Paralympic organisers have defended their anti-doping measures after admitting many medal-winning athletes would not be drugs tested.
More than a quarter of the 4,200 athletes competing at London 2012 will be assessed for performance-enhancing substances - a 20% increase from the Beijing Games, the International Paralympic Committee (IPC) said.
At least one medal winner from each Paralympics event will undergo doping tests, despite every gold, silver and bronze medallist at the Olympics being forced to provide samples.
Dr Jose Antonio Pascual, chair of the IPC's anti-doping committee, said analysing samples from every Paralympic medal winner would be "impossible" because of the vast number of events compared with the Olympics.
Some 503 golds are up for grabs in the London Paralympics compared with around 300 in their Olympic counterpart.
Dr Pascual said: "As with previous Games, we try to cover all countries, all sports.
"But the fact that in the Paralympic Games we have a lot of medal events, because of the classification of all the athletes - around 500 - makes testing for every single athlete having gold, silver and bronze impossible.
"If they have a medal, they have a high probability of being tested, but it's not 100% sure."
About 1,250 doping tests will be completed by the end of the London Paralympics, the IPC said.
A total of 207 tests were carried on athletes during the first three days of competition, while 476 tests were conducted before the start of the Games.
So far, none of the athletes have tested positive for performance-enhancing drugs, the IPC said.
Organisers said every athlete competing in power-lifting will be tested after the event was identified as a "high risk" event.
Debbie Jevans, Locog's director of sport, denied that cost was a factor in the decision not to test every medal-winning athlete at London 2012.
She said: "It's not a money issue. The process and the number of tests were agreed in advance with the International Paralympic Committee."
However, Dr Pascual said he believed the allocation of financial resources meant it was not possible for every athlete to be tested.
He added: "It has to be with money, for sure we would do all of the athletes out of competition and in competition. I would do 10,000 or so. It must be to do with money."