Anti-streptolysin O (ASO) is an antibody that is often found in the blood of people who have had a strep infection. A blood test, known as the anti-streptolysin O titer, is sometimes used to detect the presence of this antibody in the blood. This test is considered useful because certain health conditions, such as rheumatic fever, usually only occur in people who have recently had or are suffering from a strep infection. Although the antistreptolysin O test is not considered effective in diagnosing a specific disease, it can help doctors determine if a patient has a strep-related disease.
People who have had a strep infection in the past few months may still have anti-streptolysin O antibody in their blood. People who have an active strep infection may also have the antibody in their blood. Common strep infections can include strep throat, rheumatic fever, scarlet fever, and bacterial endocarditis.
Strep infections can cause serious complications, including inflammation of the membrane that surrounds the heart. Strep bacteria can also cause serious kidney infections. Rheumatic fever and scarlet fever are considered serious forms of streptococcal infection.
Testing for these antibodies with an anti-streptolysin O titer can help doctors diagnose a patient's symptoms. If the test indicates that a strep infection has occurred, then doctors are better able to diagnose scarlet fever, rheumatic fever, or infection of the heart or kidneys. Some conditions, such as rheumatoid arthritis, can cause symptoms similar to those of a strep infection, but do not usually occur as a result of this infection.
Anti-streptolysin O antibody levels in the blood are measured by taking a small blood sample, usually from a vein in the hand or arm. Normal blood levels of anti-streptolysin O are generally no higher than 160 units per milliliter of blood. When a person has high levels of ASO, doctors may consider the possibility of an ongoing strep infection. However, high levels of ASO in the blood do not necessarily mean that the infection is still present. They could mean that the person recently had a strep infection but has recovered in the last few months.
If high levels of ASO are found in the blood and there are symptoms of ongoing disease, further diagnostic testing may be needed. Doctors often tailor treatment for strep infection based on the patient's specific illness and individual needs.