Streptolysin O (SLO) is a toxic immunogenic protein secreted by most Streptococcus pyogenes group A strains Streptolysin O acts as an immunogen to increase the patients’ immune response in an infected individual. 

Clinically, elevated levels of anti-SLO antibodies (ASO) in serum have been shown to be associated with streptococcal diseases such as glomerulonephritis and rheumatic fever. A rise begins about 1 week after infection and peaks at 2-4 weeks. In the absence of complications or re-infection, the ASO concentration will usually fall to pre-infection levels within 6-12 weeks. 

Normal serum levels of ASO are usually no higher than 160 IU/ml. The person may be considered having the possibility of an ongoing streptococcus infection if the ASO level is over 160 IU/ml. The ASO tests are meaningful for streptococcus infection diagnosis.