Would explain the pictures of the darker colored blood in the blood bags after donation, the higher the iron, the darker the blood. I did blood draws for years, always checking hemoglobin before drawing, HGb is the protein in the blood that carries the iron, higher the HGb, thicker the blood, and the thicker blood would always give you trouble, as it was sludgy and often clotted before you could get to the minimum amount of 420 ml. If it was under that, we had to throw it out.