Single radial diffusion or (SRD) is an immuno-diffusion technique used in immunology to determine the quantity of an antigen.
Typically, wells are made in an agar gel that holds an antibody suspension. The antibody is directed against the antigen to be quantified. When antigen is added to the wells, complexes with the antibody will be formed. These complexes are soluble when the antigen is in excess, but as the antigen diffuses in the agar gel; and it's concentration decreases, insoluble precipitin complexes will form, which can be seen as circles.
By measuring the diameter of these circles and using a standard curve with known antigen concentrations, the quantity of the antigen can be calculated.
MicroTechniX can help you monitoring using the MacroLab II, Macroscan in combination with our software tool Icarus.