ASO – Antistreptolysin O
What is streptolysin O?
Streptolysin O is a protein toxin secreted by group A Streptococcus during an infection. It can lyse red blood cells and damage host tissues. In response to this toxin, the immune system produces specific antibodies—anti-streptolysin O—the blood levels of which gradually increase in the weeks following infection. Measuring this level therefore serves as an indirect indicator of a recent streptococcal infection, even when it has gone unnoticed or has been inadequately treated.
When is this test prescribed?
- Suspicion of acute rheumatic fever (ARF): fever, migratory polyarthritis, carditis, Sydenham's chorea after a neglected or inadequately treated sore throat.
- Suspicion of acute post-streptococcal glomerulonephritis: hematuria, edema, and hypertension occurring after tonsillitis or impetigo
- Retrospective confirmation of an unnoticed streptococcal infection
- Evaluation of unexplained polyarthritis or carditis in children or young adults
- Tracking documented post-streptococcal complications
How to interpret the result?
| Results | Probable meaning | Usual conduct |
|---|---|---|
| Normal (bass) | No recent streptococcal infection detected by this marker | Complete with other markers (anti-DNase B) if clinical suspicion persists |
| High | Probable recent streptococcal infection within the previous 2 to 6 weeks | Clinical correlation, search for post-streptococcal complications |
| Evolution of the rate over two samples | A significant increase between two dosages spaced 2 to 4 weeks apart is more significant than an isolated value | Dynamic interpretation recommended if in doubt |
What is the kinetics of ASO antibodies?
Understanding the temporal evolution of the ASO titer is essential for correctly interpreting this test.
- Onset of elevation: approximately 1 to 3 weeks after the onset of the streptococcal infection
- Peak: between 3 and 6 weeks after infection
- Normalization: progressive over several months, sometimes up to 6 months after the infectious episode
- An isolated high rate without a suggestive clinical context should be interpreted with caution, as it may reflect a past infection that has already resolved.
What are the limitations of this test?
- Possible false negatives in streptococcal skin infection (impetigo) - anti-DNase B is then more informative
- False positives: certain liver diseases, hypergammaglobulinemia, and certain non-streptococcal infections can elevate ASO levels.
- A high rate does not indicate an active infection in progress: it is evidence of recent past exposure.
- Reference values vary by age (higher in children due to more frequent exposure) and by laboratory.
- Early and adequate antibiotic treatment can attenuate or abolish the rise in ASO
What other tests are often combined?
| Additional test | Usefulness |
|---|---|
| Anti-DNase B | More sensitive than ASO for streptococcal skin infections, recommended as a supplement |
| CRP and ESR | Non-specific inflammation markers, useful for assessing associated inflammatory activity |
| Need for Speed | Search for hyperleukocytosis or anemia in the context of ARF |
| Urinalysis and creatinine | Screening for post-streptococcal kidney damage (hematuria, proteinuria) |
| Echocardiogram | Evaluation of possible rheumatic carditis in suspected ARF |
What is rheumatic fever?
Acute rheumatic fever (ARF) is a non-suppurative inflammatory complication of an untreated or undertreated streptococcal pharyngitis caused by group A Streptococcus. It typically occurs 2 to 4 weeks after the infectious episode and can affect the joints, heart, central nervous system, and skin. Rheumatic carditis is the most feared complication, as it can lead to permanent valvular damage. ARF has become rare in industrialized countries due to the routine antibiotic treatment of streptococcal tonsillitis but remains common in low-resource countries.
Consult at Clinique Omicron
Clinique Omicron doctors, at its service points in Quebec, can prescribe and interpret ASO testing as part of a comprehensive post-infectious workup. If you or your child experiences joint pain, fever, or urinary abnormalities in the weeks following strep throat, a medical consultation will help guide investigations and initiate appropriate management.
The content of this page is provided for informational purposes only and is not intended to replace the advice of a qualified healthcare professional. Consult a physician for any symptoms, questions or decisions you may have regarding your health.
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