Arsenal boss Arsene Wenger says he is unhappy with the current level of drugs tests in football has called for more stringent measures against doping.
The Frenchman revealed last week that he had seen "many" teams use performance-enhancing substances in his career, in the wake of revelations of widespread doping in athletics in Russia.
And Wenger has now further clarified his comments and says he is surprised about the lack of positive tests in football.
"I'm not satisfied with the level of testing because I believe blood tests should be done," he told beIN Sports. "If you want to go into a bit more sophistication you have to do blood checks.
"Urine checks are superficial and not deep enough to say absolutely sure that we have no doping problem in football.
"I think no and I wish no but on the other hand, can you have 740 football players at a World Cup and come out with zero alert on any doping? It’s a little bit surprising.
"I hope it is true but I think to be completely sure about it, you want to go into deeper tests."