Definition of SRID - Single Radial Immunodiffusion

Radial Immunodiffusion is a technique used extensively for the quantitative estimation of antigens.

Antibody of known specificity is distributed evenly in an agar gel and a sample containing the antigen of interest is placed in a well within the gel. Antigen then diffuses radially from the well and a precipitin ring forms at the point of antibody-antigen equilibrium. Antigen concentrations are determined by measuring the diameter of precipitin rings and extrapolating using standard curves.