ASO titre

Principle

• Measures antibodies to streptolysin O, an oxygen-labile haemolysin produced by group A (and some group C and G) streptococci.
• Classical neutralisation (Rantz–Randall): patient ASO neutralises a standard amount of reduced streptolysin O, preventing lysis of added rabbit or human group O red cells. Titre in Todd units is the highest dilution showing no haemolysis.
• Latex agglutination: streptolysin O-coated latex agglutinates with ASO; kits typically detect ≥200 IU/mL.
• Nephelometry or turbidimetry gives quantitative results in IU/mL.

Specimen

• Fresh serum, not haemolysed or lipaemic; refrigerate if delayed.

Results

• Typical adult upper limit about 200 IU/mL; values are higher in school-age children and vary by region and method. Local reference ranges apply.
• ASO begins to rise about 1 week after infection, peaks at about 3–5 weeks and falls over months.
• A rising titre in paired sera taken about 2 weeks apart is more informative than a single value.

Quality control and pitfalls

• Skin infections (impetigo) produce a weak ASO response; anti-DNase B is preferred after streptococcal skin infection.
• Early antibiotic treatment can blunt the rise.
• Hyperlipidaemic serum (β-lipoproteins inhibit streptolysin O), bacterial contamination and haemolysis can cause false results in the neutralisation method.
• Prozone may occur in latex tests; titrate strongly suspected samples.
• A raised ASO shows prior exposure, not active disease.

Clinical use

• Evidence of preceding streptococcal infection supporting diagnosis of acute rheumatic fever and post-streptococcal glomerulonephritis.