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What is being tested?

This test measures the amount of antistreptolysin O (ASO) in the blood. ASO is an antibody targeted against streptolysin O, a toxin produced by Group A streptococcus bacteria (Streptococcus pyogenes). Streptolysin O, is an oxygen-sensitive protein which bursts open red blood cells so the bacterium can get access to nutrients and continue to grow. 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 Group A streptococcal infection.

Group A streptococcus, is one of the species of bacterium responsible for causing most strep throat infections. When streptococci infections are identified and, treated with antibiotics, the infection generally resolves however, when a strep infection does not cause identifiable symptoms and, goes untreated, or is treated with ineffective antibiotics, post-streptococcal complications (sequelae) may develop. These are more common in young children and include, rheumatic fever and glomerulonephritis.

There has been a worldwide reduction in diseases caused by Group A Strep, but infections can still occur. In Australia, the Indigenous population is disproportionately affected by these diseases. Group A streptococcus infections may cause symptoms such as fever, fatigue, shortness of breath, heart palpitations, decreased urine output or bloody urine. They can damage the heart and/or cause acute kidney dysfunction, leg swelling (oedema) or high blood pressure (hypertension). Because these symptoms may also be seen with other conditions, the ASO test can be used to help determine if they are due to a recent Group A strep infection.

How is it used?

The ASO test is primarily ordered by itself or along with an anti-DNase B to help determine whether a person has had a recent streptococcal infection. 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, post-streptococcal complications (sequelae), namely rheumatic fever and glomerulonephritis, can develop in some patients, especially young children. Therefore the test 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.

When is it requested?

The ASO test is ordered when a person has symptoms that the doctor suspects may be due to an illness caused by a previous streptococcal infection. It is ordered when the symptoms emerge, usually in the weeks following a sore throat or skin infection.

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 (oedema)
  • High blood pressure

Note that 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 titres. This is done to determine if the antibody level is rising, falling, or remaining the same.

What does the result mean?

ASO antibodies are produced about a week to a month after an initial strep infection. ASO levels peak at about 4 to 6 weeks after the illness and are not detected until approximately 2 weeks after infection, then taper off but may remain at detectible levels for several months after the strep infection has resolved.

If the test is negative or if ASO is present in very low concentrations, then 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 or low level and if an anti-DNase B test is also negative. A small percentage of those with a post-streptococcal complication will not have an elevated ASO.

If the ASO level is high or is rising, then it is likely that a recent strep infection has occurred. ASO levels that are initially high and then decline suggest that an infection has occurred and may be resolving.

The ASO test does not predict if complications will occur following a streptococcal infection, nor do they 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?

Some antibiotics and corticosteroids may decrease ASO antibody levels. Other species of streptococci may also produce streptolysin O such as Streptococcus dysgalactiae, making this test not completely specific for predicting Streptococcus A infections. Although Streptococcus dysgalactiae can also cause strep throat it is not usually common woth post-streptococcal infection complications. It is important that when interpreting the test results, the clinical information is properly correlated.

Common questions

  • Can ASO be used to diagnose strep throat?

A throat culture is the best method to diagnose streptococcal pharyngitis. It is important that strep throat be promptly identified and treated to avoid complications and to avoid passing the infection on to others. Since detectible ASO levels do not appear for at least a week, they are not used to diagnose an acute infection.

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

No. The ASO test is usually only performed when someone has symptoms suggesting that a post-streptococcal complication may have developed and no culture was done to confirm an infection with this bacterium at the suspected time of infection. Most people do not experience these complications therefore the ASO test is not routinely performed.

Last Updated: Thursday, 1st June 2023

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