Antistreptolysin O Titer (ASO titer) is a Blood Test that measures Antibodies against streptolysin O, a Toxin produced by Group A Streptococcus bacteria, to help diagnose Past infections and Related complications such as Rheumatic Fever and glomerulonephritis. Elevated ASO titers suggest a Recent streptococcal infection, but they are not definitive for Current infection and should be interpreted alongside Clinical Findings and other Diagnostic Tests.