Anti-streptococcal hemolysin is an antibody produced by the body after infection with group A streptococcus, and positive anti-streptococcal hemolysin in the blood suggests group A streptococcal infection. Clinically, anti-streptococcal hemolysin is often used in the diagnosis of rheumatic fever, acute glomerulonephritis and other diseases. 1. Rheumatic fever: Rheumatic fever is a recurrent connective tissue inflammation involving the whole body after infection with group A streptococcus, with arthritis and carditis being common. Anti-streptococcal hemolysin O test is most commonly used as an aid in the diagnosis of rheumatic fever, and most patients with rheumatic fever have anti-O antibodies of about 250 units. The test can also determine the activity of rheumatic fever, and the anti-O antibody of patients with active stage can reach 400 units. 2. Acute glomerulonephritis: Acute nephritis is often induced by streptococcal antecedent infection, so the anti-streptococcal hemolysin in the blood tends to increase, accompanied by a decrease in the level of complement C3. This is a key way to distinguish acute glomerulonephritis from other kidney diseases such as nephrotic syndrome. It should be noted that a positive anti-streptococcal hemolysin only indicates that the patient has been recently infected with group A streptococcus, which may only be manifested as pharyngitis, tonsillitis, or cellulitis of the skin, and does not necessarily mean that the patient is suffering from rheumatic fever or acute glomerulonephritis. Therefore, the diagnosis needs to be made in conjunction with the clinical manifestations as well as other investigations.