What is Anti-Streptolysin O?

Anti-streptolysin O (ASO) is an antibody often found in the blood of people who have suffered a streptococcus infection. A blood test, known as the anti-streptolysin O titre, is sometimes used to test for 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 suffered or are suffering a streptococcus infection. While testing for anti-streptolysin O is not considered effective for diagnosing a specific illness, it can help doctors determine if a patient is suffering from a streptococci-related disease.

People who have had a streptococcus infection in the last few months may still have the antibody anti-streptolysin O in their blood. People who have an active streptococcus infection may also have the antibody in their blood. Common streptococcus infections can include strep throat, rheumatic fever, scarlet fever, and bacterial endocarditis.

Streptococcus infections can cause serious complications, including inflammation of the membrane surrounding the heart. Streptococci bacteria can also cause serious infections of the kidneys. Rheumatic fever and scarlet fever are considered serious forms of streptococcus infection.

Testing for these antibodies with an anti-streptolysin O titre can help doctors diagnose a patient's symptoms. If the test indicates that streptococcus infection has occurred, then doctors may be 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 streptococcus infection, but they do not usually occur as a result of this infection.

Levels of anti-streptolysin O antibodies 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 usually 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 streptococcus infection. High levels of ASO in the blood don't necessarily mean that the infection is still present, however. They could mean that the person recently suffered from a streptococcus infection, but has recovered within the past few months.

If high levels of ASO are found in the blood and symptoms of continuing illness are present, further diagnostic tests may be needed. Physicians typically tailor the treatment for streptococcus infection according to the patient's specific illness and individual needs.