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.
WHO's guidelines specify that the vaccine producers must determine the potency at the time of release and throughout the approved shelf life of the product. Single radial immunodiffusion (SRID) is currently the gold standard for influenza vaccine potency. In each case, the assays are based on the diffusion of viral antigen into an agarose gel containing specific antibodies to the antigen being measured. The interaction between antigen and antibody produces a precipitation zone whose size is directly proportional to the amount of antigen applied. A potency value for unknowns is obtained by comparing the sizes of zones produced by unknown preparations to the sizes of zones obtained with a calibrated reference of known antigen content.
Once the specific reference antigens and antibodies are prepared and the test standardized, it is a remarkably simple technique which, unlike agglutination assays, is very reproducible, relatively unaffected by minor variations in test conditions, and is far less time consuming and cumbersome than in vivo assays for potency. Similar SRID assays have been used experimentally to determine the potency of inactivated polio and rabies vaccines. For influenza and rabies, disruption of the virions with a detergent is necessary to permit the diffusion of the appropriate antigens, whereas, with polio, intact virions are allowed to diffuse.
(Other viruses or microorganisms may be available on request)
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