The antistreptolysin O (ASO) titer is used in the diagnosis of:

The antistreptolysin O (ASO) titer is the most commonly used streptococcal antibody test to establish a recent streptococcal infection and acute rheumatic fever (ARF). An ASO titer of 240 Todd units or higher in adults or 320 Todd units or higher in children is considered modestly elevated. ASO titers above 500 Todd units are uncommon in healthy individuals and therefore would serve as evidence of a recent streptococcal infection. Because ASO titers can be normal in approximately 20% of ARF patients, other streptococcal antibody tests can be used to establish a recent infection; these include anti-deoxyribonuclease B (anti-DNase B), antistreptokinase, and anti-hyaluronidase. Ref: Jaggi P., Shulman S.T. (2013). Chapter 52. Rheumatic Fever. In J.B. Imboden, D.B. Hellmann, J.H. Stone (Eds), CURRENT Diagnosis & Treatment: Rheumatology, 3e.