Anti-streptolysin O (ASO)
An antibody the body makes after an infection with group A streptococcus (the 'strep' behind many sore throats). A raised or rising level is evidence of a recent strep infection, used mainly to support a diagnosis of its immune complications — rheumatic fever and a form of kidney inflammation.
What it measures
Anti-streptolysin O is an antibody the immune system produces against streptolysin O, a toxin made by group A streptococcus — the bacterium that causes strep throat, scarlet fever and some skin infections. The test does not diagnose an active infection (a throat swab does that); instead, a raised or rising ASO level is evidence that a strep infection happened recently, typically in the past few weeks. Its main value is in investigating the delayed, immune-mediated complications of strep infection, which appear a week or more after the original infection has settled. The two classic ones are acute rheumatic fever — an inflammatory reaction that can affect the joints, heart and nervous system — and post-streptococcal glomerulonephritis, a kidney inflammation. Because a single ASO level can be raised simply from a past exposure, the most useful pattern is a rising level on paired samples taken a couple of weeks apart, or a clearly high level in the right clinical setting. It is often measured alongside a second antibody, anti-DNase B, to improve the chance of confirming recent strep. So ASO is a look-back test, confirming that strep was the trigger behind a suspected post-streptococcal illness.
What a high value can mean
- A recent group A streptococcal infection — the level rises a week or two afterwards.
- Acute rheumatic fever — supports the diagnosis alongside the clinical picture.
- Post-streptococcal glomerulonephritis — a kidney inflammation following strep.
- A rising level on paired samples — the most convincing evidence of recent infection.
- A raised level from past exposure — common and not necessarily significant on its own.
What a low value can mean
- Normal or low — no evidence of recent strep infection.
- A skin (rather than throat) strep infection — ASO can be less reliably raised, so anti-DNase B may be more useful.
- Very early in an illness — the antibody may not have risen yet; a repeat sample can help.
When to discuss with a doctor
ASO is not a test for a current sore throat — a throat swab or rapid strep test is used for that. Instead it is requested when a doctor is investigating a possible complication of a recent strep infection, such as new joint pains, a heart murmur or signs of rheumatic fever, or blood and protein in the urine suggesting post-streptococcal kidney inflammation. A raised or, better, a rising ASO supports that a strep infection was the trigger, and is interpreted with the clinical picture and often a second antibody test. A high ASO on its own, without symptoms, usually just reflects a past infection and needs no action. Mediora.AI shows ASO alongside inflammatory markers so the post-strep pattern is visible; the interpretation and any diagnosis belong with your doctor.