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-haemolytic streptococci, streptolysin-O is one of the two hemolytic exotoxins liberated from the bacteria that stimulates production of ASO antibodies in the human serum. The presence and the level of these antibodies in a serum may reflect the nature and severity of infection.
ASO latex reagent is a stabilized buffered suspension of polystyrene latex particles that have been coated with Streptolysin O.
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
- 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.
- ASO Positive Control: Human serum containing more than 200 IU/ml ASO and 0.1% sodium azide as preservative.
- ASO Negative Control: Human serum containing 0.1% sodium azide as preservative.
- Sufficient disposable pipettes.
- Glass test slide.
Procedure for Antistreptolysin O Test:
- Bring all test reagents and samples to room temperature.
- 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.
- Deliver one free-falling drop of positive and negative control into its identified circle.
- Mix the ASO latex reagent by gently shaking. Add one free-falling drop of reagent to each control and sample.
- 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.
- Discard the disposable pipette.
- Slowly rock the slide for exactly two (2) minutes and observe for agglutination under a high intensity light.
- Record results.
- 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.