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ANTI – STREPTOLYSIN O (ASO) TEST

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There are two types of serological tests that are available for the detection of antibodies produced against an invading Streptococcal pathogen.

  1. ASO latex agglutination test – This test detects raised ASO titers (200 IU or higher) in the serum of infected patients. IU = international unit.
  2. ASO tube test – This is usually used as a confirmatory test to validate positive results of the ASO latex agglutination test. Titers less than 50 IU is indicative of a negative test result.  

AIM: To detect the presence of anti – streptolysin o (ASO) antibodies in the serum of patient’s as an aid in the diagnosis of streptococcal infections e.g. pharyngitis.

PRINCIPLE: The principle is based on the immunological reaction between ASO antibodies with Streptolysin O antigen coated onto latex particles, to form visible agglutination.

MATERIAL/APPARATUS: Blood specimen, ASO antigen reagent (a suspension of latex particles coated with Streptolysin O), centrifuge machine, timer, Pasteur’s pipette, test card, mixing stick.

METHOD/PROCEDURE:

  1. Bring the ASO antigen test reagent to room temperature (23oC – 29oC).
  2. Centrifuge the blood specimen in the centrifuge machine at 400rpm for 5 minutes, in order to sediment the red cell component of the blood and obtain the serum.
  3. Place a drop of the serum onto the test card using the Pasteur’s pipette.
  4. Shake the vial containing the ASO antigen test reagent very well, and place a drop of the reagent onto the serum on the test card.
  5. Mix the mixture together using the mixing stick or the paddle end of the Pasteur’s pipette.
  6. Rock/rotate the test card for 2 minutes.
  7. Observe macroscopically, after the time interval for visible agglutination.

REPORTING OF THE RESULT:

Presence of agglutination within the 2 minutes of rocking indicates that the patient’s serum contains ASO antibodies. This is a positive result and is reported as thus: Concentration of ASO > 200 IU/ml.

Absence of agglutination within the 2 minutes of rocking indicates that the patient’s serum does not contain ASO antibodies that are up to 200 IU/ml. This is a negative result and is reported as thus: Concentration of ASO < 200 IU/ml.

IU/ml = International Unit per mil

Streptococcus pyogenes is the main human pathogen associated with local or systemic invasion, and post streptococcal immunologic disorders in the body. They are β – haemolytic and contain the group A antigen which helps in classifying them as “the β – haemolytic group A S. pyogenes” under the Lancefield classification of Streptococci. They have the exceptional ability to hemolyze red blood cells (RBC’s) completely with the release of haemoglobin in vitro, and are thus called β – haemolytic Streptococcus while those that only hemolyze RBC’s incompletely are called α – haemolytic Streptococcus.

Streptococcus pyogenes produces 2 different types of hemolysins (streptolysins) that help it to hydrolyze/breakdown RBC’s, viz: Streptolysin O and Streptolysin S. The former is antigenic while the latter is not. That is, streptolysin ‘O’ stimulates the production of antibody by the body while streptolysin ‘S’ does not stimulate the production of antibody by the body following an infection. This antibody that is produced by the body following an infection with S. pyogenes is called anti – streptolysin O (ASO) antibody.

Anti – streptolysin O (ASO) antibody is produced by the human body in response to infection with S. pyogenes and other Streptococci that produce streptolysin ’O’. ASO combines quantitatively with streptolysin ‘O’ produced by S. pyogenes in vivo. ASO blocks hemolysis by streptolysin ‘O’. This phenomenon forms the basis of a quantitative test for the antibody (ASO). An ASO serum titer in excess of 160 – 200 units is considered abnormally high and suggests either recent infection with Streptococci or persistently high antibody levels due to an exaggerated immune response to an earlier exposure in a hypersensitive person. 

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