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Anyone with a fucked T:E ratio gets their sample sent for carbon isotope ratio testing. They may target others as well. Endogenous test is produced in the body from cholesterol, so the carbon-12 and 13 isotopes in the test should be in the same ratio as in the cholesterol and other hormones produced from it. If it isn't, it didn't come from you. Most synthetic test will have a different ratio due to how it's made.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2784500/
Pretty sure USADA tests blood directly which is cheaper though less pleasant for crybaby athletes than a carbon piss test.