Also Known As
ASLO
Formal Name
Antistreptolysin O Titer
This article was last reviewed on
This article waslast modified on
May 26, 2018.
At a Glance
Why Get Tested?

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 To Get Tested?

When you have symptoms such as fever, chest pain, fatigue and shortness of breath that suggest rheumatic fever or symptoms such as edema and dark urine that are associated with glomerulonephritis, especially when you recently may have had a group A streptococcal infection that was not diagnosed and treated appropriately

Sample Required?

A blood sample drawn from a vein in your arm

Test Preparation Needed?

None

You may be able to find your test results on your laboratory's website or patient portal. However, you are currently at Lab Tests Online. You may have been directed here by your lab's website in order to provide you with background information about the test(s) you had performed. You will need to return to your lab's website or portal, or contact your healthcare practitioner in order to obtain your test results.

Lab Tests Online is an award-winning patient education website offering information on laboratory tests. The content on the site, which has been reviewed by laboratory scientists and other medical professionals, provides general explanations of what results might mean for each test listed on the site, such as what a high or low value might suggest to your healthcare practitioner about your health or medical condition.

The reference ranges for your tests can be found on your laboratory report. They are typically found to the right of your results.

If you do not have your lab report, consult your healthcare provider or the laboratory that performed the test(s) to obtain the reference range.

Laboratory test results are not meaningful by themselves. Their meaning comes from comparison to reference ranges. Reference ranges are the values expected for a healthy person. They are sometimes called "normal" values. By comparing your test results with reference values, you and your healthcare provider can see if any of your test results fall outside the range of expected values. Values that are outside expected ranges can provide clues to help identify possible conditions or diseases.

While accuracy of laboratory testing has significantly evolved over the past few decades, some lab-to-lab variability can occur due to differences in testing equipment, chemical reagents, and techniques. This is a reason why so few reference ranges are provided on this site. It is important to know that you must use the range supplied by the laboratory that performed your test to evaluate whether your results are "within normal limits."

For more information, please read the article Reference Ranges and What They Mean.

What is being tested?

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 (sequelae), 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 damage the heart, 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.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

Accordion Title
Common Questions
  • How is it used?

    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 (sequelae), namely rheumatic fever and glomerulonephritis, 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.

  • When is it ordered?

    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:

    • Fever
    • Joint swelling and pain in more than one joint, especially in the ankles, knees, elbows and wrists, sometimes moving from one joint to another
    • Small, painless nodules under the skin
    • Rapid, jerky movements (Sydenham's chorea)
    • Skin rash
    • Sometimes the heart can become inflamed (carditis); this may not produce any symptoms but also may lead to shortness of breath, heart palpitations, or chest pain

    Some symptoms of glomerulonephritis may include:

    • Fatigue, decreased energy
    • Decreased urine output
    • Bloody urine
    • Rash
    • Joint pain
    • Swelling (edema)
    • High blood pressure

    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.

  • What does the test result mean?

    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.

    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.

  • Is there anything else I should know?

    Use of some antibiotics and corticosteroids may decrease ASO antibody levels.

  • Can ASO be used to diagnose strep throat?

    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.

  • If I am diagnosed with strep, will an ASO always be performed?

    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.

  • Can the ASO test be performed in my doctor's office?

    Most doctors' offices will not perform this test. Typically, your blood will be sent to a laboratory for testing.

View Sources

Sources Used in Current Review

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