Can backflow (reflux) be prevented? You can’t be sure. Assume it happens.
Piston rebound
Change in atmospheric pressure
Causes of backflow:
Adjustment to tubing or source
The syringe piston is compressed at the bottom, then rebounds, drawing up fluid.
Adjusting the tubing or the height of the fluid source for the IV line can significantly change the pressure in the IV line.
The force from changes in atmospheric pressure are surprisingly strong–more than you can resist with a thumb on the plunger.