Which radiological test is used to detect antigen or antibody concentrations in the serum?

The correct answer is radioimmunoassay, which is a highly sensitive laboratory technique used to measure antigen or antibody concentrations in serum or other biological samples. This method relies on the use of radioactively labeled antibodies or antigens to quantify the presence of these molecules in a sample.

In a radioimmunoassay, a known quantity of a radiolabeled antigen is mixed with the sample that contains the unknown concentration of the same antigen. The binding of the antigen to specific antibodies is then measured, allowing for the determination of the concentration of the antigen in the sample based on the principle of competition between the labeled and unlabeled antigens.

Other imaging techniques such as ultrasound, X-ray analysis, and CT scans are not designed to specifically measure antigen or antibody concentrations. Instead, they are used for imaging internal structures and organs to identify abnormalities or diseases, providing visual insights rather than quantitative data on biochemical substances in the serum. Thus, radioimmunoassay is uniquely suited for the specific detection of antigens and antibodies, making it the correct choice.