Combination of solid-phase second antibody and internal sample attenuator counting techniques. Radioimmunoassay of...

A method is described for the quantitation of free and bound antigen in a new radioimmunoassay which does not require centrifugation or decantation steps. The method involves coupling a second antibody to agarose spheres which also contain entrapped crystals of bismuth oxide (Bi2O3) in order to attenuate the gamma-radiation from the bound fraction of the 125I-labelled antigen. The system is designed so that the samples may be measured in a gamma counter 15 min after the spheres have been added. This new method was compared to a conventional radioimmunoassay for the analysis of the thyroid-stimulating hormone (TSH). Data were obtained with the modified radioimmunoassay which showed both high sensitivity and precision and also had a good correlation (r = 0.951) with data obtained using a conventional method.