The Anti-Streptolysin O (ASO) test is a blood test that measures the levels of antibodies in the blood against the streptolysin O enzyme produced by the bacteria Streptococcus pyogenes. This bacterium causes a wide range of infections, including strep throat, scarlet fever, and rheumatic fever. The ASO test is primarily used to help diagnose streptococcal infections, especially if the infection has spread to other parts of the body or if the symptoms persist after treatment with antibiotics. A high level of ASO antibodies in the blood indicates a recent or ongoing streptococcal infection. However, a negative result does not necessarily rule out the possibility of an infection, as it may take some time for the immune system to produce detectable levels of ASO antibodies after an infection. The ASO test is a simple and reliable test that can aid in the diagnosis and management of streptococcal infections, but it should be used in conjunction with other laboratory tests and clinical evaluation to confirm the diagnosis and guide treatment decisions.