An **Anti-Streptolysin O (ASO) Test** is a diagnostic blood test used to detect a recent or past infection caused by **Group A Streptococcus** bacteria, such as strep throat or scarlet fever.
The test measures the level of ASO antibodies, which the body produces to neutralize streptolysin O, a toxic enzyme released by the bacteria.
Elevated or rising ASO levels are crucial for diagnosing post-streptococcal complications that arise weeks later, most notably **rheumatic fever** and **post-streptococcal glomerulonephritis** (kidney inflammation). It helps clinicians confirm a bacterial trigger when active symptoms of the initial infection have subsided.