World cycling's governing body warned on Thursday that several blood samples are undergoing expert scrutiny to determine whether blood doping or manipulation has taken place.
Using its pioneering biological passport program, the International Cycling Union (UCI) has snared several top professionals despite the fact they had not tested positive for banned substances.
UCI doctor Mario Zorzoli suggested on Thursday that more positive cases could follow after a decision to probe several samples was taken in the wake of a meeting with World Anti-Doping Agency (WADA) officials in February.
"We can see for example a change in the value of certain parameters, which are completely different in 2010 to what they were in 2008 or 2009," Zorzoli said at a French National Anti-Doping Agency (AFLD) conference. Obviously that leads us to suppose that some kind of manipulation has taken place."
The idea of the passport program is to test, chart and then compare the blood and urine values of athletes over time.
If changes are noticed, the rider can be targeted for further testing randomly or in competition. Since it was introduced in 2008, five riders in 2009 and three in 2010, including Italian Franco Pellizoti of Liquigas, were charged with doping based on the evidence of parameters on their biological passports. Although no cases have been revealed since, that could be about to change.
Zorzoli added: "For other athletes, we've been obliged to carry out more tests to see how certain parameters reacted or showed up, which was quite time-consuming.
"And some of those have just recently arrived among the elite" and are therefore automatically obliged to adhere to the biological passport program.
Zorzoli added: "The passport evolves. The important thing is, even if we have no evidence to suspect an athlete at a certain time, after subsequent results the same athlete could be facing proceedings a few months later."
Zorzoli said evidence from the program suggests changes in doping behavior in recent years, leaning towards what is commonly know in the sport as 'micro-dosing'.
"There's a tendency to use (banned) substances via micro-dosing during perhaps shorter time-scales, the aim of which is to reduce the risk of testing positive," he added.
Zorzoli said the passport is bringing benefits, compared to 2001 when authorities were first able to analyze the reticuloytes (young blood cells), another useful tool to determine if blood doping had taken place.
"In 2001, when we first started analyzing reticulocytes during blood testing ... about 10 to 11 percent of the samples revealed abnormal values. Since 2008, this has reduced to about two percent."