I think this explains this rather well. Learning something new here myself.
Testosterone is the male sex hormone and is also present in women. Epitestosterone is a related chemical with no known function in the body. The relationship between the two is unimportant for legitimate medical purposes, scientists said. But it is important to anti-doping experts.
To detect the use of synthetic testosterone as a muscle-building anabolic steroid, anti-doping officials have declared that a ratio of testosterone to epitestosterone above 6 to 1 is evidence of drug use. The test for what is known as the T/E ratio was first devised in 1982 by a German scientist, Manfred Donike, for the purpose of catching drug cheats.
In the Balco steroids case, some track and field athletes have been accused of using a sophisticated substance known as ''the cream,'' which contains testosterone and epitestosterone. Epitestosterone is said to serve as a masking agent for elevated levels of testosterone...
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(Snip)
http://query.nytimes.com/gst/fullpage.html?sec=health&res=9A0CE2D71638F935A15755C0A9629C8B63&n=Top%2fReference%2fTimes%20Topics%2fOrganizations%2fU%2fUnited%20States%20Anti%2dDoping%20AgencyAccording to the press conference (article posted in #100), the threshold ratio was recently lowered to
4:1. Doesn't say how recently, though. Also says that mere mortals typically have a
1:1 ratio.
Two things are evident from this - one, that the anti-doping authorities obviously take into account the increased levels that a world-class athlete would naturally have, and two, there's a lot of 'art' here to go along with the science.
'Art', in that they use a threshold ratio of naturally-occurring subtances to impute use of a banned substance. Much like they use haematocrit levels to impute use of banned substance, EPO, for which they have yet to develop an accepted definitive test.
Really doesn't sound anything like DNA testing....
To detect the use of synthetic testosterone as a muscle-building anabolic steroid, anti-doping officials have declared that a ratio of testosterone to epitestosterone above 6 to 1 is evidence of drug use.