Final answer:
When the collection bag only fills halfway and the vein collapses during blood collection, the phlebotomy technician should use a new collection bag to draw a new unit of blood.
Step-by-step explanation:
When a phlebotomy technician is collecting blood from a patient for donation and the collection bag only fills halfway and the vein collapses, the technician should use a new collection bag to draw a new unit of blood. This is necessary because a collapsed vein can impede the flow of blood into the collection bag, affecting the quality and quantity of the blood sample. The technician should ensure that the new collection bag is properly secured and labeled before proceeding with the blood collection.