T/E Ratio and Carbon Isotope Ratio (CIR/IRMS) Testing
How Labs Tell Your Own Testosterone From a Pharmaceutical Vial
Evidence grade 5 of 5, Settled. Read how the Atlas grades evidence.
The written rationale for this grade is still being prepared.
- FDA
- Not an FDA matter; WADA-accredited laboratory methodology
- WADA
- Detection methodology for S1 Anabolic Agents
Reference only. Not medical advice.
What can go wrong
For the marketplace, this science refutes 'undetectable testosterone' claims at a chemical level: the carbon-isotope signature persists regardless of formulation. For clinicians and patients, it underscores that endogenous-looking serum labs do not equate to anti-doping clearance, the detection biology is more discriminating than any single number.
This is something to take to a clinician. The Atlas can tell you what is known about t/e ratio and carbon isotope ratio (cir/irms) testing in general. It cannot tell you what is happening in your body, and the difference matters most for exactly the symptoms that bring people to this page.
The analytical core of testosterone doping detection. Screening starts with the urinary testosterone-to-epitestosterone (T/E) ratio: exogenous testosterone raises T without raising epitestosterone, pushing the ratio up (WADA's atypical threshold was lowered to 4:1 in 2005). Because individuals differ and some carry gene variants that distort the ratio, the decisive confirmation is the Carbon Isotope Ratio test by gas chromatography/combustion/isotope-ratio mass spectrometry (GC/C/IRMS). Pharmaceutical testosterone, synthesized from plant phytosterols (soy, yam), is depleted in carbon-13 relative to endogenous testosterone built from a typical diet, giving a distinguishable delta-13C signature.
Plant-derived pharmaceutical testosterone carries a lighter carbon-13 fingerprint than the testosterone your body builds, and the lab can read it.
What it is invoked to explain
Claims this idea is used to support. Whether it supports them is the rest of the entry.
How it works
Epitestosterone is a near-inert isomer the body makes alongside testosterone at roughly 1:1; dosing testosterone raises T but not epitestosterone, elevating T/E. But the ratio alone is fallible, so IRMS measures the stable-carbon-isotope composition (carbon-13 vs carbon-12) of the suspect testosterone metabolites versus an endogenous reference steroid in the same sample. Pharmaceutical testosterone is manufactured from plant sterols whose photosynthetic pathway leaves them carbon-13-depleted; endogenous testosterone reflects the athlete's mixed diet. A significantly depleted delta-13C in the target steroid relative to the reference indicates an exogenous origin, independent of the T/E value.
Why 'bioidentical' does not mean undetectable
Marketplace language calls compounded or branded testosterone 'bioidentical', molecularly identical to endogenous testosterone. That is true and irrelevant to IRMS, which does not read molecular structure but isotopic origin. Because the carbon atoms in pharmaceutical testosterone come from plant sterols, they carry a lighter carbon-isotope fingerprint than the carbon in testosterone the body synthesizes. The T/E ratio can be defeated by microdosing or by genetic variants (e.g., a UGT2B17 deletion that lowers baseline T/E in some individuals), which is exactly why IRMS, ratio-independent, is the confirmatory test that closes those loopholes.
What people get wrong
Often saidBioidentical testosterone is undetectable
ActuallyIRMS reads the plant-derived carbon-isotope signature of pharmaceutical testosterone, regardless of molecular identity.
Often saidStaying under a 4:1 T/E ratio means passing
ActuallyMicrodosing and genetic variants can keep T/E low, but IRMS carbon-isotope analysis is ratio-independent and can still confirm exogenous use.
Often saidA single positive ratio convicts an athlete
ActuallyT/E is a screen; an adverse finding for exogenous testosterone typically requires IRMS confirmation.
Who profits, and who pushes back
Nobody is named here yet, because we have not done that work on this entry. Read that as unfinished. Somebody almost certainly sells this, and somebody almost certainly disputes it. Assume both until we have checked.
At a glance
- Screen marker
- Urinary T/E ratio
- Atypical T/E threshold
- Lowered to 4:1 in 2005
- Confirmation
- GC/C/IRMS (Carbon Isotope Ratio test)
- Physical basis
- Plant-derived pharma T is depleted in carbon-13 vs endogenous T
- Reported as
- delta-13C difference between target steroid and an endogenous reference compound
What this entry does not yet cover
What follows is unfinished work on our side, not a finding about T/E Ratio and Carbon Isotope Ratio (CIR/IRMS) Testing. A short entry is not a safe entry.
- Timeline. We have not built the dated record of research and regulatory events.
- Cost and market. We have not documented what it sells for or who sells it.
- Supply chain. We have not mapped where it is made.
- Who uses it. We have not documented the user groups.
- What could change. No forward-looking summary yet.
- How it compares. We have not set it against the alternatives.
- Children and teenagers. We have not checked what is known about younger people.
Sources
The studies below are named and described, but not linked. You cannot open them from here and check our reading against them, which means this page is our summary of the literature rather than a document you can verify. Weigh it accordingly until the links exist.
The studies that matter
WADA lowers T/E atypical threshold to 42005
Tightened the screening cutoff for suspicious steroid profiles
GC/C/IRMS for exogenous steroid detectionestablished method
Distinguishes synthetic from endogenous testosterone by carbon-isotope ratio
UGT2B17 deletion and T/E2015
Gene deletion lowers baseline T/E in some athletes, motivating IRMS confirmation
Steroidal module of the ABP2014
Embeds T/E and related markers in longitudinal monitoring