Anti-doping specialist Mario Thevis has warned that conventional tests for blood doping are insufficient, and that up to 100 undetectable EPO variants have been designed.
The German was speaking at the Tackling Doping in Sport 2012 conference in London, and stressed the importance of developing methods to detect their use.
"They act like EPO but they are structurally different and that means the current EPO tests will not pick them up," Thevis, who is based at the German Sports University in Cologne, said, according to Reuters.
"Fortunately we know about that problem and we have to develop new tests to help to find these drugs that, according to anecdotal evidence and rumours, are already used in elite sports, although they are not officially launched yet.”
He said that the substances were not widely available, and that those seeking to obtain the substances would need ‘good connections.’
However in the past athletes have gained access to clinical trial products and black market supplies.
Thevis wants authorities to work to eliminate the danger, but also warned that the task won’t be a straightforward one.
"It is quite difficult to develop tests when you don't have an idea what the molecule really looks like,” he explained. “If you don't know what the molecule looks like, it's almost impossible to have a potential strategy."
An example of the problem is the third generation form of EPO, CERA. Tests developed for it caught out several riders during the 2008 Tour de France, including stage winners Stefan Schumacher and Riccardo Ricco, plus the King of the Mountains and third place finisher Bernhard Kohl. However it is thought to have been in use for at least two years before it.
The Operacion Puerto raids in May 2006 saw the substance seized by investigators.
Screening of that year’s Giro d’Italia samples for CERA have still not been carried out, despite commitments to do so.
The UCI’s biological passport was designed to be an indirect method to detect blood doping, looking for the symptoms of such methods rather than specific chemical traces.
This gave authorities greater possibilities to catch those breaking the rules.
However it has been a long time since any new cases were opened. Also, concerns have been expressed lately about a drop off in the scale of testing being done.
Riders recently told Belgian media that they had undergone very few out of competition controls. The UCI has said that it is target-testing suspicious riders rather than doing the broad range of tests it carried out in the past.
However this approach is problematic, as a general deterrent effect in the sport is logically dependent on the possibility that all riders could be subject to surprise controls.
Meanwhile Anna Baoutina of the National Measurement Institute in Sydney has said that tests have been developed to catch those using gene doping, However she said that they would be in place before this year's London Olympics.
"We are developing methods to fight it," she explained. "But we have yet to see the implementation of these methods. WADA has to decide when these methods should be implemented."