Antistreptolysin O (ASO) Antibodies

Antistreptolysin O (ASO) Antibodies, also known as ASO or Antistreptolysin O titer, is used to substantiate recent infection with group A Streptococcus and to support the diagnosis of poststreptococcal conditions, including acute rheumatic fever and poststreptococcal glomerulonephritis. Serial ASO measurements help demonstrate a characteristic rise after infection and a fall with convalescence and are used to gauge response to therapy. A pronounced increase, or a sustained elevation over time, indicates recent streptococcal infection or related immune-mediated sequelae.

ASO antibodies recognize streptolysin O, a cytolytic toxin produced by group A beta-hemolytic streptococci (Streptococcus pyogenes), pathogens associated with pharyngitis, scarlet fever, and cutaneous infections. Following infection, ASO titers start to rise at about one week, reach maximum concentrations at 3–6 weeks, and usually return toward baseline over 6–12 months. Elevated ASO levels are present in most patients with acute rheumatic fever. By contrast, titers may be low or undetectable after streptococcal skin infections or when renal involvement is present; in these contexts, anti–DNase B testing may provide greater diagnostic yield.