ASO Titre (Anti-Streptolysin O)
Also known as: ASO titre, Anti-streptolysin O, ASOT, ASO test, ASO titre zyada
The anti-streptolysin O (ASO) titre measures antibodies against a toxin made by group A streptococcus, the germ that causes 'strep throat'. It shows a recent strep infection, not a current one.
Why it's ordered
Suspected complications after a strep infection, such as rheumatic fever or kidney inflammation (post-streptococcal glomerulonephritis).
Preparation
No preparation needed.
Typical adult values
| Upper limit (adults) | often about 200 IU/mL; higher in children; lab- and age-specific |
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Ranges vary between labs and methods. Always use the range printed on your report.
If it's high
A high ASO means a strep infection in the past few weeks. ASO rises 1–3 weeks after the infection and peaks at about 3–5 weeks, so a rising value on a repeat test is more meaningful than one result. A high ASO alone does not mean rheumatic fever.
If it's low
A normal ASO makes a recent strep infection less likely, but it does not rule out all strep complications.
Good to know
Rheumatic fever is diagnosed with the Jones criteria (symptoms, heart findings, ECG and tests), not by ASO alone. Many children have a high ASO after ordinary sore throats; joint pain with a high ASO should be assessed by a doctor before starting long-term penicillin.
Common questions
Does a high ASO mean rheumatic fever?
No. A high ASO alone does not mean rheumatic fever; it is diagnosed with the Jones criteria.
What is a normal ASO titre?
The upper limit is often about 200 IU/mL in adults, and higher in children. Always use the range printed on your report.
Why repeat the ASO test?
ASO rises 1–3 weeks after the infection and peaks at about 3–5 weeks, so a rising value is more meaningful than one result.
Should a child with joint pain and high ASO start penicillin?
Joint pain with a high ASO should be assessed by a doctor before starting long-term penicillin.