Antistreptolysin O Titer (ASO) Test

Written by Darla Burke | Published on July 16, 2012
Medically Reviewed by George Krucik, MD

What Is an ASO Titer?

When you are exposed to harmful bacteria, your body produces antibodies. These antibodies help protect your body against future infections from the same bacteria. They are specific to the bacteria they were produced against.

These antibodies can be detected using blood tests. One such blood test is an antistreptolysin O titer (ASO) test. The ASO test measures antibodies against streptolysin O. These antibodies are made when you are exposed to group A Streptococcus (GAS).

Symptomatic infections caused by GAS are treated with antibiotics. However, many people don’t have symptoms. Therefore, their infections may not be detected and treatment may not be provided. Complications—referred to as post-streptococcal complications—can arise if GASremains in your body.

The ASO titer test is essential in detecting and treating GAS.

Why Is the ASO Test Ordered?

Your doctor will order the ASO test if you have symptoms of poststreptococcal complications. Such complications include:

  • bacterial endocarditis
  • glomerulonephritis
  • rheumatic fever
  • scarlet fever
  • strep throat

The antistreptolysin antibody remains in the body four to six weeks after you contract GAS bacteria. Your doctor can determine if symptoms are being caused by a post-streptococcal complication by checking antibody levels.

How Is the ASO Test Administered?

The ASO test is performed on a blood sample. This is typically taken by a nurse or lab technician. A needle will be used to draw blood from a vein in your elbow or hand. The blood will be collected in a tube. This tube will be sent to a lab for analysis. Your doctor will share your results with you when they become available.

What Are the Risks of the ASO Test?

The risks of the ASO test are minimal. You may experience pain during or after the blood draw.

Other potential risks of a blood draw include:

  • difficulty obtaining a sample, resulting in multiple needle sticks
  • excessive bleeding at the needle site
  • fainting as a result of blood loss
  • accumulation of blood under the skin, known as a hematoma
  • infection at the puncture site

Preparation for the ASO Test

Your doctor will tell you if there is any special preparation needed for the test. You may be told not to eat or drink anything for six hours before the test.

Your doctor may recommend that you stop taking certain medications before this test. Corticosteroids and certain antibiotics may reduce ASO antibody levels, making it difficult for your doctor to confirm your diagnosis.

Tell your doctor about all of the medications you are taking. Be sure to include both prescription and over-the-counter drugs. Do not stop taking your medication unless told to do so.

Understanding the Results of an ASO Test

Generally, an ASO test value below 200 is considered normal. In preschool-aged children, the test value should be less than 100. However, specific results will vary by laboratory.

If your results show that your ASO value is elevated, you may have a post-streptococcal complication.

Your doctor may repeat the test within 10 to 14 days to confirm your results. The body produces ASO antibodies within a week after infection with GAS. If both tests are negative, your symptoms are probably not caused by a Streptococcus infection.

If the results of your tests show that your ASO antibodies are increasing, it is likely that your infection is recent. Declining antibody levels suggest that your infection is probably getting better.

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