The ASO titre (Antistreptolysin O) is a blood test that measures the level of antibodies your immune system makes against streptolysin O, a toxin produced by Group A Streptococcus (the bacteria that commonly cause “strep throat”).

This test does not usually detect the bacteria itself. Instead, it helps show whether you have had a recent streptococcal infection—even if the sore throat or skin infection has already improved or you no longer test positive on a throat swab. ASO levels typically begin to rise about 1–3 weeks after infection, peak around 3–5 weeks, and then gradually fall over weeks to months.

What results can mean:

Higher-than-expected ASO suggests a recent or past Group A strep infection. It is especially helpful when a clinician is evaluating possible strep complications that can occur after the initial infection has resolved.

Normal or low ASO makes a recent strep infection less likely, but it does not fully rule it out. Some people do not develop a strong ASO response (for example, certain skin infections may raise ASO less), and timing matters—testing too early may miss the rise.

Because ASO levels can remain elevated for a while, doctors often interpret the result alongside your symptoms, exam findings, and sometimes a repeat ASO titre (to look for a rising or falling trend) or another antibody test such as anti-DNase B.

The ASO titre is usually ordered when there are symptoms or signs of illness that may be linked to a recent strep infection, but the original infection (like strep throat) is no longer present or was never confirmed.

Your clinician may request an ASO titre if you have symptoms that raise concern for post-streptococcal complications, including:

Possible rheumatic fever (a complication after Group A strep throat), such as:

Fever, painful or migrating joint swelling (often knees/ankles), chest pain or shortness of breath, a new heart murmur, unusual fatigue, or involuntary movements (Sydenham chorea).

Possible post-streptococcal glomerulonephritis (kidney inflammation after strep), such as:

Dark/tea-colored urine, swelling of the face or legs, reduced urine output, or high blood pressure.

Ongoing inflammation after suspected strep, including persistent joint pain, unexplained elevated inflammatory markers, or symptoms following a recent sore throat or skin infection (impetigo).

This test is also used to help differentiate causes of symptoms that could mimic other conditions (for example, viral infections or autoimmune diseases). It is not typically used to diagnose an active sore throat on its own; a rapid strep test or throat culture is preferred for that.

• Rheumatic fever (including rheumatic heart disease risk)
• Post-streptococcal glomerulonephritis
• Post-streptococcal reactive arthritis
• Scarlet fever (evidence of recent Group A strep infection)
• Recent Group A streptococcal pharyngitis (“strep throat”)
• Recent streptococcal skin infection (e.g., impetigo; may also be evaluated with anti-DNase B)

• Identifying a recent strep infection when symptoms appear later or initial testing was missed
• Evaluating strep complications such as rheumatic fever or kidney inflammation after strep
• Protecting heart health by supporting timely diagnosis of rheumatic fever and reducing risk of rheumatic heart disease
• Supporting kidney health monitoring when post-streptococcal glomerulonephritis is suspected
• Guiding follow-up care by trending antibody levels (when clinically appropriate) alongside symptoms and other lab tests