Anti-streptolysin O (ASO) is an antibody that appears in the serum of the body after infection with group A hemolytic streptococci. It is one of the hemolytic exotoxins produced by group A, type B hemolytic streptococci, and is a protein with enzymatic activity that can lyse red blood cells and destroy white blood cells and platelets. ASO is one of the tests for acute rheumatic fever, glomerulonephritis. It appears 2 weeks after streptococcal infection, peaks at 4-6 weeks, and returns to normal at 8-10 weeks. The titer correlates with geographic location and age. 80% of patients with rheumatic fever have an increased ASO, 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 this antibody can gradually decrease after recovery. Rheumatoid arthritis is essentially a reactive arthritis caused by streptococcal infection. In addition, increased ASO titers are also seen in acute and chronic pharyngitis, scarlet fever, impetigo, dengue, lymphangitis and other streptococcal infections, and in the following cases: contaminated or hemolyzed specimens, patients with macroglobulinemia, hypercholesterolemia, rheumatoid factor positivity, hepatitis, nephrotic syndrome, multiple myeloma, tuberculosis, connective tissue disease, subacute infective endocarditis and some allergic purpura, among others. Therefore, ASO is not a specific test for rheumatoid arthritis. The differential diagnosis of patients with increased ASO should be made clinically in the context of other conditions.