The Antistreptolysin O (ASO) test is a blood test that measures antibodies your body makes against Streptolysin O, a toxin from Group A Streptococcus bacteria (like strep throat). Doctors order it to check for recent strep infections, especially when complications like rheumatic fever or glomerulonephritis (kidney issues) are suspected, as these can follow untreated strep infections. Elevated ASO levels show a recent or past strep infection, even if it was asymptomatic, helping diagnose delayed problems