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Anti-Streptolysin O (ASO) Test

Last updated May 7, 2018

Approved by: Krish Tangella MD, MBA, FCAP


Anti-Streptolysin O Test is a measurement of the amount of ASO, a type of antibody, in blood.

What are the other Names for this Test? (Equivalent Terms)

  • Anti-Streptolysin O Titer Test
  • ASLO Test
  • ASOT Test

What is Anti-Streptolysin O (ASO) Test? (Background Information)

The following is background information for Anti-Streptolysin O (ASO) Test:

  • Anti-Streptolysin O Test is a measurement of the amount of ASO, a type of antibody, in blood
  • Antibodies are produced by the body, to fight infection. Specific antibodies are produced for specific types of microorganisms. These antibodies remain in the body for a long time and help prevent infection from the same microorganism, in the future
  • The human body, in response to infection by group A streptococci bacteria, generates one such type of antibody, known as ASO. Group A streptococci bacteria causes illness, which includes rheumatic fever, scarlet fever, bacterial endocarditis, glomerulonephritis, and strep throat
  • In a majority of the cases, strep throat infections (causing sore throat) are identified and treated with antibiotics, leading to resolution of the infection. However, if the infection remains untreated, may be due to a lack of symptoms, or is treated ineffectively, then the bacteria remain in the body
  • This may lead to post-streptococcal infection, with complications, such as endocarditis, glomerulonephritis, and rheumatic fever. If the healthcare provider suspects any of these conditions in an individual, then an ASO Test is ordered, to rule out a previous streptococcal infection leading to the disease
  • Often, the requirement to confirm diagnoses and monitor the progress of an infection is through a series of tests, performed every 10-14 days, for a period of 6 weeks. This is called serial ASO titre measurement

What are the Clinical Indications for performing the Anti-Streptolysin O (ASO) Test?

The primary reason to perform an Anti-Streptolysin O (ASO) Test is the presence of symptoms, suspected to be caused previously, by group A streptococcus infection. Usually, individuals are prescribed this test, when symptoms emerge, weeks after a skin infection, or a sore throat.

Signs and symptoms associated with glomerulonephritis (inflammation, which affects the kidneys) are:

  • Swelling of the face, eyes, lower extremity, or abdomen (called anasarca)
  • Brown, rust-colored, or dark urine, due to blood in urine (called hematuria)
  • Decreased urine output, problems urinating
  • Weakness and tiredness
  • Pain in the muscles and joints (called myalgia)
  • High blood pressure (Hypertension)
  • Flank pain, resulting in pain on side of the abdomen

Sign and symptoms of rheumatic fever include:

  • Fever
  • Joint pain (arthralgia)
  • Inflammation of the heart (carditis), leading to shortness of breath, chest pain, and palpitations
  • Abdominal pain
  • Skin rashes and skin nodules (called rheumatic nodules)
  • Rapid jerky movements (chorea)

The test is usually ordered twice, over a period of 2 weeks, in order to note the rising, falling, or constant levels, of the antibody. This is called serial measurement of Anti-Streptolysin O Test.

How is the Specimen Collected for Anti-Streptolysin O (ASO) Test?

Following is the specimen collection process for Anti-Streptolysin O (ASO) Test:

Sample required: Blood

Process: Insertion of needle into a vein (arm).

Preparation required: No food or drink is to be consumed, 6 hours before the test. The healthcare provider shall be informed about any current medications being taken, as some medications (like antibiotics and corticosteroids), interfere with the test results.

What is the Significance of the Anti-Streptolysin O (ASO) Test Result?

A positive Anti-Streptolysin O (ASO) Test result is when the ASO levels are >200 units/ml. The results vary with an individual’s age and with respect to individual laboratories, where tests are conducted. The healthcare provider evaluates the test results taking into consideration, all these various factors.

The significance ofASO Test is as follows:

  • The ASO level rises, until it reaches its highest level, approximately 4-weeks after the throat infection. Thereafter, the levels decline, if the infection is resolved or is eliminated. It may take several months for the levels to become undetectable
  • A preliminary low or negative ASO level, followed subsequently by low or negative ASO levels, indicates that no infection has occurred. However, a small number of post-streptococcal cases with complications may not present with elevated ASO
  • Initial high or low levels, followed subsequently by higher levels, indicate a recent or current infection
  • ASO Test, though helpful in a diagnosis of the condition, does not help predict the development of complications, after a streptococcal infection

The laboratory test results are NOT to be interpreted as results of a "stand-alone" test. The test results have to be interpreted after correlating with suitable clinical findings and additional supplemental tests/information. Your healthcare providers will explain the meaning of your tests results, based on the overall clinical scenario.

Additional and Relevant Useful Information:

  • Due to false negative results for Anti-Streptolysin O Test in some positive cases, another antibody test, called the anti-DNase B is ordered along with ASO
  • Anti-DNase B is a different type of antibody produced by the body immune system, in response to group A streptococcal infection. The combined testing helps in better evaluating the presence of group A streptococcal infection

Certain medications that you may be currently taking may influence the outcome of the test. Hence, it is important to inform your healthcare provider, the complete list of medications (including any herbal supplements) you are currently taking. This will help the healthcare provider interpret your test results more accurately and avoid unnecessary chances of a misdiagnosis.

References and Information Sources used for the Article:

Reviewed and Approved by a member of the DoveMed Editorial Board
First uploaded: April 11, 2014
Last updated: May 7, 2018