Anti-streptolysin O Antibody Test
The ASO test measures serum antibodies to streptolysin O, an exotoxin of group A Streptococcus, to confirm recent pharyngeal streptococcal infection. It is central to the diagnosis of post-streptococcal sequelae — acute rheumatic fever and post-streptococcal glomerulonephritis — where an elevated or rising titre supports serological evidence of infection in the context of the corresponding…
1
Biomarkers
Blood (serum)
Sample
3–5 business days
Turnaround
No
Fasting
What is the ASO test?
The Anti-streptolysin O Antibody Test measures serum IgG antibodies to streptolysin O, an exotoxin of group A Streptococcus. It is ordered to confirm that an illness is a post-streptococcal immunological sequela rather than a direct infection, since the antibody response lags behind the acute pharyngitis or pyoderma by weeks. The test does not identify active infection — throat culture or rapid antigen detection serves that purpose — but instead documents preceding streptococcal exposure when clinical features of acute rheumatic fever or post-streptococcal glomerulonephritis are present. Practitioners interpret ASO in the context of the clinical syndrome and, when feasible, pair it with convalescent serology to demonstrate a rising titre. Because background titres are common in children and some geographic populations, a single result above the reference limit does not by itself establish causality; the magnitude of elevation, the presence of symptoms, and correlation with inflammatory markers and echocardiographic or renal findings all contribute to the diagnostic assessment.
What does the ASO test measure?
Streptolysin O is an oxygen-labile haemolysin secreted by group A Streptococcus (Streptococcus pyogenes) during infection. Anti-streptolysin O antibodies develop as an adaptive immune response to pharyngeal or cutaneous streptococcal infection, rising within a week of acute infection, peaking at three to five weeks, and declining over several months. Background titres in healthy populations reflect prior exposure and can be elevated in school-age children and adolescents without implying current disease.
ASO is a marker of recent or ongoing group A streptococcal infection and the principal serological test for confirming post-streptococcal sequelae. It is central to the diagnosis of acute rheumatic fever (where an elevated or rising titre supports one of the Jones major criteria) and post-streptococcal glomerulonephritis. Because ASO may remain normal after skin infections (impetigo, cellulitis), anti-DNase B is the preferred marker in that setting. Isolated elevated titres are common and do not diagnose rheumatic fever or glomerulonephritis without the corresponding clinical syndrome.
Why is the ASO test ordered?
- Suspected acute rheumatic fever presenting with migratory polyarthritis, carditis, Sydenham chorea, erythema marginatum, or subcutaneous nodules, where an elevated ASO fulfils serological evidence of streptococcal infection under the Jones criteria
- Acute nephritic syndrome (haematuria, proteinuria, hypertension, oliguria) consistent with post-streptococcal glomerulonephritis following pharyngitis
- Recurrent pharyngitis or tonsillitis with concern for incomplete eradication or carrier state, though throat culture is the first-line test for active infection and ASO is adjunctive
- Paediatric neuropsychiatric syndromes (PANDAS) attributed to group A streptococcal triggers, though evidence for this association remains contested and ASO alone does not establish causality
- Evaluation of unexplained inflammatory arthritis, erythema nodosum, or glomerulonephritis to exclude a post-streptococcal aetiology when clinical history suggests preceding pharyngitis
- Follow-up of known acute rheumatic fever to document falling antibody titres during secondary prophylaxis, though clinical and echocardiographic monitoring is more important than serial serology
- Investigation of persistent fever or elevated inflammatory markers when post-infectious sequelae are in the differential and streptococcal exposure is plausible
Sample collection and turnaround
Sample type
Blood (serum)
Fasting required
No
Collection method
Venipuncture at patient service centre
Turnaround
3–5 business days
Collection notes
Phlebotomy required; no special preparation. If post-streptococcal disease is suspected and the initial titre is normal or borderline, paired convalescent serology two to four weeks later increases diagnostic sensitivity. Note any recent antibiotic use and timing relative to pharyngitis or other suspected streptococcal infection, as these influence interpretation.
Specimen requirements
Minimum 0.5 mL serum. Avoid grossly lipaemic specimens, which can interfere with nephelometric and latex-agglutination assays; if turbid, note on the requisition and consider resampling after an overnight fast.
What can affect ASO results?
School-age children and adolescents mount higher antibody responses and retain elevated baseline titres from repeated exposures; age-adjusted reference limits are essential to avoid over-interpretation.
Skin infections provoke a weaker ASO response than pharyngitis because cutaneous lipids inhibit streptolysin O; anti-DNase B is more sensitive in this setting and should be ordered alongside or instead.
Early antibiotic treatment can blunt the antibody response by reducing bacterial load and toxin release, potentially yielding a falsely low ASO titre in a true post-streptococcal syndrome.
ASO rises within a week, peaks at three to five weeks, and declines over months; timing of the draw relative to pharyngitis determines whether the titre will be elevated, so paired acute and convalescent sera increase diagnostic yield.
Lipids in serum can inhibit the haemolytic reaction used in some ASO assays, yielding falsely low titres; turbid specimens should be noted and the assay repeated after clearing if suspicion is high.
Populations with higher rates of streptococcal pharyngitis (crowded living conditions, limited access to antibiotics) carry higher baseline titres, so interpretation must account for local epidemiology.
What do ASO results mean?
Elevated
An elevated ASO titre (typically >200 IU/mL in adults or above the age-adjusted paediatric threshold) indicates recent or ongoing immune response to group A Streptococcus. It supports, but does not alone diagnose, post-streptococcal sequelae — the clinical syndrome (acute rheumatic fever, post-streptococcal glomerulonephritis) must be present. A fourfold rise between acute and convalescent samples is more specific than a single elevated value. Very high titres (>400–600 IU/mL) strengthen the likelihood of recent infection, but some healthy children carry such levels from repeated exposures.
Normal
A titre within the reference range does not exclude group A streptococcal infection if tested too early (before antibody rise) or too late (after decline), nor does it rule out skin infection. Interpret in the context of symptom timing, throat culture results, and alternative post-infectious markers. A normal ASO with a positive anti-DNase B points to cutaneous streptococcal exposure.
Reduced
A low or undetectable ASO titre (<100 IU/mL) in the appropriate timeframe argues against recent pharyngeal streptococcal infection, though it does not exclude skin infection (where anti-DNase B is more sensitive) or very early or very late convalescence. If clinical suspicion for a post-streptococcal syndrome remains high, repeat serology in two to four weeks or add anti-DNase B.
Note: ASO is a retrospective marker of infection, not a diagnostic test for acute pharyngitis or a stand-alone criterion for post-streptococcal disease. Age-adjusted thresholds, paired serology, correlation with clinical findings (Jones criteria for rheumatic fever, nephritic features for glomerulonephritis), and consideration of anti-DNase B in cutaneous infections are all essential to interpretation. Elevated titres are common in healthy children and do not imply disease without corresponding clinical features.
Frequently asked questions
A throat culture detects active group A streptococcal infection during acute pharyngitis. ASO measures antibodies that develop weeks after infection, so it is used to confirm preceding streptococcal exposure when post-infectious complications such as acute rheumatic fever or glomerulonephritis are suspected, not to diagnose acute pharyngitis.
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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Published: October 7, 2026
Last reviewed: October 7, 2026
- Gerber MA, et al. Prevention of rheumatic fever and diagnosis and treatment of acute streptococcal pharyngitis: a scientific statement from the American Heart Association. Circulation. (2009)
- Revised Jones criteria for the diagnosis of acute rheumatic fever (AHA scientific statement). 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)
- Bisno AL. Acute pharyngitis. N Engl J Med. (2001)
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.