ASO is the most common antibody produced after an infection by Group A streptococcus bacteria.
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This test can also be purchased as part of our Autoimmune panel.
PREPARATION: No fasting or other special preparation is needed for this test.
What does this test do?
This test determines the concentration of the Anti-Streptolysin O enzyme (ASO) circulating in the bloodstream.
Why is this important?
ASO antibodies are produced after an infection by Group A streptococcus bacteria, the organisms which commonly cause “strep throat.” Most of these infections cause severe pain and other symptoms that lead to medical evaluation and treatment. But if the strep infection isn’t adequately treated with antibiotics, complications can develop.
These post-streptococcal complications may be severe, and can lead to kidney damage or even kidney failure. Another serious problem is rheumatic fever, which can produce joint pains, rashes, involuntary muscle twitching, and cause permanent damage to the heart muscle and valves.
Due to the relative success with which streptococcal infections are treated in the U.S., these post-infection complications are not seen very often. However, since the symptoms of rheumatic fever or kidney damage may not occur until weeks or even months after a strep infection, an elevated ASO level may be used to help determine if they are due to a recent Group A streptococcal infection.
What do the results mean?
An elevated level of Anti-Streptolysin O Antibody (ASO) indicates a streptococcal infection sometime within the preceding 6 to 12 months. A negative value means there most likely has not been a recent strep infection.
In the case of an ASO present at very low levels, repeat testing in 10-14 days can help clarify its significance: a rising level is strong evidence of a recent strep infection, whereas a repeat negative or low level helps rule out the possibility.
This test has a high sensitivity: over 80% of patients with acute rheumatic fever, and 95% of patients with post-streptococcal kidney damage have elevated levels of ASO. However, streptococcal infections that only involve the skin (such as cellultis, erysipelas, and impetigo) may lead to post-streptococcal complications without ever causing an elevated ASO level.
There are other limitations to this test – ASO levels can be falsely elevated in patients with liver disease, and the ASO test cannot be used to predict whether complications will occur following a streptococcal infection, nor can it predict the type or severity of the disease.
This test is best used in conjunction with a physician’s evaluation to help understand its meaning.