ASO Titre (Antistreptolysin O Titre)
Sample Type: Quantitative (Serum)

Overview
The ASO titre (Antistreptolysin O titre) is a blood test that measures the amount of antibodies produced against streptolysin O, a toxin released by Group A Streptococcus (Streptococcus pyogenes). It is used to determine if a recent streptococcal infection has occurred, especially in cases where complications are suspected rather than current infection.

Purpose of the Test
Detect recent or past infection with Group A Streptococcus

Diagnose post-streptococcal complications, including:

Rheumatic fever

Post-streptococcal glomerulonephritis

Support differential diagnosis in unexplained joint pain, fever, or cardiac symptoms

Mechanism
After a streptococcal infection, the body produces antibodies against streptolysin O, a hemolysin produced by the bacteria. These antibodies can be measured quantitatively in the serum to determine the level of immune response.

Normal and Elevated Values
Normal ASO titre: < 200 IU/mL (adults), < 400 IU/mL (children)

Elevated ASO titre indicates:

Recent streptococcal infection (usually within the past 2–4 weeks)

May remain elevated for months even after infection resolves

Note: One single ASO titre is less informative than paired tests taken 2 weeks apart, showing rising or falling levels.

Interpretation
ASO Titre Level Clinical Meaning
Normal No recent streptococcal infection
Elevated Recent streptococcal infection or post-streptococcal complication
Rising titres Ongoing or recent immune response

Clinical Relevance
Rheumatic Fever: Often diagnosed with the help of an elevated ASO titre as per the Jones Criteria

Post-Streptococcal Glomerulonephritis: ASO titres help confirm a prior streptococcal throat or skin infection

Tonsillitis/Pharyngitis: ASO titre is not usually needed unless complications are suspected

Limitations
Cannot identify a current infection—it only detects past immune response

Does not confirm site of infection (e.g., skin vs throat)

False positives can occur in healthy carriers or those with unrelated illnesses

Complementary Tests
Anti-DNase B test (another streptococcal antibody)

Throat culture or rapid antigen test (for active infection)

ESR/CRP (inflammation markers)

Renal function tests (for glomerulonephritis)

Conclusion
The ASO titre test is a valuable diagnostic tool in detecting a recent streptococcal infection and in supporting the diagnosis of post-infectious complications such as rheumatic fever or glomerulonephritis. Quantitative measurement helps monitor changes in antibody levels over time, providing insights into the patient’s immune response and infection timeline.