A ASO titer blood test is a test that measures the level of antibodies to streptococcal bacteria in a person's blood. Streptococcal bacteria are a common cause of infections such as strep throat and scarlet fever. The test is used to determine whether a person has been infected with streptococcal bacteria in the past, and if so, whether their body has been able to fight off the infection or if the bacteria are still present and causing symptoms. The test results are reported as a numerical value, called the titer, which reflects the concentration of streptococcal antibodies in the blood. A high titer indicates a strong immune response to the bacteria, while a low titer may indicate a past infection that has since been cleared by the body or a chronic, ongoing infection. The test is often used in conjunction with other tests to diagnose and monitor streptococcal infections.