Anti-streptolysin O (ASO) is the main antibody produced by the body in response to a group A beta-hemolytic streptococcal infection (strep A). This bacteria is responsible for many throat infections and usually responds well to antibiotic treatment. If a strep A infection is not diagnosed or is poorly treated, the toxin produced by the bacteria (streptolysin O) can lead to complications such as acute rheumatism or glomerulonephritis (kidney inflammation). ASO antibodies develop one week to one month after a strep A infection and peak about 4 to 6 weeks after onset of the disease and then decrease, but may remain detectable for several months after recovery.
A negative or very low result, especially if repeated 10 to 14 days later, indicates that the person probably did not have a recent strep A infection, but there are rare exceptions. If the concentration is high or increasing, a recent strep A infection is very likely. An ASO blood test cannot be used to predict whether complications will occur, or to predict the type and severity of the disease. If symptoms of rheumatic fever or glomerulonephritis are present, a high ASO titre will help confirm the diagnosis.