Advance notice is key to cheating the system

It is a generally acknowledged fact among international sporting bodies that unannounced doping tests can be rendered useless by any form of advance notice. In cases where an athlete has something to hide, the period of time required to manipulate their sample can be less than an hour.

In the case of urine testing, the most effective means of manipulating the result is achieved by the process of catheterisation, in which a sportsperson can introduce either water or "clean" urine into their bladder by means of a flexible tube.

"You can do it in 30 seconds and it hides everything," says John Whetton, Britain's former European 1,500 metres champion, who lectures in physiology at Nottingham Trent University and works part-time for Independent Dope Testing and Management. "As for the effect of having up to 48 hours' notice, it could be enormous," he added. "There are various short-life chemicals which can naturally metabolise out of the system in that period of time. Water-based steroids - as opposed to oil-based ones, which hang around for a long time in the body's fat reserves - are very rapidly eliminated.

"There are also a range of stimulants, including stronger versions such as amphetamines, which can also clear in that timescale, depending upon what quantities have been taken," Whetton said.

Some athletes have attempted to disguise their use of banned substances by using others such as Probenicid or Bromantan, in order to mask them, although Whetton believes the use of such means is far from certain to achieve the required result.

Athletes have also turned to diuretics in the mistaken belief that they will clear out any unwanted evidence. One competitor was said to have used a laxative before an important event, but took so much that he or she had to spend the time on the toilet instead.

Whetton, who in the course of the last year has travelled to Russia, Uzbekistan, Spain, Switzerland, France, Nigeria and widely in the UK conducting tests on behalf of international governing bodies for sports such as athletics, swimming and rugby union, maintains that out-of-competition tests with any form of notice are "a complete and utter waste of resources".

The process of catheterisation is particularly uncomfortable for male athletes, involving as it does feeding the tube, which is usually up to a foot long, up into the bladder and then transferring the new urine either by means of squeezing it from a bag attached to the other end of the tube, or using a large syringe.

Women have an alternative option available to them. There have been cases where clean urine has been stored internally inside a sealed condom which is burst when the time comes to offer a sample.

Manipulating urine samples can sometimes fail as a policy, however. In the early 1990s, three East German sprinters, including the double European champion Katrin Krabbe, were banned for two years after the samples they provided, despite being clear, were found to be identical.