Antistreptolysin O titer

What is Anti-streptolysin O ?

  • Anti-streptolysin O (ASO or ASLO) is the antibody made against streptolysin O, an immunogenic, oxygen-labile hemolytic toxin produced by most strains of group A and many strains of groups C and G streptococci. The O in the name stands for oxygen-labile; the other related toxin being oxygen-stable streptolysin-S. The main function of streptolysin O is to cause hemolysis.
  • The antistreptolysin O titer measures the level of antistreptolysin O antibodies in the blood plasma.
    These antibodies produced against the bacteria cross-react with human antigens (mainly collagen) and attack the cellular matrix of various organs, mainly the heart and  joints.
  • The antibodies level starts to rise in 1-3 weeks after streptococcal infection, peaks in 3-5 weeks, and then goes back to insignificant level over 6-12 months, so a positive test can indicate current but more recent group A, C, and G streptococcal infection and may support the diagnosis of poststreptococcal infection complication. Rising titers over time are more indicative of infection than a single test,[1] which is why repeating the test is recommended 10 days after the initial test.
  • An antistreptolysin titer greater than 166 Todd units (or >200 IU) is considered a positive test in adults, while The normal value for adults is less than 166 Todd units, which indicates a negative test.

Normal Value

  • Adult and preschool age child: <100 IU/mL
  • Children (school age 5 – 15 years old) : <200 IU/mL

Acceptable values:

  • Adults: less than 200 units
  • Children(school age 5 – 15 years old) : less than 400 units

Important notes about the test:

  • A positive test can indicate recent or current group A, C, and G streptococcal infection and may support the diagnosis of post-streptococcal infection complication (eg, glomerulonephritis and rheumatic fever).
  • Antibiotics may give false-negative results by inhibiting streptococcal antibody response
  • Increased titer can occur in healthy carrier
  • increased beta-lipoprotein levels produce falsely positive results
  • Repeating the test 10 days after the first test is recommended
  • Consider also performing another test like anti-DNase B (anti-deoxyribonuclease B), which, when combined with the ASO titer, can detect up to 95% of streptococcal infections compared to 80% with the ASO titer test alone


Antistreptolysin O titer Methods:

  • It is done by serological methods like latex agglutination or slide agglutination. ELISA may be performed to detect the exact titre value.
Antistreptolysin O Titeration Latex Agglutination Kit

Antistreptolysin O Titeration Latex Agglutination Kit


  • For the qualitative and semi-quantitative measurement of antibodies to Antistreptolysin‐O in human serum.

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 2-8o C for up to 48 hours prior to testing. If  longer storage is necessary,  sera should be stored frozen at -20ºC.

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 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 preservative.
  3. ASO Negative Control: Human serum containing 0.1% sodium azide as preservative.
  4. Sufficient disposable pipettes.
  5. Glass test slide.

Qualitative Test (Screening)

  1. Bring all reagents and specimens to room temperature.
  2. Place one drop (˜ 50 μL) of the positive control and negative control and 50 μL of the patient serum into separate circles on the glass slide.
  3. Shake the ASO latex reagent gently and add one drop (45 μL) on each circle next to the sample to be tested and control.
  4. Mix well using disposable stirrer spreading the mixture over the whole test area and tilt the slide gently. Agitate for about 2 minutes with rotator or by hand and observe for the presence or abscence of agglutination.

Results and Interpretation

Negative result:    

  • milky suspension with No agglutination of the latex particles suspension within two minutes.

Positive result:

  • An agglutination of the latex particles suspension will occur within two minutes, indicating an ASO level of more than 200 IU/ml.
ASLO Result Interpretation

ASLO Result Interpretation

Semi-Quantitative Test

  1. Serum to be titrated is serially diluted (1:2, 1:4, 1:8 etc) in 0.9 g/L saline solution.
  2. Place one drop of positive and negative control on slide. Do not attempt to dilute the ASO positive control serum for comparative or other purposes as no correlation exists between actual titre of the control and titre of unknown sera.
  3. Place 50 μL of each serum dilution individually in successive circles on the slide and proceed as in screening methodology.
Semi-Quantitative - Serial Dilution

Semi-Quantitative – Serial Dilution


Results and Interpretation

  • The serum ASO titre can be defined as the highest dilution showing a positive result. The approximate ASO level (IU/ml) present in the sample can be optained by the following formula:

ASO Titre (IU/ml) = Highest dilution with positive reaction x Reagent sensitivity ( 200 IU/ml)
e.g. if the agglutination is present up to a titre 1:8 , the approximate
serum ASO level is 8 x 200 = 1600 IU/ml”

Common Questions


1.  Can ASO be used to diagnose strep throat?

  • ASO levels are not detectable for at least a week after an infection, so ASO tests are not used to diagnose a current, acute infection. A throat culture or a rapid strep test is the best method to diagnose strep throat (streptococcal pharyngitis). It is important that strep throat be promptly identified and treated to avoid complications and passing the infection on to others.

2.  If I am diagnosed with strep, will an ASO always be performed?

  • No. In general, the ASO test is only performed when someone has symptoms suggesting that a complication may have developed after a strep infection that was not diagnosed and treated appropriately. Most people do not experience these complications, so the ASO test is not routinely done.



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