Antistreptolysin O (ASO) titer
Measures antibodies your body makes against a toxin from group A strep bacteria, which can show a strep infection in the recent past.
AI-verified against cited sources. Awaiting expert reviewAlso called ASO, ASOT, ASLOLOINC 5370-2
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What it is
Group A Streptococcus, the bacteria behind strep throat, makes a toxin called streptolysin O. About a week after an infection, the body starts making antibodies against it. The ASO test measures those antibodies, so it shows whether you were exposed to strep in the past weeks to months. It does not show that you have strep right now. Levels are often higher in school-age children, who catch strep more often.
Why it matters
Doctors mostly use it when someone has signs that could point to rheumatic fever or to inflammation of the kidney filters (post-streptococcal glomerulonephritis), which can appear weeks after strep throat or a skin infection. A raised or rising ASO supports a recent strep infection as the trigger. On its own it cannot diagnose either condition, and a normal level does not fully rule them out, especially after skin infections.
If it’s low
A low or normal ASO suggests you have not had a strep infection recently. Antibodies take about a week to rise, so a test taken very early can be low. After strep skin infections the ASO can stay low even though the infection was real, so a normal result does not rule out a past strep infection.
Common causes
- No recent group A strep infection
- Testing too early, before antibodies have had time to rise
- A strep skin infection, which may not raise ASO much
What can help
- If symptoms continue, ask whether a repeat test or an anti-DNase B test makes sense
If it’s high
A high ASO means your body has made antibodies to strep bacteria, most likely from an infection in the past few weeks to months. Levels can stay up for months after the infection is gone, and many healthy children and adults run a little high without any problem. A level that keeps rising on repeat tests tells your clinician more than a single high number. Your clinician will read it along with your symptoms.
Common causes
- A recent strep throat or other group A strep infection, including a skin infection
- An older infection, because antibodies can stay raised for several months
- Normal variation in school-age children, whose levels are often higher
- Less often, a falsely high result linked to liver disease (raised beta-lipoprotein) or a contaminated sample
- Uncommonly, a complication that follows a strep infection, such as rheumatic fever or post-streptococcal glomerulonephritis
What can help
- Ask whether a second blood test about 2 weeks later would show if the level is rising or falling
- Ask whether an anti-DNase B test would add information, especially after a skin infection
- Tell your clinician about any recent sore throat, skin infection, joint pain or dark urine
Usual ranges
What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.
For adults: 0–200 IU/mL. Source: Labcorp, Annals of Medicine and Surgery.
- 18+ (this one applies)0–200 IU/mL
Sources: Labcorp, Annals of Medicine and Surgery
What can affect your result
- Age: levels are usually highest in school-age children and lower in adults, so labs often use age-specific limits
- Time since infection: levels rise after about 1 week, peak after several weeks and fall over months
- Type of infection: strep skin infections raise ASO less than strep throat does
- How common strep is where you live: more exposure means higher background levels
- Assay method: turbidimetric and nephelometric assays can give quite different upper limits
- Liver disease or a contaminated sample can cause a falsely high result
- Very fatty (lipemic) or grossly hemolyzed samples can be rejected by the lab.
- Liver disease with raised beta-lipoprotein, or a contaminated serum sample, can cause a falsely high result with some assays.
- Different assay methods give different numbers, so compare results only from the same lab and method.
Antibodies start to rise about 1 week after a strep infection, peak about 2-5 weeks later, and often return to pre-infection levels within 6-12 months.
When to talk to a clinician
Contact a clinician promptly if, in the weeks after a sore throat or skin infection, you or your child develops painful or swollen joints, fever, chest pain, a new rash, jerky movements, swelling of the face or legs, or dark tea- or cola-colored urine. Call your local emergency number for chest pain or trouble breathing.
Questions you could ask
- How does my lab's age-specific range apply to me or my child?
- Does this fit a recent strep infection, and should we repeat the test in about 2 weeks?
- Would an anti-DNase B test add anything?
- Do any of my symptoms need other checks?
- If we still suspect a past strep infection, which other tests could help?
Questions people ask
Does a high ASO mean I have strep right now?
Not necessarily. ASO shows that your body met strep bacteria in the past weeks to months, and it can stay raised for months. Strep throat itself is usually found with a throat swab test. Your clinician looks at ASO together with your symptoms.
Why is the upper limit higher for children?
School-age children catch strep more often, so they carry more antibodies. Studies of healthy children often find the highest levels in the school-age and early teen years. Many labs print age-specific limits, and the limit also depends on the test method, so use the range on your own report.
How long does ASO stay high after an infection?
It starts rising after about a week, peaks a few weeks later, and usually falls back toward its starting level within 6 to 12 months if there is no new infection. That is why a repeat test about 2 weeks later can be more informative than a single result.
Show the science and sources
For clinicians and researchers
ASO measures neutralizing antibody to streptolysin O, an oxygen-labile hemolysin produced by group A streptococci. Labcorp runs it by latex immunoturbidimetry and reports IU/mL; other labs use nephelometry or latex methods, and WHO established an international standard for ASO in 1961. Antibody begins to rise about 1 week after infection and peaks about 2-3 weeks later (Labcorp) or 3-5 weeks after the illness (Testing.com). Without complications or reinfection it usually falls to pre-infection levels within 6-12 months (Labcorp). ASO is raised in about 85% of patients with acute rheumatic fever (Labcorp), and in more than 80% of acute rheumatic fever and about 95% of post-streptococcal glomerulonephritis cases (Testing.com), but it may stay normal after skin or renal sequelae, where anti-DNase B is usually the better choice. The upper limit of normal is conventionally the 80th percentile of the local population (not the central 95%) and depends on age, local strep exposure and assay. An Australian healthy-children study found 80% upper limits by age that differed sharply between a turbidimetric (Roche) and a nephelometric (Siemens) assay, for example 346 versus 249 IU/mL at ages 5-9 and 245 versus 426 IU/mL at ages 15-18, with the turbidimetric peak at ages 5-9 while the nephelometric values kept rising to age 18. The best evidence of recent infection is a 4-fold (two-dilution) or greater rise between an acute and a convalescent sample taken 2-3 weeks apart. Old reports may use Todd units, which this catalog does not convert.
LOINC 5370-2 (primary), 25788-1, 22568-0, 5371-0, 9788-1. Specimen: serum.
Sources
- Antistreptolysin O (ASO) Antibodies (test 006031), Labcorp (2023)
- Antistreptolysin O, Testing.com (formerly Lab Tests Online) (2018)
- Normal values of antistreptolysin O for adults ages 30 to 70 in Syria: A cross sectional study, Annals of Medicine and Surgery (2021)
- Streptococcal Serology Reference Intervals in an Australian Pediatric Cohort, Journal of the Pediatric Infectious Diseases Society (2025)
- International Standard for Antistreptolysin-O, Bulletin of the World Health Organization (1961)
- LOINC codes verified with the NLM Clinical Tables LOINC search (e.g. 5370-2), U.S. National Library of Medicine
AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.