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Strep Antistreptolysin O Titer (Blood)

Does this test have other names?

ASO titer

What is this test?

This test looks for antibodies that your body makes while fighting group A Streptococcus bacteria. The antibodies work against a substance made by the bacteria called streptolysin O.

Group A Streptococcus can cause strep throat. It can also cause other infections that can lead to other conditions that are more severe. This includes rheumatic fever, which most often occurs in children between 5 and 15 years old.

Strep bacteria can also cause:

  • Streptococcal glomerulonephritis, a disorder of the kidneys 

  • Scarlet fever, an infection that causes a red rash

  • Impetigo, a skin infection

  • Toxic shock syndrome

  • Cellulitis and necrotizing fasciitis, also known as flesh-eating disease

Why do I need this test?

An ASO titer is done to find out if you have a current or recent strep infection that may have caused these health problems. Antibodies from a strep infection begin to increase about 1 week after a strep infection. They may get higher for several weeks before decreasing. Because antibodies don't increase right away, the better test to use to diagnose a strep infection is the rapid strep test. ASO titers can help later, if your healthcare provider needs to prove that you've had a recent strep infection.

What other tests might I have along with this test?

You may also need a throat culture to look for group A beta-hemolytic streptococci. And you may need a rapid streptococcal antigen test.

If your healthcare provider thinks that you have rheumatic fever, you may also need other antibody tests. These may include anti-DNase B, antihyaluronidase, or anti-Streptozyme. You may also have a chest X-ray, electrocardiogram, and other blood tests.

If you have an infection that has spread through your body, you may also need other cultures. These may include cultures of your blood, phlegm in your lungs (sputum), and certain tissues.

What do my test results mean?

Test results may vary depending on your age, gender, health history, the method used for the test, and other things. Your test results may not mean you have a problem. Ask your healthcare provider what your test results mean for you.  

A negative result means that you have no antibodies to the strep bacteria in your blood. Because it takes time for the number of antibodies to increase in your blood after you are infected, you may need to repeat the ASO titer 2 weeks after your first blood sample.

A positive result means that antibodies have been found. This means that you may have had a recent strep infection. But in 1 in5 cases, this test won't show an increase in antibodies when you have an illness such as rheumatic fever. You may need other tests to confirm you have an active infection.

How is this test done?

The test is done with a blood sample. A needle is used to draw blood from a vein in your arm or hand. 

Does this test pose any risks?

Having a blood test with a needle carries some risks. These include bleeding, infection, bruising, and feeling lightheaded. When the needle pricks your arm or hand, you may feel a slight sting or pain. Afterward, the site may be sore.  

What might affect my test results?

Timing is important for this test. You may have a false-negative. This means the results show you don’t have the antibodies in your blood, even when you do have an infection. This can happen if are you infected with the strep bacteria but not enough time has passed to let antibodies build up in your blood. It takes 4 to 5 weeks for the number of antibodies to reach a peak.

How do I get ready for the test?

You don't need to prepare for this test. But be sure your healthcare provider knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illegal drugs you may use.

Medical Reviewers:

  • Bass, Pat F. III, MD, MPH
  • Haldeman-Englert, Chad, MD