Swelling of the arm after blood drawing may be caused by blood pressure blood extravasation due to improper operation of blood drawing or improper pressing, and it may also be caused by blood reflux obstacle due to too long pressing, and it is also necessary to exclude the coagulation dysfunction caused by some of your own diseases. 1. Blood extravasation: swelling or bruising of the local skin where the blood is drawn is a very common phenomenon, which may be caused by improper operation or improper pressure when the blood is drawn, resulting in damage to blood vessels and blood spilling out or overflowing into the subcutaneous tissue. 2. Blood reflux obstruction: swelling of the ministries after blood sampling may also be caused by blood reflux obstruction due to prolonged pressure, which will lead to poor local blood circulation and thus swelling. 3. Coagulation dysfunction: some patients may have some blood diseases such as idiopathic thrombocytopenic purpura or hemophilia, which may lead to coagulation dysfunction and swelling of the arms after blood sampling. If the swelling of the arm after blood draw is accidental, it usually does not need too much intervention, but if it persists, it is necessary to consult a doctor in time and exclude the factors of the disease under the doctor’s guidance, so as to find out early and treat early.