The production of TG (testosterone glucuronide)is controlled by an enzyme that is, in turn, encoded by a gene called UGT2B17. This gene comes in two varieties, one of which has a part missing and therefore does not work properly. A person may thus have none, one or two working copies of UGT2B17, since he inherits one copy from each parent.
A researcher gave healthy male volunteers whose genes had been examined a single 360mg shot of testosterone (the standard dose for legitimate medical use) and checked their urine to see whether the shot could be detected.
Nearly half of the men who carried no functional copies of UGT2B17 would have gone undetected in the standard doping test. By contrast, 14% of those with two functional copies of the gene were over the detection threshold before they had even received an injection. The researchers estimate this would give a false-positive testing rate of 9% in a random population of young men.
Dr Schulze also says there is substantial ethnic variation in UGT2B17 genotypes. Two-thirds of Asians have no functional copies of the gene (which means they have a naturally low ratio of TG to EG), compared with under a tenth of Caucasians—something the anti-doping bodies may wish to take into account
Thus one third of asians could take testerone and not trigger an anti-doping detection.