Anti-streptolysin O or ASO is a blood test to measure antibodies against a toxin (Streptolysin O), produced by Group A Streptococci (GAS). GAS are responsible for diseases such as rheumatic fever, glomerulonephritis, bacterial endocarditis, and scarlet fever. It is a standard serological test for retrospective diagnosis of infection with Streptococcus pyogenes or Group A Streptococci. The ASO antibody may be detected in the blood for weeks or months after the primary infection has been eradicated. A titre of >200 IU is considered positive and indicates a previous infection with GAS. Since rheumatic fever can have long term sequelae (Rheumatic heart disease), antimicrobial prophylaxis is recommended for prevention. Joint pain (arthritis) is a feature of rheumatic fever.