Final answer:
If the collection bag only fills halfway due to a collapsed vein, the phlebotomist should adjust the needle or donor's arm, or if necessary, select a new vein after stopping the procedure and applying pressure to the site.
Step-by-step explanation:
When a vein collapses during a blood donation and the collection bag is only half full, the phlebotomist should first try repositioning the needle slightly or adjusting the donor's arm to re-establish blood flow. If these efforts fail, it may be necessary to remove the needle and apply pressure to the puncture site to prevent bruising. In some cases, a new vein may need to be selected for the procedure to continue. While the IV bags in fluid administration are placed above the arm to utilize gravity, blood collection bags are typically placed below the arm to facilitate the flow of blood from the arm into the bag. It is important to stop the procedure if the blood flow cannot be restored to ensure the safety and comfort of the donor.