Here is an informative article by David Epstein explaining why it is difficult to catch drug cheats.
Earlier this month, London’s Sunday Times and German broadcaster ARD published a joint investigation on doping in track and field that included an analysis of 12,000 leaked blood tests from 5,000 athletes between 2001 and 2012. The tests had been carried out by the IAAF, track and field’s international governing body. Two respected experts in doping methods said blood tests of 800 of the athletes were “highly suggestive of doping or at the very least abnormal.” Ten runners who won medals in endurance events at the 2012 Summer Olympics in London had suspicious test results. And a startling 80 percent of Russian medalists recorded tests that showed likely doping. The vast majority of athletes with suspicious tests were never sanctioned.
On Saturday, the 2015 track and field world championships kick off and, of course, some athletes who are doping will vie for medals. Most will not be caught; only 1 to 2 percent of tests in international Olympic sports result in sanctions each year. If doping is so rife in track and field, why are athletes penalized so rarely? It’s partly because many suspicious tests don’t quite reach the high evidence bar to be considered officially positive. But it’s also because doping athletes tend to employ methods that make drug testing extremely difficult. As Paul Scott, head of Scott Analytics, which provides testing services in multiple sports has put it: “Drug testing has a public reputation that far exceeds its capabilities.”