What is Antistreptolysin (ASO)?

Antistreptolysin-O (ASO) is one of the two hemolytic exotoxins produced by group A streptococci. When infected, a patient will produce an antibody, antistreptolysin-O, that is detectable in serum. The ASI ASO Slide Test provides a means of measuring these antibodies in serum and can assist in the determination of the severity of the infection. It is useful to detect patients with acute rheumatic fever or nephritis evolving following streptococcal infection.

Why Test for Antistreptolysin-O

The ASO test is used to document streptococcal infection including acute pharyngitis, tonsillitis, and scarlet fever. A marked rise in titer or a persistently elevated ASO titer indicates a recent or current infection with beta-hemolytic group A Streptococcus (Streptococcus pyogenes). Elevated titers are seen in 80% to 85% of patients with acute rheumatic fever and in 95% of patients with acute glomerulonephritis.

Strep antibodies become important after the patient has had multiple Strep A infections.


The ASI ASO Slide Test is a latex slide agglutination assay for the qualitative and the semiquantitative detection of Antistreptolysin-O (ASO) antibodies in human serum. No initial dilution of patient samples is required for this test. These materials are intended to be acquired, possessed and used only by health professionals. ASI’s ASO Slide Test is not a rapid test for Strep A infections.