As the stakes for top level competitive athletes become increasingly high, so does the pressure to perform better. The use of banned substances is widespread. But catching doping cheats is difficult, as highlighted by the drawn out case that resulted in the recent suspension of Alberto Contador from professional road cycling for two years. Contador tested positive for clenbuterol in a blood sample taken on 21 July 2010, but it was not until February 2012 that the Court of Arbitration for Sport ruled on the matter after much legal wrangling.1 In the intervening time Contador won the 2010 Tour de France and the 2011Giro d’Italia.

Traditionally, doping tests have focused on detecting banned substances in blood or urine samples provided either at the time of competition or at random time points outside competition (as introduced by the International Olympic Committee in 1994). The drawbacks of direct testing are many. Athletes may dodge tests, for example, and many substances have a short window in which their metabolites are excreted and detectable. Some substances that are banned in competition, such as corticosteroids, may be used to treat an injured athlete, which, as the World Anti-Doping Agency (WADA) acknowledges, can hardly be branded as cheating. It has developed guidance that requires athletes to declare the therapeutic use of banned substances.2 It’s also not uncommon for athletes to claim that they have tested positive for a substance because they have eaten contaminated food.

Another problem that grew towards the end of the last century is “blood doping”—that is the infusion of homologous or autologous red blood cells before a competition or, more recently, the use of recombinant human erythropoietin, which stimulates endogenous red …