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To help determine whether you have had a recent strep infection with the bacteria group A Streptococcus; to help diagnose complications resulting from a strep infection such as rheumatic fever or glomerulonephritis, a form of kidney disease
When you have symptoms such as fever, chest pain, fatigue and shortness of breath that suggest rheumatic fever or symptoms such as fluid accumulation (edema) and dark urine that are associated with glomerulonephritis, especially when you recently may have had a group A streptococcal (GAS) infection that was not diagnosed and treated appropriately
A blood sample drawn from a vein
You may be instructed not to eat (fast) six hours before the test
Antistreptolysin O (ASO) is an antibody targeted against streptolysin O, a toxic enzyme produced by group A Streptococcus bacteria. ASO and anti-DNase B are the most common of several antibodies that are produced by the body's immune system in response to a strep infection with group A Streptococcus. This test measures the amount of ASO in the blood.
Group A Streptococcus (Streptococcus pyogenes) is the bacterium responsible for causing strep throat and a variety of other infections, including skin infections (pyoderma, impetigo, cellulitis). In most cases, strep infections are identified and treated with antibiotics, and the infections resolve.
When a strep infection does not cause identifiable symptoms, goes untreated, or is treated ineffectively, however, complications, namely rheumatic fever and a type of kidney disease (glomerulonephritis), can sometimes develop, especially in young children. These secondary conditions have become much less prevalent in the U.S. because of routine strep testing, but they still do occur. These conditions can cause serious complications such as heart damage, acute kidney dysfunction, tissue swelling (edema), and high blood pressure (hypertension). The ASO test can be used to help determine if these are due to a recent group A strep infection.
The ASO test is primarily used to help determine whether a recent strep infection with group A Streptococcus:
The test may be ordered by itself or along with an anti-DNase B, another test used to detect recent strep infections.
In most cases, strep infections are identified and treated with antibiotics and the infections resolve. In cases where they do not cause identifiable symptoms and/or go untreated, however, complications can develop in some people, especially young children. The test, therefore, is ordered if a person presents with symptoms suggesting rheumatic fever or glomerulonephritis and has had a recent history of sore throat or a confirmed streptococcal infection.
Since the incidence of post-streptococcal complications has dropped in the U.S., so has the use of the ASO test.
The ASO test is ordered when a person has symptoms that a health practitioner suspects may be due to an illness caused by a previous strep infection. It is ordered when the symptoms emerge, usually in the weeks following a sore throat or skin infection when the bacteria are no longer present in the throat or on the skin.
Some symptoms of rheumatic fever may include:
Some symptoms of glomerulonephritis may include:
However, these symptoms can be seen in other conditions.
The test may be performed twice, with samples collected about two weeks apart, for acute and convalescent ASO titers. This is done to determine if the antibody level is rising, falling, or remaining the same.
ASO antibodies are produced about a week to a month after an initial strep infection. The amount of ASO antibody (titer) peaks at about 3 to 5 weeks after the illness and then tapers off but may remain detectable for several months after the strep infection has resolved. Over 80% of patients with acute rheumatic fever and 95% of patients with acute glomerulonephritis due to streptococci have elevated ASO.
A negative ASO or ASO that is present at very low titers means the person tested most likely has not had a recent strep infection. This is especially true if a sample taken 10 to 14 days later is also negative (low titer of antibody) and if an anti-DNase B test is also negative (low titer of antibody). A small percentage of people with a complication related to a strep infection will not have an elevated ASO. This is especially true with glomerulonephritis that may develop after a skin strep infection.
An elevated titer of antibody (positive ASO) or an ASO titer that is rising means that it is likely that the person tested has had a recent strep infection. ASO titers that are initially high and then decline suggest that an infection has occurred and may be resolving.
The ASO test does not predict whether complications will occur following a strep infection, nor does it predict the type or severity of the disease. If symptoms of rheumatic fever or glomerulonephritis are present, an elevated ASO level may be used to help confirm the diagnosis.
ASO levels are not detectable for at least a week after an infection, so ASO tests are not used to diagnose a current, acute infection. A throat culture or a rapid strep test is the best method to diagnose strep throat (streptococcal pharyngitis). It is important that strep throat be promptly identified and treated to avoid complications and passing the infection on to others.
No. In general, the ASO test is only performed when someone has symptoms suggesting that a complication may have developed after a strep infection that was not diagnosed and treated appropriately. Most people do not experience these complications, so the ASO test is not routinely done.
Most doctors' offices will not perform this test. Typically, your blood will be sent to a laboratory for testing.
If you healthcare practitioner thinks you have rheumatic fever, you may need other tests such as anti-DNAse B, anti-hyaluronidase or anti-streptozyme.
Use of some antibiotics and corticosteroids may decrease ASO antibody levels.
Sources Used in Current Review
2018 review performed by Kim L. Diehl, MLS(ASCP)CM, Laboratory Director
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