Antistreptolysin

Antistreptolysin refers to antibodies directed against streptolysin O, a toxin produced by group A Streptococcus (Streptococcus pyogenes). Streptolysin O is a cholesterol-dependent cytolysin that damages cell membranes and contributes to virulence; it is oxygen-labile, in contrast to streptolysin S. Following a streptococcal infection, the immune system produces antibodies against streptolysin O, known as antistreptolysin O (ASO). The presence of ASO in serum is used as a marker of recent Streptococcus pyogenes infection, especially when clinical history or signs of rheumatic fever or glomerulonephritis are present. The antistreptolysin O titer typically rises 1–3 weeks after infection, peaks around 3–6 weeks, and may remain elevated for several months. It is less useful for distinguishing current infection because many people have baseline ASO titers; serial testing showing rising titers is more informative. Normal reference ranges vary by age and assay; in adults, upper limits are commonly around a few hundred international units per milliliter, with lower thresholds in children.

Testing is performed by various immunochemical methods, including turbidimetric, nephelometric, or agglutination assays, and sometimes ELISA. High ASO titers may be seen after scarlet fever, streptococcal pharyngitis, or skin infections, and can be associated with rheumatic fever and post-streptococcal glomerulonephritis. Interpretation should consider clinical context, prior infections, and vaccination status if relevant.