The ASO titer blood test measures the level of antibodies in the blood that are produced in response to a bacterial infection called Streptococcus pyogenes. These antibodies are known as anti-streptolysin O (ASO) antibodies.

When the body is infected with Streptococcus pyogenes, it produces ASO antibodies to help fight the infection. After the infection has been cleared, the level of ASO antibodies in the blood gradually decreases. The ASO titer blood test is used to measure the level of ASO antibodies in the blood and to determine whether someone has had a recent streptococcal infection.

The ASO titer blood test is most commonly used to diagnose a recent streptococcal infection that can cause diseases such as strep throat, scarlet fever, and rheumatic fever. The test can also be used to monitor the effectiveness of treatment for a streptococcal infection and to detect any recurrence of the infection.

Normal levels of ASO antibodies in the blood can vary depending on the laboratory and the testing method used. Generally, a low ASO titer is considered normal, while a high ASO titer indicates a recent or ongoing streptococcal infection. The test is not specific to a particular disease, so it cannot be used to diagnose any particular condition on its own. The ASO titer blood test is often used in combination with other tests, such as a throat culture, to diagnose a streptococcal infection.