A common genetic variation allows many people to take testosterone while testing negative. 67% of asian men tested and 10% of europeans carry this variant.
What happens is that the body responds testosterone's anabolic stimulus (builds muscle) but does not metabolize the testosterone to the metabolites commonly tested for. This means that tests show normal T/E ratios for people who are seriously doping.
There's no easy way to devise a better test at this point. Isotope Mass Spectroscopy could detect doping in people with these genes, but it's too expensive to use routinely.
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