They tested him all the time- that was why he was so believable... but he had a very complex blood exchanging scheme in place.
They would give themselves a whole blood transfusion - changing 100% of his blod.
Where you get that much blood should be questioned. I imagine you have to have enough to completely drain you an make sure only fresh blood goes in.
First of all that is a heck of a process just to cheat.
It’s very expensive and takes out a blood bank. Unless they were cleaning his blood somehow and putting it back in his body kinda on a kidney dialysis machine.