If needing blood for laboratory analysis, in addition to the usual technique of cannula insertion, a syringe of appropriate size (10 ml will suffice for most things, 20 ml if blood cultures are to be taken) can be used to take blood directly from the cannula. This is done directly following removal of the needle and before the cannula is flushed. The tourniquet should not be removed (but should be loosened) while the syringe is being inserted and removed.
This technique requires patients to have only one needle inserted, and is thus far less painful. It is often important to insert cannulas and take blood at the same time, for example when seeing acutely unwell medical patients, or acute trauma patients.