It is very easy to detect the presence or absence of some radioactive materials even when they exist in very low concentrations. Radioisotopes can therefore be used to label molecules of biological samples in vitro (out of the body). Pathologists have devised hundreds of tests to determine the constituents of blood, serum, urine, hormones, antigens and many drugs by means of associated radioisotopes. These procedures are known as radioimmunoassays and, although the biochemistry is complex, kits manufactured for laboratory use are very easy to use and give accurate results. In Europe some 15 million of these in vitro analyses are undertaken each year.