The ASO (Anti-Streptolysin O) test measures the level of antibodies your immune system produces in response to Group A Streptococcus, the bacteria that cause strep throat and several skin infections. This test helps determine whether you recently had a strep infection—even if symptoms were mild or went unnoticed—and is often used to evaluate possible post-streptococcal complications, such as rheumatic fever or post-streptococcal glomerulonephritis, a form of kidney inflammation.
What is Anti-Streptolysin O?
Anti-streptolysin O (ASO) is an antibody directed against streptolysin O, a toxin produced by Group A Streptococcus (Streptococcus pyogenes). ASO—along with anti-DNase B—is one of the main antibodies the immune system creates after a strep infection. Measuring ASO levels in the blood helps show whether your body has mounted a recent immune response to this type of bacteria.
About Group A Streptococcus
Group A Streptococcus is best known for causing strep throat, but it can also lead to skin infections such as impetigo, cellulitis, and pyoderma. Most strep infections are diagnosed early and resolve with appropriate antibiotic treatment.
However, when a strep infection is untreated, not fully treated, or produces few noticeable symptoms, the immune response can sometimes trigger more serious conditions—including rheumatic fever and post-streptococcal glomerulonephritis—particularly in children. While these complications have become less common due to routine testing and early treatment, they still occur and can result in heart inflammation, kidney dysfunction, swelling, and high blood pressure.
The ASO test helps clinicians determine whether these issues may be related to a recent Group A strep infection.
Understanding Your Results
A negative or very low ASO level usually means there has not been a recent streptococcal infection. This is especially reliable if a second sample taken 10–14 days later is also low and if the anti-DNase B test is negative as well.
A small number of people with post-streptococcal complications may not show elevated ASO levels—this is more common in cases of kidney inflammation following a skin-based strep infection.
What does it mean if your Anti-Streptolysin O result is too high?
A high ASO level indicates that your immune system has recently responded to a Group A Streptococcus infection. Elevated results are often seen after strep throat, impetigo, or other strep-related infections. High ASO levels do not diagnose an active infection but suggest that a strep infection occurred within the past weeks or months.
Clinicians may use a high ASO result to help evaluate symptoms such as joint pain, fever, swelling, or kidney issues that may indicate rheumatic fever or post-streptococcal glomerulonephritis. A rising ASO level across two tests taken 10–14 days apart provides even stronger evidence of a recent strep infection.
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What does it mean if your Anti-Streptolysin O result is too low?
A low or negative ASO level usually means no recent strep infection. This is especially reliable if a repeat test 10–14 days later is also low.
However, some individuals with post-streptococcal complications—especially kidney inflammation following a skin infection—may not show elevated ASO levels. In these cases, an anti-DNase B test may provide additional information.
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