Antistreptolysin O titre

How is it tested?
This test measures the amount of antistreptolysin O (ASO) in the blood. ASO is an antibody directed against streptolysin O, a toxin produced by group A streptococcus bacteria. ASO and anti-DNase B are the most common antibodies produced by our immune system in response to group A streptococcal infection.
Group A streptococcus (Streptococcus pyogenes) is the bacterium responsible for strep throat and tonsillitis. In most cases, strep infections are identified and treated with antibiotics until the infection clears. However, when a strep infection does not cause recognisable symptoms and is not treated or is treated ineffectively, post-streptococcal complications (sequelae), known as rheumatic fever or glomerulonephritis, can sometimes develop in young children. These secondary complications have become much less common in the United States since strep testing became routine, but they still occur. Secondary complications cause symptoms such as fever, tiredness, difficulty breathing, heart arrhythmia, reduced urine output and blood in the urine. They can damage the heart and/or cause acute kidney dysfunction, swelling of the legs (oedema) and high blood pressure (hypertension). Because these symptoms are shared with other conditions, the ASO test can be used to determine whether they are actually the result of a recent group A streptococcus infection.

How is the sample collected for testing?
By drawing blood from a vein in the arm.

Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed

1. How is it used?
2. When is it requested?
3. What does the result mean?
4. Is there anything else I should know?

1. How is it used?
The ASO test is mainly performed, on its own or together with anti-DNase B, to determine whether a person has had a recent strep infection. In most cases, strep infections are identified and treated with antibiotics and the infection clears up. However, when there are no recognisable symptoms and/or the infection is not treated, post-streptococcal complications (sequelae) called rheumatic fever and glomerulonephritis can develop in some patients, especially young children. The test is also prescribed when a person has symptoms suggesting rheumatic fever or glomerulonephritis and has a recent history of sore throat or a confirmed strep infection. The incidence of post-streptococcal complications has fallen in the United States since the ASO test came into use.

2. When is it requested?
The doctor prescribes the ASO test when they suspect that a patient has an illness resulting from a previous strep infection. The test is requested when symptoms appear, usually in the weeks following a sore throat or skin infection. It may be requested twice over a period of 10-14 days to see whether the antibody level has risen, fallen or stayed the same.

Some symptoms of rheumatic fever include:

• Fever
• Swelling or pain in more than one joint, especially the knees, elbows and wrists, sometimes moving from one joint to another
• Small, painless lumps under the skin
• Rapid, jerky movements (Sydenham’s chorea)
• Skin rash
• Sometimes inflammation of the heart (carditis), which may cause no symptoms or may cause difficulty breathing, chest pain or palpitations.

Some symptoms of glomerulonephritis include:

• Tiredness, reduced energy
• Reduced urine output
• Blood in the urine
• Rash
• Aches and pains
• Swelling (oedema)
• High blood pressure

However, these symptoms can also arise in other conditions.

The test may be carried out twice, with samples collected about two weeks apart, to obtain what are called acute and convalescent ASO titres. Testing twice makes it possible to establish whether the antibody level has risen, fallen or stayed the same.

3. What does the result mean?
ASO antibodies are produced from one week to one month after the initial strep infection. ASO levels peak about 4-6 weeks after the illness and then fall, although they can remain at measurable levels for several months after the infection has cleared.
If the test is negative, or if ASO is present at very low levels, the person has probably not had a recent strep infection. This is especially true if a sample taken 10-14 days later is still negative or low and if the anti-DNase B test is also negative. Only a small percentage of patients with a post-streptococcal complication will not also have a raised ASO level.
If the ASO level is high or rising, a recent strep infection has probably occurred. When ASO levels are initially high and then fall, a recent infection is resolving.
The ASO test cannot predict the likelihood of complications after a strep infection, nor the type or severity of the illness. If symptoms of rheumatic fever or glomerulonephritis are present, a high ASO level is useful in confirming the diagnosis.

4. Is there anything else I should know?
Some antibiotics and corticosteroids can lower ASO antibody levels

1. Can ASO be used to diagnose strep throat?
2. If I am diagnosed with strep, will I always need an ASO test?

1. Can ASO be used to diagnose strep throat?
A throat culture or a rapid strep test is the best way to diagnose strep throat. It is important for strep throat to be diagnosed and treated promptly to avoid complications and prevent the infection from spreading to other people. ASO levels cannot be measured until a week after infection, so they are not useful for diagnosing acute infection.

2. If I am diagnosed with strep, will I always need an ASO test?
No. In general, the ASO test is only performed when someone has symptoms suggesting post-streptococcal complications and the bacterial infection was not confirmed by culture. Most people do not develop these complications, so the ASO test is not performed routinely