Single radial immunodiffusion (SRID) is an agar gel precipitation test used to determine the quantity or concentration of an antigen in a sample. 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. https://antiviral.creative...