classification criteria, is not the only criterion, is not unique to the RA It is not the only criterion, but not unique to RA; it is used as the differentiation criterion for seronegative spondyloarthropathies; it is 2% positive in normal people, and up to 5% positive in the elderly, and the RF positive rate can increase with age; RF has cross-reactivity with other proteins and semi-antigens; it arises only half a year after the disease, and those with extra-articular manifestations, such as interstitial lesions of the lungs, will have a high titer of RF. Persistent high titer RF suggests that RA is a serious disease with poor prognosis. 2, RF and a variety of diseases, can be summarized as "CHRONIC" (see below): (1) CH (Chronic diseases) chronic diseases, especially liver and lung diseases such as cirrhosis, chronic active hepatitis, diffuse interstitial pulmonary fibrosis, tuberculosis, macroglobulinemia, etc.; (2) R (Rheumatoid arthritis). Rheumatoid arthritis) RA; (3) O (Other rheumatic diseases) Other rheumatic diseases such as SLE, Scleroderma (SSc), Mixed Connective Tissue Disease (MCTD, Dry Syndrome (SS), Polymyositis/Dermatomyositis (PM/DM); (4) N (Neoplasms) Tumors; (5) I ( Infections) Infections such as AIDS, mononucleosis, parasitic infections, chronic viral infections, chronic bacterial infections (tuberculosis, subacute bacterial endocarditis, others); (6) C (Cryoglobulinemia) Cryoglobulinemia.