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What is Antistreptolysin O (ASO) test?

Our immune system makes Antistreptolysin O (ASO) antibodies to fight infection caused by group A Streptococcus bacteria, the most common culprit in throat and skin infections. Most cases of infection are detected and treated with antibiotics. However, the infection can go untreated or inadequately treated in patients who do not present the typical symptoms (see below). This can lead to complications such as rheumatic fever and glomerulonephritis (a kidney disease). If untreated, these complications can cause further damage to the heart and kidneys. Several patients who have been diagnosed with rheumatic fever and acute glomerulonephritis from streptococcal infection have increased ASO levels. 

ASO antibodies are usually produced a week to 10 days after the initial infection; they peak at three to five weeks and subside within six to 12 months. For this reason, they are not effective for detecting acute or existing infections. While the ASO test can help detect prior infections, acute streptococcal infections should be diagnosed by direct streptococcal cultures or the presence of streptococcal antigens. 

The name Antistreptolysin O (ASO) comes from streptolysin O, a toxic enzyme produced by group A Streptococcus bacteria.

  1. Why is an ASO test performed?
  2. How do you prepare for an ASO test?
  3. How is ASO test performed?
  4. What do ASO test results indicate?

ASO titre test is done specifically to determine a Streptococcus infection by assessing the levels of antibodies produced in response to streptolysin O.

Although streptococcal infections can be easily controlled by antibiotics that kill the infection-causing bacteria, there can be cases where the treatment may get delayed due to an absence of apparent symptoms, leading to post-streptococcal complications.

An ASO titre test is recommended when the individual shows symptoms of post-streptococcal complications, such as bacterial endocarditis, glomerulonephritis and rheumatic fever.

Antibodies tend to peak in the body for 3-8 weeks following a Streptococcus infection and would remain so for several months.

Your doctor will provide instructions related to any preparations before the test. You will likely be asked to fast for 6 hours before the test.

Intake of certain medications such as antibiotics may also be restricted a few hours before the test. However, one must not stop taking any routine medication unless asked by a physician. 

  • A lab technician will place an elastic band around your upper arm to stop the flow of blood, which enables easy identification of a vein for withdrawal of blood sample
  • The needle injection site is cleaned using alcohol for sanitisation
  • A needle is then inserted into the vein. One or more needle sticks may be required occasionally
  • A tube is attached to the needle to collect the blood sample, and once enough blood is collected, the band is removed from the arm.
  • Cotton is placed over the site as the needle is removed and some pressure is applied to stop bleeding
  • A bandage is placed on the site thereafter

You may experience some tightness when the band is wrapped around the upper arm. Few individuals report no pain from the needle, and some may feel a quick sting or pinch.

Besides the fact that the individual may experience some pain while blood is drawn, some other risks associated with a blood test are:

  • Difficulty in obtaining the sample
  • Excessive bleeding at the site from where the blood is drawn
  • Fainting
  • Haematoma (accumulation of blood under the skin)
  • Infection at the site where the needle is inserted

However, these risks can easily be reduced when proper precautionary measures are taken.

Normal results: A negative test result indicates the absence of a Streptococcus infection. However, the test may be performed again after two weeks. Normal values may vary slightly among laboratories. The results must be discussed with a doctor to get a clear diagnosis.

Abnormal results: An elevated antibody titer indicates a recent Streptococcus infection even though you may not have experienced any symptoms.

This test does not predict the complications that may occur due to the infection, and it does not assess the severity of the infection. However, it may help in confirming the diagnosis of a Streptococcus infection complication.

Disclaimer: All results must be clinically correlated with the patient’s complaints to make a complete and accurate diagnosis. The above information is provided from a purely educational perspective and is in no way a substitute for medical advice from a qualified doctor. 

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References

  1. MedlinePlus Medical Encyclopedia: US National Library of Medicine; Antistreptolysin O titer
  2. University of Rochester Medical Center [Internet]. Rochester (NY): University of Rochester Medical Center; Strep Antistreptolysin O Titer (Blood)
  3. Stanford T. Shulman. Clinical Practice Guideline for the Diagnosis and Management of Group A Streptococcal Pharyngitis: 2012 Update by the Infectious Diseases Society of America. Clinical Infectious Diseases, Volume 55, Issue 10, 15 November 2012, Pages e86–e102, https://doi.org/10.1093/cid/cis629
  4. Benioff Children's Hospital [internet]: University of California, San Francisco; ASO Titer
  5. UF Path labs-Department of Pathology [internet]: University of Florida; Antistreptolysin O (ASO) Antibodies
  6. Ricardo G. Hahn, Lynda M. Knox, and Todd A. Forman. Evaluation of Poststreptococcal Illness. Am Fam Physician. 2005 May 15;71(10):1949-1954.
  7. Patterson MJ. Streptococcus. In: Baron S, editor. Medical Microbiology. 4th edition. Galveston (TX): University of Texas Medical Branch at Galveston; 1996. Chapter 13.