Antistreptolysin O (ASO)
Last updated: October 8, 2014
CPT Codes: ASO Screen 86063; ASO titer 86060
Description: ASO is antibody to an extracellular streptococcal product, hemolysin O.
Method: A rapid latex-agglutination slide test is currently used most commonly. Older methods used tube dilutions with RBC hemolysis. Todd units are expressed as the reciprocal of the last tube showing hemolysis. International units (IU) using newer rapid latex are equivalent to Todd units.
Normal Values: Normal values are <200 to 240 IU (varies by laboratory) using the latex method. With the older methodology, normal values are <240 Todd units in adults and 320 Todd units in children.
Increased in: With streptococcal infection, titers peak 4 to 5 weeks after infection (this is usually the second or third week after the onset of acute rheumatic fever). Titers are elevated in 80% of patients with acute rheumatic fever and are usually not increased after streptococcal skin infection. ASO may be non-specifically elevated in patients with hypergammaglobulinemia or with heightened immunologic activity.
Confounding Factors: Titers of ASO vary with age, season, and geography. Higher values are seen in children and in those living in crowded living conditions and in temperate climates. Serum contamination and cross-reactivity with muscle sarcolemma can rarely yield false-positive results.
Indications: ASO is useful along with other antistreptococcal antibodies such as anti-DNAse B, anti-DPNAse, antistreptokinase, and antihyaluronidase to demonstrate evidence of recent streptococcal infection.
Cost: ASO, $40–75; DNAse B, $50–75.
Comments: There is a 90% to 95% likelihood that at least one of three anti-streptococcal antibodies will be elevated in the setting of acute rheumatic fever.