Radial immunodiffusion

Definition
noun
A quantitative immunodiffusion technique used to detect the level of protein (antigen) in a sample by measuring the diameter of the ring of precipitin formed by the complex of the protein (antigen) and the antiserum (antibody).
Supplement
The procedure involves placing of a solution of protein (antigen) into a well cut into an agarose gel containing the evenly distributed antiserum (antibody). As the protein (antigen) diffuses out of the well, it complexes with the antiserum (antibody) and forms a precipitin ring, the diameter of which is proportional to the quantity of the protein (antigen) in the well.
Compared with microbial diffusion assays, this technique is less precise as it gives a rather relatively insensitive estimation of the concentration of antigen. However, it has the advantage of specificity. Thus, it can be applied when sensitivity is not required but specificity is, and antibody is plentiful. It is commonly used in clinically detecting levels of blood proteins in a patient.
See also: antigen.