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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 made while fighting off group A Streptococcus bacteria. These are antibodies against a substance called streptolysin O, made by the bacteria.

Group A Streptococcus can cause strep throat and other infections that can eventually lead to other, more serious, conditions. These include rheumatic fever and streptococcal glomerulonephritis, a disorder of the kidneys. Rheumatic fever most commonly strikes children between 5 and 15 years old.

Strep bacteria can also cause:

  • Scarlet fever, an infection that causes a red rash

  • Impetigo, a skin infection

  • Toxic shock syndrome

  • Cellulitis and necrotizing fasciitis, or flesh-eating disease

An ASO titer is done to find out whether 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.

Why do I need this test?

You may need this test if you have symptoms of another illness a few weeks after having a sore throat that could have been caused by group A streptococcus. 

What other tests might I have along with this test?

Your healthcare provider may also order a throat culture to look for group A beta-hemolytic streptococci and a rapid streptococcal antigen test.

If your provider suspects that you have rheumatic fever, he or she may also order other antibody tests, including anti-DNase B, antihyaluronidase, or anti-Streptozyme. He or she may also order a chest X-ray, electrocardiogram, and other blood tests.

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

What do my test results mean?

Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your healthcare provider.

Normal results are negative, meaning 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 and 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. Your healthcare provider may order another antibody test or cultures of your blood and tissues to confirm you have an active infection.

How is this test done?

The test requires a blood sample, which is drawn through a needle from a vein in your arm.

Does this test pose any risks?

Taking a blood sample with a needle carries risks that include bleeding, infection, bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight stinging sensation or pain. Afterward, the site may be slightly sore.

What might affect my test results?

Timing is important for this test. You may have a false-negative if are you infected with the strep bacteria but not enough antibodies have built 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 illicit drugs you may use.

Medical Reviewers:

  • Hanrahan, John, MD
  • Sather, Rita, RN