Choice of monitoring isotope in double label radioimmunoassays with nonimmunological separation techniques.

We discuss the suitability of some radioactive isotopes as volume markers in radioimmunoassays, from a radiochemical point of view. For three eligible isotopes (22Na, 60Co, and 75Se) we studied the concentration of the marker in the precipitate formed in the separation phase of radioimmunoassays. For all those kinds of separations tested (charcoal, ammonium sulfate, polyethylene glycol, and ethanol), binding or coprecipitation was virtually absent or negligible with 22Na but 75Se was strongly concentrated in the precipitate. Concentration of 60Co occurred only with charcoal and ethanol precipitation. Because heavy metals tend to bind to serum proteins, we conclude that of all radioactive isotopes commercially available only 22Na should be used in radioimmunoassays with nonimmunological separation methods.