The Anti-Streptolysin O (ASO) test is a blood test used to detect a recent infection caused by Streptococcus bacteria, which are responsible for conditions like strep throat. A positive ASO result indicates that you have had a recent strep infection. This test helps in diagnosing strep-related infections and monitoring the body's immune response. A decline in ASO levels suggests that the treatment is effective and the infection is resolving.