In a radioimmunoassay (RIA) drug test, urine samples are mixed with antibodies that attach to illegal drugs. The antibodies have radioactive chemicals attached, which give off radiation that is seen by a machine. If no drugs are in the sample, no radiation is given off.
The most popular RIA test is Abuscreen, the military's preferred drug test. RIA requires complex machines and highly trained laboratory people, but the military likes it because it produces fast results.