ASO Titre (Anti-Streptolysin O)
Anti-streptolysin O (ASO) is an antibody against streptolysin O, a toxin produced by Group A streptococcus. ASO rises about 1 week after streptococcal throat infection, peaks at 3–5 weeks, and declines over months. It is used to confirm recent streptococcal infection in the context of suspected rheumatic fever, post-streptococcal glomerulonephritis, or reactive arthritis — conditions where the strep infection itself may have resolved by the time these complications appear.
Reference Ranges
Male
0 – 200
IU/mL
Female
0 – 200
IU/mL
Child
0 – 320
IU/mL
School-age children up to 320 IU/mL
Reference ranges may vary by laboratory. Always compare with the range printed on your lab report.
What Low Levels Mean
A low ASO titre (below 200 IU/mL in adults, below 320 in children) suggests no recent streptococcal infection. In suspected rheumatic fever or post-strep disease, a single low titre is not enough to rule out — consider paired titres, anti-DNase B, and clinical criteria. Some patients clear strep without mounting a strong ASO response.
What High Levels Mean
An elevated ASO suggests recent streptococcal infection. A rising titre on paired samples (acute and convalescent, 2–4 weeks apart) is more diagnostic than a single elevated value. High ASO with arthritis, carditis, chorea, or erythema marginatum supports rheumatic fever. With hematuria, hypertension, and edema it supports post-streptococcal glomerulonephritis.
Frequently Asked Questions
Does a positive ASO mean I have rheumatic fever?
No. A positive ASO simply shows recent streptococcal infection. Rheumatic fever is a clinical diagnosis using the Jones criteria — carditis, arthritis, chorea, erythema marginatum, and subcutaneous nodules. ASO supports the diagnosis but is neither necessary nor sufficient.
Should ASO be checked after a sore throat?
No. Acute streptococcal pharyngitis is diagnosed by rapid strep test or throat culture. ASO rises only a week after infection and is not useful for acute diagnosis. ASO is reserved for suspected post-streptococcal complications that appear 2–6 weeks after the initial infection.
Why is anti-DNase B tested alongside ASO?
ASO is more often elevated after streptococcal pharyngitis, while anti-DNase B rises more reliably after streptococcal skin infection (impetigo) — especially relevant in post-streptococcal glomerulonephritis. Together, the two antibodies detect over 90% of recent streptococcal infections; either one alone can miss cases.
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This information is for educational purposes only and should not replace professional medical advice. Always consult your doctor for interpretation of your test results.
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