Blood draw
Antistreptolysin Titer (ASO)
ASO; recent streptococcal exposure.
€ 41,00
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Infections · Blood
Antistreptolysin titer (AST)
Antibodies against a toxin from the group A streptococcus; shows whether you have had this bacterium in recent months.
At a glance
- Measured in blood
- In 1 test panel
- No special preparation
Explanation
Antistreptolysin titer (AST): what is measured?
The group A streptococcus is the bacterium behind strep throat, scarlet fever and impetigo. It makes streptolysin O, a substance that destroys red blood cells. The immune system makes antibodies against it; these can be measured in the blood. The laboratory reports the result as a titer: how far the blood can be diluted and still react. A higher titer means more antibodies.
What can it detect?
- A past infection with the group A streptococcus in the preceding weeks to months
- Support when acute rheumatism (acute rheumatic fever) is suspected, a rare aftereffect of such an infection
- Support when kidney inflammation after a streptococcal infection is suspected
Result
What does the result mean?
Antistreptolysin titer (AST) too high
According to the Dutch Society for Clinical Chemistry (NVKC), a raised titer points to a recent, often untreated infection with the group A streptococcus. The titer only rises about one to three weeks after the infection, then peaks and slowly falls over the following months. A raised value therefore says that you have had the bacterium, not that you have an infection now. In acute rheumatism the titer is also raised, but this condition is rare in the Netherlands; by far most people with a raised titer have simply had a throat infection. Children and young people have higher values on average than older people, because they come into contact with the bacterium more often.
Antistreptolysin titer (AST) too low
A low or normal titer fits someone who has not had the group A streptococcus recently. This is not completely ruled out: in a very early infection the titer has not yet risen, and in a skin infection, such as impetigo, the titer often rises less. Anyone who received antibiotics quickly may also keep a lower titer. To find out whether you now have a throat infection caused by this bacterium, the titer is not the right test; a throat culture or rapid test is used for that.
After an abnormal result
With a raised titer together with matching symptoms, the doctor usually repeats the test after a few weeks to see whether the titer is rising or falling. If acute rheumatism is suspected, referral follows to a pediatrician, internist (specialist in internal medicine) or cardiologist, among other things for an ultrasound of the heart. If kidney inflammation is suspected, urine, kidney function and blood pressure are checked.
Reliability
How reliable is the test?
The AST says something about the past, not the present: it is not suitable for detecting an acute throat infection. The result is mainly meaningful when acute rheumatism or kidney inflammation after a streptococcal infection is suspected. In people without symptoms, the result is hard to interpret, because a raised titer is common in healthy people: streptococcal infections are everyday events and the antibodies remain measurable for months. Moreover, one measurement says less than two measurements a few weeks apart, where the doctor looks at the rise.
What can affect the result?
- The titer only rises about one to three weeks after the infection; early in the illness it may still be normal.
- After a skin infection with the group A streptococcus, such as impetigo, the titer often rises less than after a throat infection.
- The titer stays raised for months and falls slowly; a raised value therefore does not mean that the infection is still active.
- What counts as raised depends on age and on the laboratory; children have higher values on average than adults.
Who it is for
Who is the test useful for?
Useful for
Useful for a targeted question: joint symptoms, fever or heart symptoms a few weeks after a throat infection, or blood in the urine and fluid retention after a throat or skin infection. For an ordinary sore throat now, the test adds nothing; a throat culture or rapid test is used for that if needed. In people without symptoms, the test is not useful, because a raised titer in them usually only reflects a past, harmless infection.
How common is it?
Infections with the group A streptococcus are everyday events, especially in children: throat infections, scarlet fever and impetigo occur in many people every year. The serious aftereffects this test looks for are, by contrast, rare in the Netherlands. Acute rheumatism has become very uncommon here, and kidney inflammation after a streptococcal infection is also uncommon.
RIVM/LCI guideline on group A streptococcal infection
Drawbacks and risks
- A raised titer without symptoms usually only means that you once had an ordinary throat infection. Without that explanation, it can lead to unnecessary worry about the heart or kidneys and to follow-up testing that is not needed.
- The test is not suitable for detecting an acute throat infection; a normal result does not rule out a streptococcal infection at this moment.
Get tested
Antistreptolysin titer (AST): test panels
Antistreptolysin titer (AST) is part of these test panels. You order online and have your blood drawn at a collection point or collect the sample at home.
Evidence
Sources
This explanation is based on Dutch guidelines and independent information. It is general information, not a diagnosis. Discuss an abnormal result with your GP.
More about risks and resultsMore explanations
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