Definition

Antistreptolysin O Titer, or ASTO, is a blood test used to measure the level of antibodies against streptolysin O, a toxic enzyme produced by the group A Streptococcus (GAS) bacteria. Antibodies are proteins that the body produces in response to harmful foreign substances, such as bacteria.

Group A Streptococcus bacteria are known to cause common infections, including sore throats and skin infections. These bacteria produce a harmful enzyme called Streptolysin O, which can trigger the immune system to produce antibodies in an attempt to neutralize the infection.

When infected with group A Streptococcus (GAS) bacteria, the body produces antibodies to combat the toxic enzyme known as Streptolysin O. These antibodies are referred to as Antistreptolysin O (ASO). The level of ASO antibodies typically rises about one week after the onset of infection symptoms and remains elevated for several weeks before gradually decreasing over time.

The ASTO test is primarily used to determine if there is a history of GAS bacterial infection. This is important because untreated GAS infections can lead to serious complications such as rheumatic fever and glomerulonephritis.

Indication

Your doctor may recommend an ASTO test if you are suspected of having the following conditions:

• Infection. If the GAS infection is not treated properly, it can spread to other parts of the body, such as the sinuses, ears, and blood.

• Scarlet fever

• Rheumatic fever

• Glomerulonephritis

• Endocarditis caused by bacterial infection

This test may also be recommended if you have symptoms that suggest a past GAS infection, including:

• Fever

• Sore throat

• Infected skin wounds

• Red rash on the skin

• Joint pain

• Blood or dark-colored urine

• Chest pain

Contraindication

There are no contraindications or conditions that would prevent someone from undergoing this test.

Preparation Before the Test

Before taking the ASTO test, you should fast for approximately 6 hours. During this fasting period, you should avoid eating or drinking.

It is also important to inform your doctor about any medications you are currently taking, as certain medications can affect the results of the test. 

Test Procedure

The ASTO test usually requires a blood serum sample of 0.25 - 0.5 ml. This blood sample is drawn from a vein, typically from the elbow crease, and collected into a special tube. The laboratory staff will wear gloves and Personal Protective Equipment (PPE) during the procedure.

Before taking the blood, the healthcare provider will place an elastic band around your upper arm and clean the area where the needle will be inserted with antiseptic gauze. After collecting the blood into the special tube, the band will be removed, and the puncture site will be cleaned and pressed with antiseptic gauze.

This procedure takes only a few minutes. Once the blood sample is obtained, it will be analyzed in the laboratory using specialized equipment, and results are typically available within a few hours.

Normal and Abnormal Values

The normal value range for the ASTO test varies by age. Here are the typical normal value ranges for ASTO results:

As shown in the table above, ASTO levels vary according to age, with upper and lower limits for each group. ASTO blood levels are considered abnormal if the results fall outside of the normal range indicated.

Some laboratories may report results as positive or negative, where normal results are considered negative, and abnormal results are indicated by a positive result.

Results and Recommendations (Follow-up Tests)

Negative Results

If the ASTO test shows negative results or very low values, it means that there is no history of GAS bacterial infection, which is considered normal. The body typically produces ASO antibodies within one week to a month following a GAS infection.

It is recommended that you continue to maintain a healthy lifestyle to prevent future GAS bacterial infections, which may cause symptoms such as fever, sore throat, joint pain, and skin rashes. These symptoms may indicate abnormal parameters and require further medical attention if they arise.

Positive Results

If the ASTO test results are positive or show an increase, it likely means you have recently been infected with GAS bacteria. A positive result can also suggest that your body is experiencing complications from a previous GAS infection.

In such cases, it is important to consult a doctor promptly. The doctor may recommend lifestyle changes and may start drug therapy to address the symptoms caused by the GAS infection.

Consult the Right Doctor

If the ASTO test shows abnormal results in adults, it is advisable to consult a general practitioner for further tests or to begin therapy for an appropriate diagnosis. You can also seek advice from an internist if you wish to discuss your ASTO results further. For pediatric patients, any abnormal test results should be discussed with a pediatrician.

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