What is a vented needle used for?

A vented needle is specifically designed to equalize the pressure inside a vial during the process of drawing up or injecting liquid. When diluent is injected into a vial, the introduction of the liquid can create positive pressure inside the vial, which could lead to difficulties in achieving a smooth flow of medication. The vented needle allows air to enter the vial simultaneously, which helps balance the pressure and facilitates a smoother transfer of the liquid both in and out. This pressure equilibrium is critical for maintaining the integrity of the vial and ensuring a consistent, accurate measurement of the medication being drawn or injected.

While extracting liquid from a vial without pressure might be an intended effect of using a needle, it does not provide the full context of how a vented needle operates during the injection of diluent. Other options, such as solely injecting without withdrawing or filtering solutions before use, do not accurately reflect the primary function of a vented needle. The primary purpose is to allow for simultaneous venting of air pressure during the injection of liquid, which is why the second choice accurately describes the function of a vented needle.