The main reasons for slow blood draws are usually a thin needle, thin blood vessels, or high blood viscosity. When the needle is thin, slow blood draws occur because less flow is drawn. If a thin blood vessel is chosen for blood collection, the negative pressure during aspiration will cause the upper and lower walls of the vessel to close, thus making it difficult for the blood at the distal end to flow into the blood collection needle. If the blood vessel is thin it is also easy for the bevel of the puncture needle to attach to the inner membrane of the vessel wall, causing the vessel wall to obstruct the blood collection needle during aspiration, and it is not easy to draw blood out. Large blood viscosity, a condition caused by the body’s own influence, can also cause slower blood draws.