One unit has greater than 3×1011 platelets. Therefore, it takes 2 liters of blood having a platelet count of 150,000/mm³ to produce one unit of platelets. Some regular donors have higher platelet counts (over 300,000/mm³); for those donors, it only takes about one liter of their blood to produce a unit. Since the machine used to perform the procedure uses suction to draw blood out of a donor's body, some people who can give whole blood may have veins too small for platelet donation. Blood centers evaluate each donor's veins prior to donation.