What technique is used to maintain equalized pressure in a vial while adding or withdrawing a solution?

The milking technique is specifically designed to maintain equalized pressure within a vial while adding or withdrawing a solution. This technique involves gently squeezing or compressing the vial's flexible walls, which allows for the controlled addition or withdrawal of a solution without creating negative pressure or vacuum conditions. When the vial is compressed, the pressure inside is equalized with the outside atmospheric pressure, preventing the formation of bubbles or foam and ensuring that the solution can be drawn up or expelled smoothly.

In compounding sterile preparations, maintaining proper pressure within a vial is crucial to avoid potential contamination and to ensure that the correct volume of medication is delivered. This technique is particularly useful in situations where air must be introduced into the vial to replace the volume of liquid being withdrawn, helping to prevent the collapse of the vial or the creation of a vacuum that could hinder the process.

Other techniques mentioned, such as piercing, vacuum, and pressure equalization, do not specifically describe the controlled and deliberate method of maintaining equalized pressure while manipulating solutions in vials. Therefore, the milking technique stands out as the most effective method for this purpose.