Anti-Streptolysin O Unit Converter

Anti-Streptolysin O (ASO) is an antibody your immune system produces in response to streptolysin O, a toxin released by Group A Streptococcus bacteria (the same bacteria behind strep throat and scarlet fever).

The ASO titer measures the level of these antibodies in your blood. It does not detect the bacteria themselves but rather confirms that your body mounted an immune response to a recent streptococcal infection.

ASO levels typically begin rising 1-3 weeks after a strep infection, peak at 3-5 weeks, and gradually return to baseline over 6-12 months. The test is most commonly ordered when a doctor suspects post-streptococcal complications such as rheumatic fever or glomerulonephritis.

ASO antibodies are produced by B lymphocytes and plasma cells of the immune system in response to streptolysin O, a hemolytic toxin released by virtually all strains of Group A Streptococcus and many strains of Groups C and G streptococci.