A serological test to measure the concentration (titer) of Antistreptolysin O (ASO) antibodies in blood serum. Streptolysin O is an enzyme produced by Streptococcus pyogenes bacteria (Group A Streptococcus). This test is used as serological evidence of a previous infection with this bacteria and helps diagnose non-suppurative complications that follow it, such as rheumatic fever and post-streptococcal glomerulonephritis.