Final answer:
The most likely cause of blood flow starting and then stopping during venipuncture with an evacuated tube is that the needle is not inserted deep enough into the vein. Other potential causes, such as the needle being too deep, a faulty seal on the tube, or an expired tube, could also affect flow, but they would likely present with different flow patterns.
Step-by-step explanation:
If blood begins to flow and then stops when an evacuated tube is pushed into the needle, the most likely cause is that the needle is not inserted deep enough. When venipuncture is performed correctly, blood should flow smoothly into the evacuated tube by vacuum action. However, if the needle does not penetrate adequately into the vein, this might cause an initial flow that stops because the needle might be partially within the vein, allowing a vessel wall to seal off the opening, or it might slip out of the vein altogether.
Other potential causes might include the needle being inserted too deep, causing it to go through the vein entirely. An improper seal on the evacuated tube could also result in blood flow ceasing, but this would usually prevent the flow entirely rather than allowing it to start then stop. Expired evacuated tubes may have a reduced vacuum, but they typically do not cause a start-stop flow. Instead, they might cause a slower overall flow or incomplete filling of the tube.