Anti-streptolysin O

Anti-streptolysin O (ASO) is the antibody produced against an antigen produced by Lancefield group A streptococci. The enzyme is called "streptolysin O", wherein the "O" stands for "oxygen-labile"; the other antigen being oxygen stable streptolysin-S. Both the enzymes are involved in producing hemolysis, i.e., digestion of blood. In particular they cause, beta-hemolysis.

Significance

When the body is infected with streptococci, it produces antibodies against the various antigens that the streptococci produce. ASO is one such antibody. So, a raised or rising levels can indicate past or present infection. Historically this bacterial marker was one of the first markers used for diagnosis and follow up of rheumatism or scarlet fever. Its importance in this regard has not diminished.

Values

Since these antibodies are produced as a delayed hypersensitive reaction to the above mentioned bacteria, there is nothing called a "normal value." Any presence of these antibodies indicates an exposure to these bacteria. As many people get exposed to these bacteria, but remain asymptomatic, the mere presence on ASO does not indicate a serious disease.

Acceptable values, where there is no clinical feature of rheumatism are as follows:
*Adults: less than 200 units
*Children: less than 300 units

This titre (value) has a significance, only if it is greatly raised, or if a rise in titre can be demonstrated in paired blood samples taken days apart.

Values need to be correlated with a clinical diagnosis.

Estimation

It is done by serological methods like latex agglutination or slide agglutination. ELISA may be performed to detect the exact titre value.

To detect the titre value, by a non-ELISA method, one has to perform the above agglutination using a serial dilution technique.

Mechanism of action

These antibodies produced against the bacteria cross react with human antigens (mainly collagen) and hence attack the cellular matrix of various organs, mainly the heart, joints, skin, brain, etc.

See also

*Antistreptolysin O titre