Antistreptolysin O (ASO) Unit Converter
Antistreptolysin O (ASO) Unit Converter
IU/mL and U/mL are the same number and IU/L is a thousand times larger — so the value of this page is the interpretation, not the arithmetic.
Antistreptolysin O (ASO) converter
IU/mL ⇄ IU/LASO titre 320 IU/mL
Formula and conversion factors
IU/mL = IU/L ÷ 1000
U/mL = IU/mL (numerically identical)
- IU
- an international unit defined against a WHO reference preparation of antistreptolysin O, not a mass or a molar quantity
- × 1000
- a litre is 1000 mL, so the only real conversion here is between per-millilitre and per-litre
- no molar unit
- the titre measures antibody activity against a biological standard, so there is no molecular mass and no molar conversion
- Todd units
- the older unit that IU/mL replaced; the two are broadly comparable in magnitude but are not formally interchangeable
Worked example
ASO titre 320 IU/mL
320 IU/mL = 320 IU/mL = 320 U/mL
320 × 1000 = 320,000 IU/L
Above the adult upper limit of 200 IU/mL, but at the upper limit for a school-age child
The three units
| Unit | Relationship | Example |
|---|---|---|
| IU/mL | The reporting convention almost everywhere | 320 |
| U/mL | Numerically identical to IU/mL | 320 |
| IU/L | IU/mL × 1000 | 320,000 |
Interpreting an ASO titre
| Question | Answer |
|---|---|
| When does it rise? | About a week after infection, peaking at three to five weeks |
| How long does it stay up? | Months, falling gradually — so a single value cannot date the infection |
| What is the adult upper limit? | Around 200 IU/mL |
| What is the limit in school-age children? | Higher — commonly up to about 320 IU/mL |
| Does a raised titre mean current infection? | No. It shows past exposure only |
| What is more useful than one value? | A paired sample two weeks apart, showing a rising or falling titre |
| What if the infection was in the skin? | Anti-DNase B is more sensitive; ASO responds poorly to pyoderma |
Why the interpretation, not the conversion, is the work
There is very little arithmetic on this page: IU/mL and U/mL are the same number, and IU/L is simply a thousand times larger, since a litre is a thousand millilitres. ASO is standardised in international units against a WHO reference preparation rather than by mass, so there is no molecular weight and no molar conversion to offer. The older Todd unit that IU/mL replaced is broadly comparable in magnitude but is not formally interchangeable. Everything difficult about an ASO result is in reading it.
The titre has a slow and characteristic time course. It begins to rise about a week after a streptococcal infection, peaks at three to five weeks, and then falls gradually over months. That means a single value cannot date an infection: a high titre is equally compatible with an illness three weeks ago and one four months ago, and a normal titre early in an illness simply means the antibody has not risen yet. Paired samples taken two weeks apart, showing a titre that is rising or falling, are far more informative than any single figure and are what should be requested when the timing matters.
The upper limit is age-dependent, and ignoring that is the commonest reason an ASO is over-interpreted. Healthy school-age children carry higher titres than adults through repeated pharyngeal exposure, so a value that would be clearly abnormal in an adult can sit within the expected range for a child of eight. Applying the adult limit across the board generates unnecessary anxiety and unnecessary referrals.
Finally, a raised ASO indicates past exposure, not current infection, and it is never a reason to treat in isolation. Its proper uses are as supporting evidence of preceding streptococcal infection within the revised Jones criteria for acute rheumatic fever, and in the assessment of post-streptococcal glomerulonephritis. In both settings it is measured alongside anti-DNase B, which is more sensitive after skin infection — ASO responds poorly to pyoderma, so a normal ASO after impetigo does not exclude a streptococcal cause.
Frequently asked questions
Is U/mL the same as IU/mL for ASO?
Yes, they are numerically identical and no conversion is needed. IU/L is a thousand times larger, so an ASO of 320 IU/mL is 320 U/mL and 320,000 IU/L.
Why is there no molar unit for ASO?
The result is an antibody activity standardised in international units against a WHO reference preparation, not a concentration of a defined molecule. There is no molecular mass to convert with, so no µmol/L figure exists.
Can a single ASO titre date a streptococcal infection?
No. The titre rises about a week after infection, peaks at three to five weeks and falls over months, so one value is compatible with a wide range of timings. Paired samples two weeks apart, showing a rising or falling titre, are much more informative.
Why do children have higher ASO titres?
Repeated pharyngeal exposure to group A streptococcus means healthy school-age children carry higher baseline titres than adults, commonly up to about 320 IU/mL. Applying the adult limit of around 200 IU/mL to a child is the commonest reason a result is over-interpreted.
When should anti-DNase B be requested instead?
After skin infection. ASO responds poorly to pyoderma, so a normal ASO does not exclude a streptococcal cause following impetigo. Anti-DNase B is more sensitive in that setting and the two are often measured together.
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References
- Gewitz MH, Baltimore RS, Tani LY, et al. Revision of the Jones Criteria for the diagnosis of acute rheumatic fever in the era of Doppler echocardiography: a scientific statement from the American Heart Association. Circulation. 2015;131(20):1806–1818.
- Shet A, Kaplan EL. Clinical use and interpretation of group A streptococcal antibody tests: a practical approach for the pediatrician or primary care physician. Pediatr Infect Dis J. 2002;21(5):420–426.
- Sen ES, Ramanan AV. How to use antistreptolysin O titre. Arch Dis Child Educ Pract Ed. 2014;99(6):231–238.