If you experience numbness in your arm after a blood draw, it is usually caused by spasm of the nerve vessels in your arm. The damage to the veins in the arm can lead to spasm and contraction of the veins, which can lead to a weakened venous return and poor blood circulation in the arm, which can affect the blood flow to the nerves and lead to nerve spasm, resulting in a weakened sensation and numbness in the nerve area. Therefore, when the arm is numb after blood sampling, massage of the arm or treatment with hot compresses around the arm can be performed to relieve the spasm of the nerve vessels and thus relieve the symptoms of arm numbness.