This test measures the amount of antistreptolysin O (ASO) in the blood. ASO is an antibody targeted against streptolysin O, a toxin produced by Group A streptococcus bacteria. ASO and anti-DNase B are the most common of several antibodies that are produced by the body's immune system in response to a Group A streptococcal infection.

Group A streptococcus (Streptococcus pyogenes), is the bacterium responsible for causing strep throat. In most cases, strep infections are identified, treated with antibiotics, and the infections resolve. When a strep infection does not cause identifiable symptoms, goes untreated, or is treated ineffectively, however, post-streptococcal complications (sequelae), namely rheumatic fever and glomerulonephritis, can sometimes develop, especially in young children. These secondary conditions have become much less prevalent in the U.S. because of routine strep testing, but they still do occur. They cause symptoms such as fever, fatigue, shortness of breath, heart palpitations, decreased urine output, and bloody urine. They can damage the heart and/or cause acute kidney dysfunction, leg swelling (edema), and high blood pressure (hypertension). Because these symptoms may also be seen with other conditions, the ASO test can be used to help determine if they are due to a recent Group A strep infection.

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