ASO (latex)

ASO (antistreptolysin O) is a blood test measuring antibodies the body produces after beta-haemolytic streptococcal infection, most often streptococcal throat infection. A raised result indicates an infection in recent weeks or months and helps a doctor when late complications such as rheumatic fever or kidney inflammation are suspected.

Who the test is for

• people developing fever and painful, swollen joints several weeks after a sore throat or skin infection, when a doctor suspects rheumatic fever;
• people developing dark tea-coloured urine, swelling around the eyes and legs or high blood pressure after a throat or skin infection, when a doctor suspects post-streptococcal glomerulonephritis;
• people with other symptoms a doctor considers possibly related to a recent streptococcal infection;
• people whose doctor repeats the test after two to four weeks to check whether streptococcal infection was recent.

What ASO is and what it shows

Streptolysin O is a toxin produced by group A beta-haemolytic streptococci, less often groups C and G. The immune system responds by producing antibodies, antistreptolysin O. Their level begins rising about a week after infection, peaks after three to five weeks, then falls gradually, so an elevated result can persist for months.

ASO therefore indicates past rather than current infection. It does not identify where infection occurred or whether it caused disease. ASO rises less after skin streptococcal infections such as impetigo than after throat infection, so a doctor may then also request another antibody test, anti-DNase B.

In rheumatic fever, painful swollen joints often occur sequentially, sometimes with signs of heart involvement. Post-streptococcal glomerulonephritis usually occurs one to three weeks after throat infection or several weeks after skin infection.

ASO is not used to diagnose acute tonsillitis because it has not yet risen in the first days. When the throat hurts now, the pathogen is identified by a throat swab or a rapid streptococcal antigen test. Routine checking after recovered tonsillitis is also unnecessary if symptoms have resolved.

If you notice blood in urine, suddenly reduced urination or marked swelling, see a doctor immediately without waiting for the result.

Preparation and sample

Blood is taken from a vein; collection in children is adapted to age. Fasting is unnecessary for ASO. If other fasting tests, such as glucose or blood lipids, are performed the same day, come in the morning fasting. Tell your doctor if you took antibiotics for the sore throat because early treatment can blunt the antibody rise.

How to interpret the result

ASO is expressed in international units per millilitre (IU/mL). The adult upper limit is most often 200 IU/mL, but the precise method-dependent limit is printed on the report.

• Children: school-age children commonly have higher values than adults because streptococcal infections are more frequent. A doctor therefore interprets a mildly raised result in a child alongside age and symptoms.
• One raised result means infection was probably recent, but does not establish rheumatic fever or kidney disease. Raised ASO without symptoms most often does not itself require treatment.
• A rise in two samples most reliably confirms recent infection. A doctor repeats testing after two to four weeks: at least a twofold rise supports recent infection, while a fall supports infection that has passed.
• Normal result does not exclude infection in the first week or skin infection.

A doctor diagnoses rheumatic fever or glomerulonephritis based on examination and other findings, such as heart examination, urinalysis and kidney function tests. ASO is only one piece of evidence that streptococcal infection preceded the disease.

How to arrange testing

You can have blood taken without an appointment or referral at one of our branches across Croatia (listed under Contact). Prices for all tests and panels are available in the service price list.

Frequently asked questions

What does raised ASO mean?

Most often that you had streptococcal infection within recent weeks or months, often without pronounced symptoms. Its significance depends on symptoms: joint pain, heart symptoms or urinary changes prompt further investigation.

Are antibiotics needed if ASO is raised?

Not necessarily. Antibiotics treat active streptococcal infection, while ASO indicates a past infection. A doctor decides on treatment based on symptoms, examination and, when necessary, a throat swab.

How long does ASO stay raised after tonsillitis?

It peaks several weeks after infection and then falls slowly over several months. Repeat measurement after a short interval therefore often remains raised even though infection is no longer active.

What does “latex” mean in the test name?

It refers to the tiny latex particles used in the laboratory to measure antibodies. It has nothing to do with latex allergy and requires no special preparation.