1. Because the blood vessels are too thin and the blood flow is not smooth, the repeated aspiration of the syringe is too long, so that the blood cells are destroyed and hemolyzed;

2. When the blood is injected into the blood collection tube, the pressure is too high, and it is not slowly injected along the wall of the tube, which causes the blood cells to be damaged;

3. After the blood hits the blood collection tube, it shakes too much.