In the fight against doping, disciplinary sanctions have up to now been primarily based on the discovery of an exogenous substance in a biological fluid of the athlete. However, indirect markers of altered erythropoiesis can provide enough evidence to differentiate between natural variations and blood doping. Forensic techniques for the evaluation of the evidence, and more particularly Bayesian networks, allow antidoping authorities to take into account firstly the natural variations of indirect markers - through a mathematical formalism based on probabilities - and secondly the complexity due to the multiplicity of causes and confounding effects - through a distributed and flexible graphical representation. The information stored in an athlete's biological passport may be then sufficient to launch a disciplinary procedure against the athlete. The strength of the passport is that it relies on a statistical approach based on sound empirical testing on large populations and justifiable protocols. Interestingly, its introduction coincides with the paradigm shift that is materializing today in forensic identification science, from archaic assumptions of absolute certainty and perfection to a more defensible empirical and probabilistic foundation.