Which statement describes the end-point method of RID?

• A. Readings are taken before equivalence.
• B. Concentration is directly in proportion to the square of the diameter.
• C. The diameter is plotted against the log of the concentration.
• D. It is primarily a qualitative rather than a quantitative method.

Answer: (B)

In radial immunodiffusion end-point, the amount of antigen that has diffused into the gel determines the size of the precipitin ring. The key idea is that the ring’s area grows with the amount of antigen, and area is proportional to the square of the ring’s diameter. With a fixed well volume and constant diffusion conditions, the antigen concentration is therefore proportional to the ring diameter squared. That’s why the end-point readout uses the relationship that concentration ∝ diameter squared.

This method is quantitative, not qualitative, and it doesn’t rely on readings taken at equivalence or on plotting diameter against the log of concentration. The emphasis is on the ring size reflecting amount, which translates to concentration via the D^2 relationship.