An antistreptolysin-O (ASO) test is a blood test that measures the amount of ASO in the blood. ASO tests are generally used to diagnose and monitor the treatment of a strep infection. A series of tests every 10 to 14 days for six weeks is often required to confirm diagnoses and monitor the progress of an infection.

ASO is an antibody generated in response to infection by a specific type of bacteria called Group A streptococci. A serious strep infection will cause ASO levels to rise significantly. Therefore, a physician may order an ASO test to determine whether the person has a condition (e.g., endocarditis or rheumatic fever) related to a serious strep infection.