Quantitative measurement of precipitating antibodies and standardization of Streptococcus group B antisera by the reverse radial immunodiffusion technique.

A comparative study was made of the reverse radial immunodiffusion (RRID) technique and the quantitative precipitin test for determining the amount of precipitable antibodies present in streptococcus group B antisera. The coefficient of correlation between the 2 tests was 0.963 when a purified carbohydrate antigen was used. The results of this study show that: (1) a group B antiserum should contain at least 3 mg of precipitable antibodies per ml to ensure that it will react with acid extracts of group B diagnostic cultures; and (2) the RRID technique is an accurate and simple procedure for determining the amount of precipitable antibody present in an antiserum, and it could be used for the standardization of these diagnostic reagents. A method for preparing the group B carbohydrate antigen used in the RRID and quantitative precipitin tests is described.