Anti-Streptolysin O (ASO): Meaning and Results

What Is Anti-Streptolysin O (ASO)?

Anti-streptolysin O (ASO) is an antibody that the body produces when it encounters streptolysin O, a toxin released by Group A Streptococcus bacteria. These bacteria can cause illnesses like strep throat, scarlet fever, or skin infections.

ASO levels rise as part of the immune response to this infection. Measuring them helps doctors understand whether someone recently had a streptococcal infection, even if the symptoms have already disappeared.

This test doesn’t detect the bacteria itself. Instead, it checks your body’s reaction to the infection. It’s especially useful when diagnosing conditions that appear weeks after the infection, such as inflammation of the heart, joints, or kidneys.

In simple terms, ASO works like a footprint - it shows that your immune system fought a streptococcal infection recently.

When Is Anti-Streptolysin O (ASO) Testing Recommended?

The ASO test is usually ordered when a doctor suspects a recent or past streptococcal infection that may have led to complications.

It may be recommended if you experience symptoms that suggest an inflammatory condition.

Common situations include:

• Joint pain or swelling - may indicate inflammation after a strep infection.
• Fever with fatigue - could signal post-infection reactions.
• Skin rashes or red patches - often linked to immune responses.
• Shortness of breath (SOB) or chest pain - possible sign of heart inflammation.
• Swelling around the eyes or ankles - may point to kidney involvement.
• History of untreated strep throat - especially if complications appear weeks later.

Doctors may also repeat the test over time to check whether antibody levels are increasing or decreasing, which helps track recovery.

How Is Anti-Streptolysin O (ASO) Testing Conducted?

The ASO test is a blood test. A small sample is taken from a vein in your arm and sent to the lab for analysis.

In the laboratory, the blood sample is tested to measure the concentration of ASO antibodies.

The result shows whether your immune system has reacted to a recent streptococcal infection.

Some laboratories may include ASO testing in a panel with other antibody tests like the anti-DNase B test. Together, they help confirm or rule out complications caused by Group A Streptococcus.

Preparation for the Test

There’s usually no special preparation needed for an ASO test. However, here are a few tips:

• Avoid eating right before the test if your doctor requests fasting.
• Inform the healthcare provider about any recent infections or medical procedures.
• Stay hydrated to make the blood draw easier.

The test takes only a few minutes, and you can return to your normal activities immediately afterward.

What Are the Normal Anti-Streptolysin O (ASO) Levels?

Normal ASO levels can vary slightly among laboratories, but the general reference ranges are:

• Adults: less than 200 IU/mL
• Children: less than 250-300 IU/mL

ASO levels typically start to rise about 1 week after infection, peak around 3 to 5 weeks, and gradually return to normal over a few months.

Note: The units of measurement and the normal ranges can differ depending on the laboratory and the specific methods used.

What Do Abnormal Levels Mean?

Abnormal ASO levels usually indicate that your body has recently been exposed to Group A Streptococcus. The higher the value, the more likely a recent infection occurred.

However, test results should always be interpreted alongside your symptoms and medical history, since high or low ASO levels alone don’t confirm or exclude a condition.

Anti-Streptolysin O (ASO) High Levels

High ASO levels mean your immune system recently fought a streptococcal infection. The levels can stay elevated for several weeks or months after the infection resolves.

Common Causes of Elevated ASO Levels

• Recent strep throat infection: The most common reason for a rise in ASO levels.
• Scarlet fever: A strep-related illness that can cause a significant antibody response.
• Rheumatic fever: An inflammatory reaction that can develop after untreated strep infection, affecting joints, heart, or skin.
• Post-streptococcal glomerulonephritis: A kidney condition caused by immune system reactions following infection.
• Skin infections (impetigo, cellulitis): Some skin-related strep infections can also trigger antibody production.

High ASO levels don’t always mean the infection is still active - they may simply reflect that your immune system reacted recently.

• Follow up with a healthcare provider: A doctor can evaluate symptoms and order additional tests if needed.
• Rest and recover: Support your immune system with adequate rest, hydration, and nutrition.
• Track symptoms: Note any persistent joint pain, swelling, or fatigue.
• Avoid self-diagnosis: ASO levels alone can’t confirm a disease; a complete evaluation is needed.
• Schedule repeat testing: Sometimes, two ASO tests a few weeks apart show whether levels are increasing or decreasing.

Anti-Streptolysin O (ASO) Low Levels

Low ASO levels are generally normal and suggest no recent streptococcal infection. However, very low or undetectable levels can sometimes occur even after an infection, depending on individual immune response.

Common Causes of Low ASO Levels

• No recent strep infection: The body hasn’t been exposed to streptolysin O toxins.
• Early testing: ASO levels may still be low if the test is done too soon after infection.
• Weakened immune response: Some people produce fewer antibodies due to individual or temporary immune factors.
• Use of antibiotics early in infection: Treatment may prevent the immune system from producing high antibody levels.

Recommendations

• Retest after 1-2 weeks: If infection is suspected, a follow-up test can show if antibody levels rise.
• Share your full health history: Inform your doctor about recent symptoms, illnesses, or treatments.
• Maintain healthy habits: A strong immune system helps your body respond effectively to infections.

Which Specialist Should You Consult?

If your ASO test results are abnormal or if you experience related symptoms, the right specialist depends on your condition:

• Primary care doctor: For initial evaluation and coordination of further testing.
• Infectious disease specialist: If there’s evidence of ongoing or complex bacterial infection.
• Rheumatologist: For joint pain, swelling, or suspected autoimmune reactions after strep infection.
• Nephrologist: For kidney-related symptoms such as swelling or changes in urine.
• Cardiologist: If there are signs of heart inflammation or complications following infection.