Which method requires that all antigen diffusion rings are measured at a fixed time of 18 hours?

The Fahey-McKelvey method, also known as the kinetic radial immunodiffusion (RID), is specifically designed to measure antigen diffusion rings at a predetermined time, which is typically 18 hours. This method allows for the quantification of antigens by measuring the diameter of the diffusion ring formed by the antigen-antibody reaction in a gel. The timing is crucial as it standardizes the measurement and ensures that results are consistent and comparable across different tests and conditions.

In the Fahey-McKelvey method, the diffusion of the antigen into the agarose gel continues for a set duration, and at the 18-hour mark, the diffusion rings are measured for analysis. This time frame is part of the methodology to obtain reliable data on the antigen concentration.

On the other hand, the Mancini method, or endpoint RID, measures diffusion rings at a point when the reaction has reached equilibrium, but this does not have a fixed time associated with it like the kinetic method does. Therefore, the requirement of measuring at 18 hours is specific to the Fahey-McKelvey approach, validating why this choice is the correct answer.