When does the ASO titre peak after a streptococcal infection?

ASO titres begin to rise one to three weeks after infection, peak at three to five weeks, and decline over months. The test is most informative when drawn during the convalescent phase, not during acute pharyngitis.

Can ASO be normal in someone with post-streptococcal disease?

Yes. ASO may be normal or low if the specimen is drawn too early, if the infection was cutaneous (which produces a weaker ASO response), or if early antibiotics blunted the rise. Anti-DNase B should be added in these cases.

Is fasting required for an ASO test?

No. However, severely lipaemic serum can spuriously elevate the result in nephelometric and turbidimetric assays, so practitioners may consider fasting if the patient has known hyperlipidaemia.

Why is a rising ASO titre more useful than a single value?

A fourfold rise between acute and convalescent specimens (two to four weeks apart) is more specific for recent infection than a single elevated titre, which may reflect past exposure or high baseline levels in populations with endemic streptococcal disease.

Does an elevated ASO mean the patient needs antibiotics?

No. ASO reflects past infection and immune response, not active infection requiring treatment. Acute pharyngitis is diagnosed by culture or rapid antigen test, and antibiotics are given during the acute phase to prevent complications, not based on ASO serology weeks later.

What is the role of ASO in diagnosing acute rheumatic fever?

An elevated or rising ASO titre supports one of the modified Jones criteria (evidence of recent streptococcal infection) required to diagnose acute rheumatic fever, alongside major and minor clinical features. It does not diagnose rheumatic fever by itself.

Why might ASO be ordered with anti-DNase B?

Anti-DNase B rises more reliably after cutaneous streptococcal infection and remains elevated longer than ASO, so the two are often ordered together when clinical suspicion is high and ASO is normal or equivocal, ensuring that recent streptococcal exposure is not missed.

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Antistreptolysin O (ASO) Antibodies

Q: What is the difference between an ASO test and a throat culture?

A throat culture diagnoses acute streptococcal pharyngitis by detecting the organism, while the ASO test measures antibodies that rise weeks after infection and is used to confirm recent streptococcal exposure when post-streptococcal complications are suspected.

The ASO antibodies test measures serum immunoglobulin against streptolysin O, a group A streptococcal toxin. It provides serological evidence of recent pharyngeal streptococcal infection and is central to diagnosing post-streptococcal sequelae — acute rheumatic fever and post-streptococcal glomerulonephritis — when throat culture is negative. A rising titre is more specific than a single value.

Biomarkers

Blood (serum)

Sample

2–3 business days

Turnaround

Fasting

What is the ASO test?

The Antistreptolysin O (ASO) Antibodies test measures serum immunoglobulin directed against streptolysin O, a toxin produced by group A streptococci. It is ordered to provide serological evidence of recent streptococcal infection when a patient presents with suspected post-streptococcal complications — chiefly acute rheumatic fever or post-streptococcal glomerulonephritis — at a time when throat or skin cultures are typically negative. ASO is not used to diagnose acute pharyngitis; for that, practitioners rely on rapid streptococcal antigen tests or throat culture. Instead, the test is ordered retrospectively, weeks after the initial infection, when immune-mediated sequelae emerge. A single elevated titre suggests recent exposure, but paired acute and convalescent specimens demonstrating a fourfold rise provide stronger evidence. When clinical suspicion is high and ASO is normal or equivocal, anti-DNase B is often added, because it rises more reliably after skin infection and remains elevated longer.

What does the ASO test measure?

ASO(Antistreptolysin O Titre)

Range: Typically <200 IU/mL in adults, <100 IU/mL in children under five; assay- and age-specific.Optimal: There is no therapeutic 'optimal' ASO titre; the value is interpreted in the context of suspected post-streptococcal disease, and a rising titre (fourfold change between acute and convalescent specimens) is more informative than a single elevated result.IU/mL / Todd units/mL

Antistreptolysin O antibodies are polyclonal immunoglobulins directed against streptolysin O, an oxygen-labile haemolysin produced by group A beta-haemolytic streptococci. Following pharyngeal or cutaneous streptococcal infection, ASO titres rise within one to three weeks, peak at three to five weeks, and decline over months, though they may remain detectable at low levels for a year or longer. The antibody response is stronger and more consistent after pharyngeal than after skin infection, and titres reflect recent or recurrent exposure rather than active infection.

ASO is the most widely used serological marker of recent group A streptococcal infection and plays a central role in diagnosing post-streptococcal sequelae — acute rheumatic fever and post-streptococcal glomerulonephritis. Because throat and skin swabs may be negative by the time these immune-mediated complications appear, a rising or elevated ASO titre provides retrospective evidence of antecedent streptococcal exposure. ASO does not diagnose acute streptococcal pharyngitis; culture or rapid antigen detection is used for that purpose.

Why is the ASO test ordered?

Evaluation of suspected acute rheumatic fever in a patient presenting with migratory polyarthritis, carditis, or chorea, meeting modified Jones criteria
Investigation of post-streptococcal glomerulonephritis in a patient with acute nephritic syndrome (haematuria, hypertension, oedema) one to three weeks after pharyngitis or impetigo
Work-up of paediatric movement disorders, including suspected paediatric autoimmune neuropsychiatric disorders associated with streptococcal infections (PANDAS), though ASO alone does not confirm this controversial entity
Assessment of recurrent pharyngitis to distinguish true streptococcal infection from chronic carriage or non-streptococcal sore throat, when paired titres are obtained
Follow-up of patients with known rheumatic fever to monitor for re-infection during secondary prophylaxis
Unexplained polyarthritis or glomerulonephritis in settings where streptococcal infection is endemic and clinical suspicion is high

Sample collection and turnaround

Sample type

Blood (serum)

Fasting required

Collection method

Venipuncture at patient service centre

Turnaround

2–3 business days

Collection notes

Phlebotomy at LabCorp draw site; no special preparation required. For optimal interpretation of a suspected post-streptococcal complication, an acute specimen should be drawn at presentation and a convalescent specimen two to four weeks later to demonstrate a rising titre. Avoid grossly lipaemic specimens, which may spuriously elevate nephelometric or turbidimetric assays.

Specimen requirements

Minimum 0.5 mL serum (gel separator acceptable). Refrigerate if not processed immediately; ASO antibodies are stable refrigerated for several days and frozen for months.

What can affect ASO results?

elevatesRecent streptococcal infection (within one to three weeks)

ASO titres rise during the convalescent phase, peaking at three to five weeks, so the timing of the draw relative to infection determines whether an elevation is detected.

variableCutaneous streptococcal infection (impetigo, pyoderma)

Skin infection produces a weaker and less consistent ASO response than pharyngeal infection; anti-DNase B is a more reliable marker in this setting.

reducesAntibiotic treatment of the initial streptococcal infection

Early antibiotic therapy can blunt the antibody response, reducing peak ASO titres, though it does not eliminate them entirely.

elevatesHyperlipidaemia and lipaemic serum

Turbidimetric and nephelometric ASO assays can be spuriously elevated by lipaemia; severely lipaemic specimens may require ultracentrifugation or assay by an alternative method.

elevatesLiver disease and hypergammaglobulinaemia

Polyclonal immunoglobulin elevation can non-specifically raise ASO titre; correlation with clinical context is essential.

variableGeographic and seasonal variation

Baseline ASO titres are higher in populations with endemic streptococcal exposure and during seasons of high streptococcal transmission, so reference limits may reflect local epidemiology.

What do ASO results mean?

Elevated

Recent or recurrent group A streptococcal infection, with titres peaking three to five weeks post-infection. An isolated elevated titre supports, but does not prove, post-streptococcal disease; interpret with clinical criteria for acute rheumatic fever (modified Jones) or post-streptococcal glomerulonephritis. A fourfold rise in paired samples is more specific than a single value. Non-specific elevation can occur with hypergammaglobulinaemia or lipaemia.

Normal

A normal ASO titre makes recent pharyngeal streptococcal infection less likely but does not exclude it, especially if the draw was timed early or the infection was treated promptly. Combine with anti-DNase B if post-streptococcal disease remains suspected, and interpret alongside throat culture or rapid antigen results from the acute phase.

Reduced

A low or absent ASO titre does not exclude recent streptococcal infection, particularly if the specimen was drawn too early (before the antibody response), if the infection was cutaneous (weak ASO response), or if early antibiotics blunted the rise. In these scenarios, anti-DNase B should be added.

Note: ASO is a marker of immune response to recent infection, not active infection; it does not diagnose acute pharyngitis and is not useful for deciding whether to treat with antibiotics. Titres vary by age, assay, and population, and a single elevated result is less informative than a rising titre. Always correlate with clinical criteria (Jones, nephritic syndrome) and consider anti-DNase B when ASO is equivocal.

Frequently asked questions

A throat culture diagnoses acute streptococcal pharyngitis by detecting the organism, while the ASO test measures antibodies that rise weeks after infection and is used to confirm recent streptococcal exposure when post-streptococcal complications are suspected.

How to access the ASO test

The ASO reference page is free and open to all practitioners. Create a free practitioner account to see pricing, save tests to your list, and generate requisition forms. Verification takes 1–2 business days.

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About this page

Published: October 7, 2026

Last reviewed: October 7, 2026

References:

Gewitz MH, 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)
Carapetis JR, et al. Acute rheumatic fever. Lancet. (2005)
Rodriguez-Iturbe B, Musser JM. The current state of poststreptococcal glomerulonephritis. J Am Soc Nephrol. (2008)
Shet A, et al. Immune response and disease severity of acute post-streptococcal glomerulonephritis. Pediatr Nephrol. (2003)
Kaplan EL, Rothermel CD, Johnson DR. Antistreptolysin O and anti-deoxyribonuclease B titers: normal values for children ages 2 to 12 in the United States. Pediatrics. (1998)

This content is for licensed healthcare practitioners only and does not constitute medical advice. Clinical decisions should be based on the full clinical picture and not on any single test result.