A bruise on the arm from a blood draw is most commonly caused by the following two factors First, when blood is drawn, because a needle must be punctured into a vein in the arm in order to draw blood out. If there is a leak in the gap between the vein wall and the needle, the venous blood flows into the subcutaneous tissue causing the bruise to appear. Second, usually after the needle is removed, the nurse usually recommends using a cotton swab or a cotton ball to press on the area until the bleeding stops. However, if the pressure is not long enough and the bleeding has not stopped, the punctured vessel wall may leak blood and bruise the subcutaneous soft tissues. Usually, in both cases, after the bruising has completely stopped, local hot compresses can be used to promote its elimination.