When do ASO antibodies rise after infection?

ASO antibodies begin to rise one to two weeks after the onset of group A streptococcal infection, peak at three to six weeks, and decline gradually over several months, remaining detectable for six to twelve months or longer in some individuals.

Why is a rising titre more useful than a single result?

Because baseline ASO titres from past exposures are common in the population, a single elevated result may reflect old rather than recent infection. A fourfold or greater rise between acute and convalescent samples (drawn two to four weeks apart) provides definitive evidence of recent infection.

Does the ASO test diagnose acute strep throat?

No. Acute streptococcal pharyngitis is diagnosed with throat culture or rapid antigen detection. ASO is used retrospectively, weeks after the acute infection, to confirm recent streptococcal exposure when evaluating post-infectious sequelae.

Is fasting required for the ASO test?

No special preparation or fasting is required, though severely lipaemic serum can interfere with some assays. Practitioners may request fasting if concurrent lipid testing is ordered or if the patient has known hyperlipidaemia.

What is the difference between ASO and anti-DNase B?

Both are antibodies to group A streptococcal antigens. ASO rises more reliably after pharyngeal infections, while anti-DNase B responds more strongly to skin infections such as impetigo. Ordering both increases diagnostic sensitivity for post-streptococcal disease.

How long do ASO results take?

Results are typically available within 2–3 business days from specimen receipt at Quest, though batch processing may extend turnaround in some regions.

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Anti-Streptolysin O Antibody (ASO)

Q: Can a normal ASO result rule out recent streptococcal infection?

No. A normal ASO does not exclude recent infection, especially if the infection was cutaneous (where anti-DNase B is more sensitive), if antibiotics were given early, or if the sample was drawn too soon or too late. Pairing ASO with anti-DNase B increases sensitivity.

The ASO test quantifies serum antibodies to streptolysin O, a group A streptococcal toxin, rising one to two weeks post-infection and peaking at three to six weeks. It provides serological evidence of recent Streptococcus pyogenes infection in the work-up of post-streptococcal sequelae such as acute rheumatic fever and glomerulonephritis;

Biomarkers

Blood (serum)

Sample

2–3 business days

Turnaround

Fasting

What is the ASO test?

The Anti-Streptolysin O Antibody test quantifies serum antibodies to a group A streptococcal toxin and serves as serological evidence of recent Streptococcus pyogenes infection. It is ordered not to diagnose acute pharyngitis — for which culture and antigen testing are standard — but to support the diagnosis of post-streptococcal sequelae, chiefly acute rheumatic fever and post-streptococcal glomerulonephritis, in patients who present weeks after a throat or skin infection with rheumatic or renal manifestations. Because ASO titres rise during convalescence rather than at the time of acute symptoms, and because baseline seroprevalence is common in school-age populations, interpretation demands consideration of clinical context, timing, and ideally a paired acute and convalescent sample to demonstrate a significant rise. A single elevated ASO is suggestive but not diagnostic; a rising titre is more compelling evidence of recent infection.

What does the ASO test measure?

ASO(Anti-Streptolysin O Antibody)

Range: ≤200 IU/mL in adults and school-age children; varies by age and season.Optimal: No 'optimal' range applies; the clinical question is whether a titre is elevated relative to age-specific norms and whether it rises or falls between serial samples.IU/mL / Todd units

Streptolysin O is an oxygen-labile haemolysin secreted by group A streptococci (Streptococcus pyogenes). Following pharyngeal or cutaneous group A streptococcal infection, the immune system produces antibodies against this and other streptococcal antigens. Anti-streptolysin O antibodies begin to rise approximately one to two weeks after the onset of infection, peak at three to six weeks, then decline gradually over several months, although they may remain detectable for six to twelve months or longer in some individuals.

ASO is not used to diagnose acute streptococcal pharyngitis — throat culture or rapid antigen detection serve that purpose — but rather to document recent group A streptococcal infection when a clinician suspects a post-infectious sequela such as acute rheumatic fever or post-streptococcal glomerulonephritis. A rising titre between acute and convalescent samples provides stronger evidence than a single elevated value, because background seroprevalence in the general population means that many healthy individuals carry detectable titres from past exposures.

Why is the ASO test ordered?

Work-up of suspected acute rheumatic fever in a patient presenting with migratory polyarthritis, carditis, chorea, erythema marginatum, or subcutaneous nodules following a pharyngitis episode
Investigation of post-streptococcal glomerulonephritis when a child or young adult develops haematuria, proteinuria, oedema, and hypertension one to three weeks after a streptococcal throat or skin infection
Evaluation of recurrent joint symptoms or unexplained carditis in a paediatric or adolescent patient with a history suggestive of recent streptococcal exposure
Paired acute and convalescent serology to document a significant rise in titre when a single sample is equivocal or when timing is optimal for demonstrating an antibody response
Retrospective confirmation of group A streptococcal aetiology in a patient with compatible clinical features but negative or unavailable throat culture
Monitoring the decline of ASO titre in a patient with documented post-streptococcal disease, to confirm resolution of the immune response over subsequent months
Consideration alongside anti-DNase B when ASO is borderline or negative but post-streptococcal disease is suspected, since anti-DNase B rises more reliably after skin infections

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 via Quest patient service centre; no special preparation. For optimal interpretation, consider drawing paired acute and convalescent samples two to four weeks apart to document a rising titre. Lipaemic samples may interfere; notify the laboratory if the patient is non-fasted or has known hyperlipidaemia.

Specimen requirements

Minimum 0.5 mL serum (gel separator acceptable). Separate from cells promptly and refrigerate; ASO antibodies are stable refrigerated for at least one week. Note the assay platform for serial comparability.

What can affect ASO results?

variableTiming of the sample relative to infection

ASO rises during the second to fourth week post-infection and peaks at three to six weeks; samples drawn too early in the acute phase or many months later may be falsely low or returned to baseline.

reducesSkin infections with group A streptococci

Cutaneous streptococcal infections provoke a weaker ASO response than pharyngeal infections, so a normal ASO does not exclude recent infection in impetigo or cellulitis; anti-DNase B is a more sensitive marker in that setting.

reducesAntibiotic treatment of the acute infection

Prompt antibiotic therapy can blunt the antibody response, leading to a lower peak titre or slower rise; however, treated infections can still generate detectable ASO in most cases.

elevatesAge and background seroprevalence

School-age children and adolescents living in crowded settings have higher baseline titres from cumulative exposures, so an isolated mildly elevated result may reflect past rather than recent infection.

elevatesHyperlipidaemia and lipaemic serum

Severely lipaemic samples can cause technical interference in turbidimetric or nephelometric ASO assays, yielding spuriously elevated results; fasting or ultracentrifugation may be required.

reducesLiver disease

Severe hepatic synthetic dysfunction can impair antibody production, leading to a blunted or absent ASO response even in the presence of recent streptococcal infection.

What do ASO results mean?

Elevated

A titre above the age-specific upper reference limit (often ≥200–250 IU/mL in adults) suggests recent or ongoing immune response to group A streptococcal infection and supports a diagnosis of acute rheumatic fever or post-streptococcal glomerulonephritis when the clinical picture fits. A fourfold or greater rise between paired sera (acute and convalescent, drawn two to four weeks apart) is the most definitive evidence; a single elevated titre is less specific.

Normal

A normal titre in a patient with suspected post-streptococcal sequelae may reflect mistiming of the sample, a non-pharyngeal (skin) infection, early antibiotic treatment, or an alternative aetiology. Serial or convalescent sampling and correlation with anti-DNase B are essential if clinical suspicion remains high.

Reduced

A normal or low ASO titre does not exclude recent streptococcal infection, particularly if the infection was cutaneous (where anti-DNase B is more sensitive), if antibiotics were given very early, if the sample was drawn too soon (within the first week) or too late (many months post-infection), or if the patient is immunocompromised. Combining ASO with anti-DNase B increases diagnostic sensitivity.

Note: ASO is a marker of immune response, not of active infection or carriage, so it cannot guide decisions about antibiotic treatment of acute pharyngitis. Interpretation must account for age, geography, season, and interval since symptoms. Reference limits and units differ between laboratories and assay methods; Quest's current platform and interval should be documented for serial comparisons.

Frequently asked questions

The ASO test documents recent group A streptococcal infection by measuring antibodies to streptolysin O. It is ordered not to diagnose acute pharyngitis but to support the diagnosis of post-infectious complications such as acute rheumatic fever or post-streptococcal glomerulonephritis.

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:

Gerber MA, et al. Prevention of rheumatic fever and diagnosis and treatment of acute streptococcal pharyngitis: a scientific statement from the American Heart Association Rheumatic Fever, Endocarditis, and Kawasaki Disease Committee. (2009)
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)
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.