What this test measures
ASO (Anti-Streptolysin O) is an antibody the immune system produces in response to streptolysin O — a toxin secreted by Group A Streptococcus (Streptococcus pyogenes). Titres rise within 1–3 weeks of streptococcal infection, peak at 3–6 weeks, and fall over 6–12 months. A raised or rising titre is evidence of recent streptococcal infection.
The test is reported as an absolute titre (e.g. 200 IU/mL) or as a dilution ratio. Adult normal upper limit varies by region but is typically <200 IU/mL; children commonly run higher (up to 320 IU/mL). For Indian children, where streptococcal infection is endemic, baseline titres are higher than in Western populations and isolated elevation is less diagnostic. A rising titre between two samples (4-fold rise) is more meaningful than a single value.
Why it matters
India still has one of the highest burdens of rheumatic heart disease (RHD) in the world — estimated 0.5–1 per 1,000 schoolchildren — driven by untreated streptococcal sore throats progressing to acute rheumatic fever and chronic valvular heart disease. ASO is a key piece of evidence in the modified Jones criteria for diagnosing acute rheumatic fever in a child presenting with arthritis, carditis, chorea, or erythema marginatum.
It is also useful in the workup of post-streptococcal glomerulonephritis (haematuria + hypertension + raised creatinine in a child 1–3 weeks after a throat or skin infection) and reactive arthritis. ASO is NOT useful for diagnosing active streptococcal pharyngitis — for that, a throat culture or rapid antigen test is the right test. The clinical context guides interpretation.
How to prepare
No fasting required. Disclose recent sore throat, skin infection, or current antibiotic use. If acute rheumatic fever is suspected, paired samples 2 weeks apart (looking for a rising titre) are more diagnostic than a single test.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| ASO Titre (IU/mL)[1][2] | Adults < 200 · Children up to 320 (varies by region) | Low or absent ASO does not exclude past streptococcal infection but makes recent significant infection less likely. Anti-DNase B or anti-streptokinase can be checked if streptococcal infection is still suspected. | Raised titre — evidence of recent streptococcal infection (within last 6 months). A 4-fold rise between paired samples is more specific. In a child with arthritis, carditis, chorea: supports diagnosis of acute rheumatic fever (one of the Jones minor criteria). In glomerulonephritis: supports diagnosis of post-streptococcal glomerulonephritis. |
ASO interpretation
| Scenario | ASO interpretation |
|---|---|
| Acute sore throat | Not useful — throat culture / rapid antigen is the right test |
| Child with arthritis or carditis 2–4 weeks after sore throat | Raised ASO supports rheumatic fever diagnosis |
| Child with haematuria 1–3 weeks after throat / skin infection | Raised ASO supports post-streptococcal glomerulonephritis |
| Paired samples 2 weeks apart, 4-fold rise | Strong evidence of recent infection |
| Asymptomatic adult, isolated high ASO | Common — recent / past infection; not diagnostic of disease alone |
Frequently asked questions
When is ASO ordered?
When acute rheumatic fever, post-streptococcal glomerulonephritis, or reactive arthritis is suspected after a recent streptococcal infection. It is NOT used to diagnose acute strep throat.
Do I need to fast?
No.
My ASO is 400 — does my child have rheumatic fever?
A raised ASO supports recent streptococcal infection but does not by itself diagnose rheumatic fever. The diagnosis needs clinical features (arthritis, carditis, chorea, erythema marginatum) per the modified Jones criteria.
Why are Indian children's normal values higher?
Streptococcal infections are very common in India, especially in school-age children. Many children have baseline ASO titres above Western normal limits without active disease.
My ASO is high but I have no symptoms — should I worry?
A single isolated high ASO in an asymptomatic person is usually evidence of past streptococcal infection. Without clinical features of rheumatic fever or glomerulonephritis, no treatment is needed. The titre will fall over 6–12 months.
Can antibiotics affect the ASO result?
Antibiotic treatment for the original streptococcal infection can blunt the antibody rise. If clinical suspicion is high and ASO is normal, anti-DNase B can be measured (it is less affected by antibiotic treatment).
How often is ASO repeated?
When monitoring response to treatment or excluding recent infection, a repeat at 2 weeks looking for a rising or falling titre is more informative than a single value.
Is ASO useful for adults?
Yes — for diagnosing post-streptococcal complications. Rheumatic fever is rarer in adults but can occur. Reactive arthritis after streptococcal infection is occasionally seen.
Related Coagulation tests
Tests commonly ordered alongside ANTI STREPTOLYSIN - O (ASO), or that help interpret an unexpected result.
Sources & references
- NCBI StatPearls — Anti-Streptolysin O · accessed 2026-05-30T00:00:00.000Z
- WHO — Rheumatic Heart Disease · accessed 2026-05-30T00:00:00.000Z
- AHA Scientific Statement — RHD Diagnosis (Jones criteria) · accessed 2026-05-30T00:00:00.000Z
- NIH MedlinePlus — Antistreptolysin O Titer Test · accessed 2026-05-30T00:00:00.000Z
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