Carbon Isotope Ratio, or CIR, a test used to distinguish between naturally produced (endogenous) testosterone and its synthetic (exogenous) counterpart using molecular weight, was the method used to ping Cannondale-Garmin rider Tom Danielson, and is being used in a new way, VeloNews reported Monday.

Danielson tweets innocence after "positive" test
Cannondale-Garmin rider Tom Danielson has taken to Twitter today to deny any wrongdoing after he was told by the United States Anti-Doping Authority that he had tested positive for “synthetic testosterone”.

While the traditional method of detecting testosterone abuse looks for an imbalance in the ratio of testosterone and epitestosterone, known as the T/E ratio test, testosterone/epitestosterone ratios return to normal very quickly, and often overnight, limiting the test's efficacy. The ban on overnight testing, which has since been partially lifted, had also reduced the window of opportunity for drug testers.

"How it got there will be the crux of Danielson's defense, but its presence is undisputed thanks to CIR."

Instead of looking for a physiological response, as is the case with the T/E ratio test, CIR is accurate enough to detect the product itself, by first determining the quantities of two isotopes, or types, of carbon within an individual's testosterone molecules, known as carbon-12 and carbon-13.

"The ratio between carbon-12 and carbon-13 is constant within an individual," writes VeloNews' Caley Fretz. "This is the crux of the CIR test: Labs compare the ratio of carbon-12 to carbon-13 in an individual's testosterone with the carbon-12/carbon-13 ratio in some other, non-performance-enhancing compound, like cholesterol. The two ratios should match. If they don't, that's a positive test.

Tan Lines: A Loss For Clean Cycling
The news that a Cannondale-Garmin rider returned a positive doping test is not a sign that professional cycling is getting better at catching cheats, but a billboard to say some things remain unchanged, writes Anthony Tan.

"USADA determined that the ratio between carbon-12 and carbon-13 isotopes in Danielson's testosterone molecules did not match the ratio that occurred naturally in the rest of his body. A synthetic form of the hormone had to be present. How it got there will be the crux of Danielson's defense, but its presence is undisputed thanks to CIR."

Danielson is still waiting for his B-sample results, who said on Twitter on August 3 he intends to "have the supplements I take, tested to see if this is what caused it", implying he intends to use contamination as a defence against his doping infraction.