ASO blood test: what the result can show

ASO stands for antistreptolysin O. The immune system produces these antibodies against streptolysin O, a substance made by certain streptococcal bacteria. Clinicians use the result alongside symptoms, examination and other tests.

A raised result supports recent exposure to the bacteria, but it cannot show where the infection occurred or prove that current symptoms are caused by it. Antibodies rise after the initial illness and may remain detectable after the infection has settled.

A result within the laboratory reference range does not always exclude recent infection, particularly if the sample was taken early. Doctors sometimes compare samples taken at different times because a changing level can be more informative than one measurement. Reference ranges vary by laboratory and age.

The sample is taken from a vein, and special preparation is generally not required unless other blood tests have been ordered at the same visit. Tell the clinician about recent sore throat, skin infection, fever, joint pain, swelling, dark urine and all medicines.

Do not start antibiotics solely because the antibody result is raised. Antibiotics treat a current bacterial infection when clinically indicated; they do not remove antibodies already produced. The next step depends on the reason for testing and may include urine, kidney, inflammation or heart assessments.

Seek urgent medical advice for reduced urine, blood-coloured or cola-coloured urine, swelling around the eyes or legs, breathlessness, chest pain, fainting, severe weakness, fever with marked joint swelling or uncontrolled jerking movements. These symptoms require assessment regardless of the ASO value.

After a blood draw, brief tenderness or bruising can occur. Apply gentle pressure to persistent oozing; obtain care if swelling, redness, warmth or pain at the puncture site is getting worse.

Ask why the test was ordered, how your value compares with that laboratory's range, whether the timing could affect it, which other findings matter and whether a repeat sample would change management.