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RF因子 (Rheumatoid Factor)
       RA (RF) 因子 (類風濕性關節炎因子)是要檢查是不是罹患有類風濕性關節炎 (Rheumatoid Arthritis) 的檢驗，因為要檢查的是類風濕性因子
(RF; Rheumatoid Factor) 因此也叫做 RF 檢查。
      RA (RF) 因子是一種對人體本身的丙種球蛋白的一種蛋白成份所產生的自體抗體，因此在自體免疫疾病病人的血液中的RA因子濃度會增加，而在類風濕性關節炎的病人血液中呈示陽性的機率很高，約有百分之七十的類風濕性關節炎病人會有陽性反應，所以被叫做類風濕性關節炎因子，而這一檢驗也叫做類風濕性關節炎試驗。
       但約有百分之二十的類風濕性關節炎的病人會呈示陰性反應，這一種病人叫做 RA-陰性類風濕性關節炎。另一方面要注意的是，雖然是 RA
陽性，也並不一定是類風濕性關節炎，因為有些不是風濕性關節炎的疾病也會呈示陽性反應：如各種結締組織病(膠原病)－口乾眼燥症、硬皮症、皮肌炎及紅斑性狼瘡等以及慢性肝臟病時也有一部分病人會呈示陽性，甚至在一小部份(約 5 %)的正常人(尤其是老人)也會呈示輕度的陽性反應。在大約百分之七十五的類風溼性關節炎病人可以偵測到 IgM 型的類風濕性關節炎因子。有血管炎及皮下結節的活動性慢性類風溼性關節炎病人，RA 因子的效價都很高，另外在 80-90% 的口乾眼燥症病人也可測定到高效價的類風溼性關節炎因子，而血液中的脂質高的人也會呈示陽性。
做 RA 因子檢驗的主要目的是為了支持類風濕性關節炎的診斷。
參考值：< 1: 20；  定性：陰性 ；定量：< 40 IU/mL Rheumatoid Factor (RF)
       Rheumatoid factor (RF) is the autoantibody (antibody directed against an organism's own tissues) that is most relevant in rheumatoid arthritis. It is defined as an antibody against the Fc portion of IgG. RF and IgG join to form immune complexes that contribute to the disease process.
       Rheumatoid factor can also be a cryoglobulin (antibody that precipitates on cooling of a blood sample); it can be either type 2 (monoclonal IgM to polyclonal IgG) or type 3 (polyclonal IgM to polyclonal IgG) cryoglobulin.
      Rheumatoid factor can be of any isotype of immunoglobulins, i.e. IgA, IgG, IgM, IgE, IgD.
Testing
       RF is often evaluated in patients suspected of having any form of arthritis
even though positive results can be due to other causes, and negative results do not rule out disease. But, in combination with signs and symptoms, it can play a role in both diagnosis and disease prognosis. It is part of the usual disease criteria of rheumatoid arthritis.
     The presence of rheumatoid factor in serum can also indicate the occurrence of suspected autoimmune activity unrelated to rheumatoid arthritis, such as that associated with tissue or organ rejection. In such instances, RF may serve as one of several serological markers for autoimmunity.
Interpretation
      High levels of rheumatoid factor (in general, above 20 IU/mL, 1:40, or over the 95th percentile; there is some variation among labs) occur in rheumatoid arthritis (present in 80%) and Sjögren's syndrome (present in 70%). The higher the level of RF the greater the probability of destructive articular disease. It is also found in Epstein-Barr virus or Parvovirus infection and in 5-10% of healthy persons, especially the elderly.
      There is an association between rheumatoid factor and more persistently active synovitis, more joint damage and greater eventual disability.
       Rheumatoid factor may also be elevated in: chronic hepatitis, primary biliary cirrhosis, any chronic viral infection, bacterial endocarditis, leukemia, dermatomyositis, infectious mononucleosis, systemic sclerosis, and systemic lupus erythematosus (SLE).