Anti Streptolysin O (ASO) Test: Principle, Procedure and Interpretation

Last updated on May 10th, 2020

The test principle, procedure, and interpretation presented here are based on Latex Anti-Streptolysin O (ASO) test.  

It is a rapid latex agglutination test for the qualitative and semi-quantitative determination of anti-streptolysin-O antibodies (ASO) in serum. In infections caused by β-hemolytic streptococci,  streptolysin-O is one of the two hemolytic exotoxins liberated from the bacteria that stimulates the production of  ASO antibodies in the human serum.  The presence and the level of these antibodies in the serum may reflect the nature and severity of the infection.

ASO latex reagent is a stabilized buffered suspension of polystyrene latex particles that have been coated with Streptolysin O.

ASO Latex test
ASO Latex test

When the latex reagent is mixed with a serum containing ASO, agglutination occurs. The sensitivity of the latex reagent has been adjusted to yield agglutination when the level of ASO is greater than 200 IU/ml, a level determined to be indicative of disease by epidemiological and clinical studies. Sera having titers of between 200 IU/ml to 3500 IU/ml will be reactive.

Sample Collection and Handling

Only fresh serum specimens should be used. Plasma must not be used since fibrinogen may cause non-specific agglutination of the latex. It is preferable to test samples on the same day as collected. Serum samples may be stored at 2-8°C for up to 48 hours prior to testing. If longer storage is necessary,  sera should be stored frozen at -20°C.

Materials used in the ASO Test

  1. ASO Antigen: A stabilized buffered suspension of polystyrene latex particles coated with Streptolysin O and 0.1% sodium azide as a preservative. Shake well prior to use.
  2. ASO Positive Control: Human serum-containing more than 200 IU/ml ASO and 0.1% sodium azide as a preservative.
  3. ASO Negative Control: Human serum containing 0.1% sodium azide as a preservative.
  4. Sufficient disposable pipettes.
  5. Glass test slide.

Procedure for Antistreptolysin O Test

  1. Bring all test reagents and samples to room temperature.
  2. Use a disposable pipette to draw up and place one free-falling drop of each undiluted sample into its identified circle of the slide. Retain each pipette for mixing in step 5.
  3. Deliver one free-falling drop of positive and negative control into its identified circle.
  4. Mix the ASO latex reagent by gently shaking. Add one free-falling drop of a reagent to each control and sample.
  5. Using the flattened end of the appropriate plastic pipette as a stirrer (step 2), thoroughly mix each sample with reagent within the full area of the circle.
  6. Discard the disposable pipette.
  7. Slowly rock the slide for exactly two (2) minutes and observe for agglutination under a high-intensity light.
  8. Record results.
  9. Re-wash glass slide for future use

Test Result

A test sample is considered to contain ASO antibodies in excess of 200 IU/ml when agglutination (clumping) is observed when compared to the result of the negative control.

About Acharya Tankeshwar 467 Articles
Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. I am working as an Asst. Professor and Microbiologist at Department of Microbiology and Immunology, Patan Academy of Health Sciences, Nepal. If you want me to write about any posts that you found confusing/difficult, please mention in the comments below.


  1. Any possibility to get a reliable source for those information to use them in a seminar paper? Very good info like an overview but sources needed…thanks.

    • Dear Karo
      Thank you for your inspiring comment. This blog is only for the education/academic purpose to facilitate undergraduate students, so not written in the scientific ways. You can get similar information in published papers indexed in PUBMED among others.

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