The possible reasons for the arm to become purple after drawing blood are as follows: 1, when drawing blood, the needle pierced small blood vessels, subcutaneous small blood vessels rupture lead to blood seepage at the blood vessels, after drawing blood for a period of time, due to the increase in blood seepage will appear purple, this situation generally do not have to worry about it too much, blood seepage to reach a certain amount of blood, due to the subcutaneous accumulation of blood is more, it will be pressed to stop the bleeding of the small blood vessels on their own, the blue and purple after a period of time will be by itself. 2, the patient’s blood platelet reduction leads to bruises, after the blood draw the patient’s body due to the reduction of platelets can not stop the bleeding, this is a common clinical thrombocytopenic purpura, you can check the patient’s blood routine to further clarify the cause of the disease.