Anti streptolysin O (ASO) is an antibody that appears in the serum of the body after infection with hemolytic streptococci, and is one of the hemolytic exotoxins produced by group A beta hemolytic streptococci. ASO is one of the tests for rheumatic fever and glomerulonephritis, and it appears 2 weeks after streptococcal infection, peaks at 4-6 weeks and returns to normal at 8-10 weeks. ASO is increased in 80% of patients with rheumatic fever, often above 1:800. Patients with rheumatoid arthritis can have markedly elevated ASO, and the titer of ASO can increase progressively as the disease worsens, and can gradually decrease after recovery. Rheumatoid arthritis is essentially a reactive arthritis caused by streptococcal infection. The main manifestation is fever, which can be accompanied by sore throat and arthritis, mainly manifesting multiple large joints with wandering swelling and pain, as well as circumscribed erythema and subcutaneous nodules of the skin. In addition, increased ASO titers can be seen in acute and chronic pharyngitis, scarlet fever, impetigo, dengue, lymphangitis and other streptococcal infectious diseases, as well as in the following cases: contaminated or hemolyzed specimens being tested, patients with macroglobulinemia, hypercholesterolemia, positive rheumatoid factor, hepatitis, nephrotic syndrome, multiple myeloma, tuberculosis, connective tissue disease, subacute infectious endocarditis, and some allergic purpura. Therefore, ASO is not a specific test for rheumatoid arthritis. Clinically, patients with increased ASO should be diagnosed in conjunction with other conditions and with a differential diagnosis.