This occurs when the compression hemostasis time after blood collection is short. When blood is drawn, a hole is created in a capillary vessel, and if the hole is not pressed with an alcohol swab to stop bleeding, blood will seep into the skin layer and cause bruising. Also, if the blood vessels are weak, there may be bruising, but it will disappear in 3 to 4 days.
Occasionally, swelling or bruising and pain may persist after blood is drawn. Most of the time, it will improve if you put ice on it and rest. In rare cases, the side effects may persist, and in such case treatment can be obtained by consultation with a doctor.