The detection of steroids originating from synthetic precursors in relation to their chemically identical natural analogues has proven to be a significant challenge for doping control laboratories accredited by the World Anti-Doping Agency (WADA). Endogenous steroid abuse may be confirmed by utilising the atomic specificity of gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS) that enables the precise measurement of differences in stable isotope ratios that arise as a result of fractionation patterns inherent in the source of steroids. A comprehensive carbon isotope ratio (δ 13 C) profiling study (n=1262) of urinary ketosteroids is reported that demonstrates the inter-individual variation that can be expected from factors such as diet, ethnicity, gender and age within and between different populations (13 countries). This δ 13 C distribution is shown by principal component analysis (PCA) to provide a statistical comparison to δ 13 C values observed following administration of testosterone enanthate. A limited collection of steroid diol data (n=100; consisting of three countries) is also presented with comparison to δ 13 C values of excreted testosterone to validate criteria for WADA accredited laboratories to prove doping offences.