How to pull out the blood collection needle after blood drawing

There are different ways to draw blood with a blood collection needle and a disposable needle. Blood collection needles suck blood in by negative pressure in test tubes, so the blood collection needles should be separated first to prevent blood from flowing into test tubes and ensure the accuracy of blood collection. (blood sample needs 0.5 or 1.0 or 2.0, and different test items need 5.0 ml) and then the needle is pulled out at the venous end. So the doctor was right.