A recent investigation suggests serious speculation over a key drug test only two weeks before the start of the Beijing Olympics.

The investigation indicates that labs are classing positive tests for the blood-boosting drug EPO as negatives. Scientists say some samples have been described as suspicious, giving rise to fears that no action will be taken against cheats.

Experts say often undetectable "copycat" versions of the drug are available on the Internet for as little as $50 and many of the finalists in Olympic endurance events would be using EPO.

During the Sydney Olympic Games in 2000, a test was introduced to detect recombinant EPO (erythropoietin) but a growing number of athletes were soon challenging the results in the courts.

U.S. sprinter Marion Jones had her first sample test positive but was cleared on the second or B test.

In 2004, the World Anti-Doping Agency (WADA) tightened the criteria by which an EPO positive could be declared.

As a result, the number of legal challenges fell. But experts believe athletes continued to abuse EPO as they became more adept at medicating themselves, causing the number of positives to decline.

Dr. Rasmus Damsgaard, who runs the anti-doping program for the International Ski Federation and for the Astana Cycling team, says he has clear evidence that positive EPO tests are being declared as negative or suspicious.

He sent five samples from skiers to a WADA lab for analysis and they all came back negative. But when he demanded the gels or electronic printouts on which the determination of guilt or innocence was made, he was astonished to see what he believed was clear evidence of EPO use.

"It was very obvious that the gels were very unnatural or very different from natural distributions. But I also saw that they were declared negative because they didn't fulfill the WADA criteria of a positive test; although they looked suspicious and had no natural bands at all, they were still declared negative."

He believes there are many more such samples in WADA labs.

"From a little work with a lot of blood profiles, I found maybe five positives. I wonder that maybe hundreds, maybe even thousands of EPO positive samples are lying around in WADA-accredited labs."

Professor Bengt Saltin, a leading anti-doping expert and a former winner of the IOC Olympic Prize, the highest honor in sports science, said the new WADA criteria allow most EPO cheats to get away with it.

"The reason that I am still a little bit upset with the whole situation is that I have seen too many suspicious samples that are clearly abnormal. Athletes are getting away with it. Look how many have been caught for EPO misuse recently," said Saltin.

The numbers of athletes being prosecuted for EPO use has declined significantly - by two-thirds between 2003 and 2006.

Dr Olivier Rabin, WADA's science director, said he is happy that the test is catching all the drug cheats.

"Now, it would be very presumptuous on my part to say that we are absolutely 100% sure we are going to get everyone. But I can assure you that if you were to take recombinant EPO and that would be in your urine - then, yes, we would detect it."

Rasmus Damsgaard, a Danish researcher, says the current testing regime is fatally flawed.

"WADA is sitting on a mountain of positive EPO. They have these very strict rules, and declare that everything is working fine. But it's not working at all! You can more or less do whatever you like with EPO and you will not be charged," he said.

There is also concern that scientists in some WADA labs may be actively colluding with athletes or their medical advisers.

Senior academics are currently investigating possible collusion between doctors who were working with cycling teams and WADA-accredited labs. They are worried that these doctors were gaining inside information on the latest test procedures.

One growing problem is the rise of copycat versions of EPO. These cheap versions of EPO, often called biosimilars, can be easily bought over the Internet.

Since EPO has been so successful financially, companies in India, China and Cuba have developed drugs that do a similar job in the body, but have a slightly different molecular fingerprint.

There could be up to 80 different versions now being manufactured in different parts of the world, according to scientists who track and monitor the development of "copycat" EPO drugs.

Experts say WADA should widen its criteria in declaring a positive.

They believe anti-doping officials should determine if a sample has any evidence of naturally produced EPO. If not, it should be classed as suspicious because the use of artificial EPO for doping causes the body's own production to shut down.

Scientists also say tests should examine blood profiles as well as the urine sample. An analysis of the number of young red blood cells can also indicate doping.

This could happen in the future, but it will not prevent cheating at Beijing.

Bengt Saltin says he has never been more depressed about the use of this drug.

"I would think that most of the medal winners and many in the finals of endurance events - there is a big risk for them having used EPO. Of course, they have to be clever but they don't have to be very clever."