I have had to go back when my platelets were so high that they couldn't get blood to flow -- the blood would clot in the needle almost instantly. I had one session where they went in five different locations trying to get it to flow but the clotting happened so quickly they couldn't get anything out of me. They asked me to drink enormous amounts of liquids and come back in a few days to try again. That time they opted to just draw the blood out with syringes instead.