Antistreptolysin O (ASO) is an antibody that the body produces in response to a streptococcal infection. A quantitative ASO test measures the level of ASO antibodies in the blood, which can help diagnose a recent or past streptococcal infection. The test involves taking a blood sample from the arm and sending it to a laboratory for analysis. The results of the test are reported in units per milliliter (U/mL). Normal ASO levels vary depending on the laboratory's reference range, but they are generally less than 200 U/mL for adults and less than 300 U/mL for children. Higher levels of ASO in the blood may indicate a recent or past streptococcal infection, such as strep throat or scarlet fever. Note that not all streptococcal infections lead to elevated ASO levels, and other factors such as age, medications, and other medical conditions can affect ASO levels. If a streptococcal infection is suspected based on symptoms such as fever, sore throat, or skin rash, the doctor may order an ASO test to help confirm the diagnosis. If an infection is confirmed, the doctor may prescribe antibiotics to treat the infection and prevent complications.