There is too much of a chance of a stuff up chub, that's why, saliva and urine swabs are non intrusive, where as blood clearly u are stcking a needle into a dog, anyone who has handled greys knows how hard they can be to handle especially if they don't want to do something, who's gunna hold thedog while it is done? All these things come into play especially at a track environment when the dogs canbe jittery at the best of times, alot of vets don't like it and won't do it