ANTI STREPTOLYSIN O (ASO)

The Anti Streptolysin O (ASO) test is a blood test that detects antibodies produced by the immune system against streptolysin O, a toxin released by Group A Streptococcus bacteria. These bacteria commonly cause throat infections such as strep throat and may also cause skin infections. The body begins producing ASO antibodies after exposure, and the level can remain elevated for weeks or months after the original infection has resolved.

The test is mainly used to look for evidence of a recent streptococcal infection when a patient develops symptoms that may represent a delayed complication. These complications can include acute rheumatic fever, which may affect the heart, joints, skin or nervous system, and post-streptococcal glomerulonephritis, which affects the kidneys. The ASO test does not directly detect the bacteria and is not the preferred test for diagnosing an active sore throat.

Benefits of the Test

• Detects antibodies associated with a recent Group A streptococcal infection.
• Supports the evaluation of suspected acute rheumatic fever.
• May assist in investigating post-streptococcal kidney inflammation.
• Can provide evidence of an earlier infection after throat symptoms have disappeared.
• May be repeated to identify a rising or falling antibody pattern.
• Provides useful supporting information when combined with symptoms and other investigations.

Why Doctors Recommend This Test

A doctor may recommend an ASO test when symptoms develop several weeks after a possible or known streptococcal infection. These symptoms may include fever, painful or swollen joints, joint pain that moves from one area to another, unusual involuntary movements, skin changes, shortness of breath, chest discomfort or signs of heart inflammation.

The test may also be requested when kidney inflammation is suspected following an infection. Possible warning signs include dark or cola-coloured urine, blood in the urine, swelling around the eyes or ankles, reduced urine output and elevated blood pressure. Such symptoms require prompt medical evaluation and should not be assessed using an ASO result alone.

Because ASO antibodies take time to develop, the test may be negative during the early stage of an infection. Antibody levels commonly rise after one to three weeks, peak later and gradually decline over several months. In selected cases, doctors request a second sample after approximately two to four weeks to determine whether the titre is increasing.

Preparation Before Test

A blood sample is required. No special preparation is usually necessary, and fasting is generally not required when the ASO test is ordered by itself. If it is combined with investigations that require fasting, follow the instructions provided by your doctor or Focus Diagnostics.

Inform your doctor about recent infections, antibiotic treatment, anti-inflammatory medicines and existing medical conditions. Antibiotic use and the timing of sample collection may influence interpretation. Do not stop any prescribed medicine unless instructed by a qualified healthcare professional.

Normal Reporting Time

The Anti Streptolysin O test is generally reported on the same day or within 24 hours after the sample reaches the laboratory. Turnaround time can vary according to collection time, laboratory workflow and whether a quantitative titre or additional testing is required. Focus Diagnostics will confirm the expected reporting schedule during collection.

Who Should Take This Test?

This test may be suitable for patients whose doctor suspects a delayed complication of a recent Group A streptococcal infection. It may be considered for people with symptoms suggesting acute rheumatic fever or post-streptococcal glomerulonephritis, particularly when the original throat or skin infection has already resolved.

The ASO test is not intended as a general screening test and is not usually the first choice for diagnosing a current sore throat. A throat swab, rapid antigen test or molecular test may be more appropriate for an active throat infection, depending on clinical guidance.

Detailed Information

A positive or elevated ASO result indicates that the immune system has produced antibodies against streptolysin O. It supports recent exposure to Group A Streptococcus but does not prove that current symptoms are caused by the infection. Some healthy people may retain elevated levels after an earlier infection, and reference limits may vary with age, population and laboratory method.

A negative result does not always exclude a previous streptococcal infection. Testing may have been performed too early, antibody production may be limited, or the infection may have involved a bacterial strain that did not cause a strong ASO response. Antibodies against another streptococcal enzyme, measured by an anti-DNase B test, may provide additional evidence, especially after some skin infections.

Doctors interpret ASO alongside symptoms, examination findings and other investigations. These may include C-reactive protein, erythrocyte sedimentation rate, complete blood count, anti-DNase B, urine analysis, kidney function tests, electrocardiography or echocardiography. Neither a positive ASO result nor a single numerical value independently confirms rheumatic fever, kidney disease or an active streptococcal infection.