Rheumatoid factor is one of the criteria for the classification of rheumatoid arthritis, but it is not the only criterion and is not specific. It can also be used as a differentiating criterion for seronegative spondyloarthropathies. Rheumatoid factor is cross-reactive with other proteins and semi-antigens, and persistent high-valent rheumatoid factor is produced only six months after the disease. The higher the potency of rheumatoid factor, the higher the specificity of rheumatoid arthritis diagnosis, but rheumatoid factor can also be found in other autoimmune diseases and infectious diseases, such as bacterial endocarditis and tuberculosis. In addition, non-infectious diseases such as diffuse interstitial lung fibrosis, liver cirrhosis, chronic active hepatitis, and nodular disease can also be positive for rheumatoid factor.