Antistreptolysin O (ASO) blood test in Chiswick

What the test measures

Antistreptolysin O is an antibody your immune system makes against streptolysin O, a toxin released by group A streptococcus. These bacteria cause strep throat, scarlet fever and some skin infections. ASO levels usually start to rise about a week after infection, peak a few weeks later and then gradually fall over several months.

The test does not look for the bacteria themselves, so it is not used to diagnose a sore throat. Instead, it helps show whether a recent strep infection may explain later problems, such as post-streptococcal joint pain, rheumatic fever, a particular type of kidney inflammation (glomerulonephritis) or guttate psoriasis. The price includes the blood sample taken at our Chiswick clinic, analysis at the Randox accredited laboratory, a review of your result with an Omnia practitioner and a GP feedback report on request.

Who it is for

• You had a sore throat or scarlet fever in recent weeks and now have joint pain or swelling.
• You have noticed dark or cola-coloured urine or puffiness after a throat infection.
• You have a new outbreak of small drop-shaped psoriasis patches after an illness.
• Your GP or specialist has asked for an ASO level to follow up a previous result.
• You want to understand whether recurring symptoms could be linked to strep infections.

Understanding your result

Your report shows your ASO level with the laboratory reference range. A raised result suggests a recent group A streptococcal infection, and a rising level on two samples a few weeks apart is stronger evidence than a single result. Levels vary with age and can stay raised for months, so a high reading does not tell exactly when the infection happened.

A normal ASO makes a recent strep infection less likely but does not completely exclude it, as not everyone produces a strong response. Your practitioner will interpret the result with your symptoms and history, and a result outside the range is a prompt for discussion, not a diagnosis.

Preparing for your test

No fasting or special preparation is needed. Let us know when your sore throat or other infection started, and whether you took antibiotics, as early antibiotic treatment can reduce the ASO response.

ASO is one of the immune markers in Advanced GP3 and Omnia Signature, which look at many areas of health together. Elsewhere you will find every blood test and price in Chiswick.

Questions

How much is an ASO blood test in Chiswick?

The antistreptolysin O (ASO) test costs £79. The price includes your sample at our W4 clinic, analysis at an accredited laboratory, a practitioner review and a GP feedback report if requested.

Can ASO tell me if I have strep throat now?

Not reliably. ASO takes about a week to rise, so it reflects infections in recent weeks rather than today. A throat swab is the usual test for a current sore throat.

How long does ASO stay raised?

It usually peaks a few weeks after infection and then falls gradually over several months, which is why timing matters when reading the result.

When should I seek urgent help?

If you or a child have a high fever with a rash, severe sore throat with difficulty swallowing, or are very unwell, contact NHS 111 or your GP promptly.