Which radial immunodiffusion method is primarily quantitative and relies on the diameter of the precipitin ring to estimate concentration?

• A. Prozone assay
• B. End-point radial immunodiffusion
• C. Ouchterlony diffusion
• D. Turbidimetric assay

Answer: (B)

In this type of immunodiffusion, the essential idea is turning a physical measurement—the size of a precipitation ring—into a quantitative estimate of how much antigen is present. In end-point radial immunodiffusion, antibody is fixed in the agar. The antigen diffuses outward from the sample, and at the point where enough antibody and antigen meet to form a visible precipitate, a circular ring appears. The diameter of that ring increases with the amount of antigen in the sample, and you compare it to a standard curve built from samples with known concentrations to estimate the unknown. This setup makes the ring diameter a direct, quantitative readout.

The other approaches don’t provide that same ring-based quantitative readout. The prozone effect can cause unusually small rings or no ring at high antigen or antibody levels, complicating quantitation. Ouchterlony diffusion is mainly qualitative, showing patterns of identity or non-identity rather than a numeric concentration. Turbidimetric assays measure cloudiness to estimate concentration, not the diameter of a precipitin ring.

So the method that best fits a quantitative readout based on ring diameter is end-point radial immunodiffusion.