ASO titre test explained (antistreptolysin O)
- Updated
- 2026-09-10
- Every number with its source
What antistreptolysin O is
Antistreptolysin O is an antibody that the immune system produces in response to streptolysin O — a toxin made by group A streptococcus bacteria (Streptococcus pyogenes). These bacteria cause sore throat (streptococcal pharyngitis), scarlet fever and skin infections. The amount of antibody in the blood can still be detectable weeks or months after the infection itself has passed, which is why the test is useful for establishing an infection that is already over, rather than for detecting one during the acute period. The ASO test is most often done when a recent streptococcal infection is suspected of having caused a late complication — rheumatic fever, post-streptococcal glomerulonephritis or, in rarer cases, endocarditis. A single test is often not enough, so a doctor may ask for it to be repeated after a few weeks to see whether the antibody level is rising or falling — this helps to tell a recent infection from an old one.
When the ASO test is done
- When joint pain, swelling, changes in the heartbeat or a skin rash appear after an unexplained fever or a sore throat — where rheumatic fever is suspected
- When dark urine, swelling or raised blood pressure appear after a recent streptococcal infection — where kidney damage (glomerulonephritis) is suspected
- Where an infection of the heart valves (endocarditis) is suspected after a streptococcal infection
- When it needs to be confirmed that symptoms (for example in the joints or the kidneys) are linked to a recent rather than a current streptococcal infection
- To follow how treatment is going — repeating the test after a few weeks, so that the change in the antibody level can be seen
How to prepare
- Do not eat for 6 hours before the test
- No other special preparation is usually needed, but it is worth telling your doctor if you have recently had a sore throat or another infection
Reference range
The limits of the normal range depend on the method used by the particular laboratory, so the exact interval is the one printed on your result sheet. In general, a negative or low result means that no signs of a recent streptococcal infection were found, while a raised result can be linked to conditions such as rheumatic fever, glomerulonephritis, endocarditis or guttate psoriasis.
A raised (positive) ASO: what it can mean
A raised (positive) ASO result shows that there has recently been an infection caused by group A streptococcus, even if there were no symptoms. The antibody can still be detectable in the blood weeks or months after the infection itself has passed, so the test is often used to confirm an infection that is already over rather than to diagnose an acute one.
- A recent group A streptococcal throat infection (streptococcal pharyngitis)
- Scarlet fever or another streptococcal skin infection
- Rheumatic fever as a complication of a streptococcal infection
- Post-streptococcal glomerulonephritis (inflammation of the kidneys)
- In rarer cases — an infection of the heart valves (endocarditis) linked to streptococci
- Guttate psoriasis linked to a streptococcal infection
Points usually weighed alongside a raised ASO result:
- Whether there has recently been a throat or skin infection — it is worth recalling recent symptoms, since the antibody can stay detectable for weeks or months after the infection
- Repeating the test after a few weeks, to assess whether the antibody level is rising or falling
- An assessment of the heart, the joints and the kidneys, if a complication is suspected
- Tests done alongside, for example anti-DNase B or a streptococcal culture, for a more precise picture
Full site in Lithuanian: proactiva.app
Frequently asked questions
›What does a raised ASO mean?
Per MedlinePlus, a raised (positive) ASO result means that there has recently been an infection caused by group A streptococcus, even if there were no symptoms. It can be linked to rheumatic fever, post-streptococcal glomerulonephritis or, rarely, endocarditis. The precise assessment comes from a doctor, together with other tests.
›Why is the ASO test often repeated?
The antibody can still be detectable in the blood weeks or months after the infection itself has passed. A single test is often not enough — repeating it after a few weeks shows whether the level is rising (an active process) or falling (the infection is already passing).
›Does ASO show a current infection?
Not always. ASO antibodies rise after an infection, so the test is more often used to identify a streptococcal infection that has already passed, or its complications, rather than to diagnose an acute throat infection — for that, a rapid strep test or a culture is usually used.
›When does a doctor order an ASO test?
Most often when joint pain, heart symptoms, dark urine or swelling appear after an unexplained fever or a recent throat infection — to assess whether this is linked to a streptococcal infection and its complications.
›Do I need to fast before an ASO test?
MedlinePlus says not to eat for 6 hours before the test. Other special preparation is usually not needed.
›What is the normal range for the ASO test?
The limits of the normal range depend on the method used by the particular laboratory, so the exact interval is given on the result sheet. In general, a negative or low result means no signs of a recent streptococcal infection were found, while a raised result can be linked to conditions such as rheumatic fever, glomerulonephritis, endocarditis or guttate psoriasis.
Sources
Related markers
This site is not an emergency service. If severe chest or head pain, breathlessness, confusion, difficulty speaking, heavy bleeding or another acute problem has come on suddenly — do not wait for tests, call 112.
This information is not a diagnosis and does not replace a consultation with a doctor. Reference ranges depend on the laboratory, your age and sex — always compare your result with the range printed on your own report.