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ASO (anti-streptolysin O)
Antibodies proving a recent streptococcal infection — used when rheumatic fever or post-streptococcal complications are suspected.
Other names: Anti-streptolysin O titer, ASLOSection: Immunology
Updated: August 31, 2026·MedProtect editorial team Reference information — not a diagnosis or a prescription
In shortASO is an antibody against streptolysin O, a toxin of group A streptococcus. A rising or high titer confirms a recent streptococcal infection and supports the diagnosis of rheumatic fever or post-streptococcal glomerulonephritis. A single elevated value without symptoms needs no treatment.
Reference ranges
Group
Reference range
Result — typical range (international reference, not a national standard)
negative / low titer by the laboratory's reference
MedlinePlus describes the test qualitatively; cutoffs vary by method and age — the report's reference applies. The trend over 2–4 weeks matters more than a single value. Reference ranges vary between laboratories and assays — use the range printed on your own lab report.
Check your value
Enter your result — we will show how it compares with a typical range. Match the units of the selected row.
Approximate comparison with a typical range, not a diagnosis. Reference intervals vary by laboratory and method — check your report; result interpretation is done by a doctor.
What this marker is
After a streptococcal throat infection, ASO rises over 3–8 weeks and then slowly falls. The test answers a retrospective question — was there a streptococcal infection? — rather than diagnosing an active one.
Doctors order it when complications are suspected: joint pains and carditis after a sore throat (rheumatic fever), or dark urine and edema (post-streptococcal glomerulonephritis).
When it is high
Recent streptococcal infectionThe expected and most common cause of an elevated titer.
Rheumatic feverA high or rising titer supports the diagnosis alongside the clinical criteria.
Post-streptococcal glomerulonephritisKidney inflammation 1–3 weeks after infection, with edema and dark urine.
When it is low
Expected findingMost people without a recent streptococcal infection have low titers.
What can skew the result
These factors can shift the value without any disease — worth considering before a retest:
For the trend — a second sample 2–4 weeks after the first; a rising titer is more informative than a single number.
Who usually reviews this result
A result outside the reference range is usually reviewed by: your GP or primary care doctor; a rheumatologist or nephrologist for complications. Bring your lab report and any previous results to the appointment.
Names used on lab reports
Your report may use any of these names: ASOASLOAnti-streptolysin O
Units and conversion
Measured in: IU/mL
IU/mL=kIU/L
1 IU/mL = 1 kIU/L — the number stays the same. Results are rounded to 2 decimal places.
Several markers on your report?
See how MedProtect explains results in the app: the whole report, a check of the recognized values, and a clear breakdown.
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