Antistreptolysin O (ASO)

What it is

Antistreptolysin O, often shortened to ASO, is a type of antibody that your body produces in response to a specific toxin called streptolysin O. This toxin is made by a common type of bacteria known as Group A Streptococcus. These are the same bacteria that can cause infections like strep throat and scarlet fever.

Think of antibodies as your immune system's defence force. When these bacteria invade, your body recognises the streptolysin O toxin as foreign and starts producing ASO antibodies to fight it off. The ASO test measures the level of these antibodies in your blood.

Why it matters

Measuring ASO levels can be helpful for your doctor to understand if you've had a recent or past infection with Group A Streptococcus. It's not typically used to diagnose an active infection itself, but rather to see if your body has mounted an immune response to it.

Sometimes, even after an infection like strep throat has cleared up, the bacteria can trigger a delayed reaction in other parts of the body. This is known as a post-streptococcal complication. These complications can affect the heart, joints, kidneys, or skin. The ASO test can help your doctor determine if a recent streptococcal infection might be linked to these later issues.

The reference range for ASO levels in the UK is generally up to 200 IU/ml. Levels above this might suggest a recent or past exposure to the bacteria.

What can cause it to be high or low

High ASO levels are most commonly associated with a recent or past infection by Group A Streptococcus. This could include:

• Strep throat: A common bacterial infection causing a sore throat.
• Scarlet fever: An illness that often follows strep throat, characterised by a rash.
• Skin infections: Such as impetigo, caused by the same bacteria.

It’s important to note that ASO levels don't usually rise immediately with an infection. It can take a few weeks for the antibodies to develop and become detectable in the blood. The levels then tend to fall gradually over several months. Therefore, a high ASO level is more indicative of a past infection rather than an ongoing one.

In some instances, ASO levels might remain elevated for a longer period, even after the infection has fully resolved.

Low ASO levels are generally considered normal and indicate that there's no evidence of a recent or past significant exposure to Group A Streptococcus that would trigger a detectable antibody response.

Lifestyle factors that may help

While ASO levels themselves are a marker of past infection and not directly modifiable by lifestyle, maintaining good general health can support your immune system's ability to fight off infections. Discussing the following with your GP might be beneficial:

• Good hygiene practices: Regular handwashing can help prevent the spread of bacteria.
• Balanced diet: Eating a nutritious diet supports overall immune function.
• Adequate sleep: Sufficient rest is crucial for the body's recovery and immune response.
• Managing stress: Chronic stress can impact the immune system.

Questions to discuss with your GP

If your ASO test results are higher than the expected range, or if you have concerns about a past infection and its potential consequences, there are several points you might wish to discuss with your GP:

• What do my specific ASO results mean in the context of my symptoms and medical history?
• Could my symptoms be related to a past streptococcal infection?
• Are there any further tests or investigations that might be helpful?
• What are the potential long-term implications of a past Group A Streptococcus infection?
• What steps can I take to support my overall health and immune system?
• Are there any specific signs or symptoms I should be aware of that might indicate a complication?