ASO (antistreptolysin O) test
Quick answer: An ASO test measures antibodies against streptolysin O, a substance made by group A streptococcus bacteria. Doctors usually order it to check for a recent strep infection when they suspect rheumatic fever or kidney inflammation after strep throat.
- SampleBlood
- TakesAbout 5 minutes
- Before the testNo special preparation is usually needed.
How the test is done
- 1A nurse confirms your details
- 2A vein inside your elbow is cleaned
- 3A needle draws a small tube of blood
- 4The lab measures antistreptolysin O in your serum
What your results can mean
Reading your report
Levels rise a few weeks after infection and normal levels differ by age, so discuss your result with your doctor.
If the result is outside the usual range
A raised or rising ASO can point to a recent group A strep throat infection, and supports a diagnosis of rheumatic fever or post-strep kidney inflammation. A raised level on its own does not mean you have arthritis.
Who usually has this test
Children and young adults with joint pain, fever, a new heart murmur, swelling or dark urine after a recent sore throat.
Risks and aftercare
A small bruise may form; children can return to school or play straight away.
Names and options
Also called: ASO, ASLO, antistreptolysin O titre
Listed by the hospital as
- ASLO (ANTI STREPTOLYSIN O)
Questions about this test
Is a high ASO a sign of arthritis?
Not by itself. ASO shows a recent strep infection, which can sometimes lead to rheumatic fever with joint pain. Many people with joint pain and a mildly raised ASO do not have rheumatic fever.
Does a high ASO need antibiotics?
A raised ASO alone usually does not need treatment. Your doctor decides based on your symptoms and examination.
When does ASO rise after a sore throat?
It usually starts to rise about a week after infection and peaks a few weeks later. Testing too early can miss it.
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This page explains each test in general terms so you know what to expect. It is not medical advice: your doctor decides which tests you need and what your results mean. Reference ranges differ between laboratories. Updated 4 November 2026.