Before we go into further details, we must first answer this question: what justifies the application of radioactive substances in medicine at all? Quite frequently, very low concentrations of substances have to be measured either in the body or in biological samples, or we have to follow the physiological or pathological metabolic, secretory, or excretory processes, making otherwise invisible phenomena observable by applying suitable tracers.

12.2.1 Comparison of Methods for In Vitro Measurement of Concentrations

Although more and more accurate laboratory (photometric, fluorometric, and enzymatic) methods had been developed for the measurement of concentrations in biological fluids, about two decades ago, there was still no method apart from RIA and immunoradiometric assay (IRMA; for more details about this, see Section 12.3), which were simple and cheap enough for routine measurements of concentrations in the nmol/L range.

Note that recently various “alternative” (nonradiotracer) methods have been intro­duced that have similar sensitivity to that of IRMA. However, in vitro nuclear diag­nostic methods are still indispensable parts of laboratory medicine (and probably will be for a long time), primarily due to their low cost. (From an environmental point of view, the waste produced by the alternative assays is no less hazardous than the radiotracer.)