For the preparation of Non-Hazardous Drugs, the buffer room needs to be at a higher pressure than the ante-area, which, in turn, must be at a higher pressure than the surrounding areas.
This is to ensure outward flow of the (at least) ISO Class 7 air of the buffer room to the (at least) ISO Class 8 air of the ante-area and in turn to the surrounding areas whenever doors or pass-throughs are opened.
USP <797> states, “a minimum positive pressure of 0.02- to 0.05-inch water column is required,” for rooms with a physical separation of ante area from buffer area by means of walls, doors, and pass-throughs.
By mechanical control or through your SOPs and staff training, you should ensure that doors from the buffer room to the ante-area and doors from that ante-area to the outside environment are never open at the same time.
Ante room pressures for Hazardous Drug compounding have become more complex and detailed with the publication of USP <800>.
Under <800>, the (at least) ISO Class 7 buffer room (C-SEC) must operate at a negative pressure between 0.01- and 0.03-inches of water column relative to the ante-area. This means that for HD sterile compounding the ante-area air must be as clean as the buffer room air, because when the door between the rooms is opened, air will move from the ante-area toward the buffer area. Therefore, the ante-area must also maintain no worse than ISO Class 7 quality under dynamic conditions.
Additionally, in an <800> cleanroom design, the ISO Class 7 ante-room must become a barrier to the movement of dirty air from the outside environment toward the Direct Compounding Area (DCA). This is achieved by ensuring that the ante-area maintains at least 0.02-inch water column of positive pressure relative to adjacent unclassified areas.
As must always be emphasized, even the best-designed cleanroom environment cannot protect our patients from a careless operator. As noted in <797>, “direct touch contamination is the most likely source of introducing microorganisms into CSPs prepared by humans.”
My advice – and this is over and above USP requirements – is that you ensure one extra “layer” of cleanliness by surrounding the buffer room AND the ante-area with what I call a “work room” or a “prep room” that has HEPA-filtered air supply, smooth, impervious surfaces (floors, walls, ceilings, shelves, and fixtures), minimal particulate-shedding contents (i.e., no cardboard, no wood, no pressed board, etc.) minimal paper, and a strict cleaning protocol. I don’t suggest that you attempt to qualify the air in this outer room, but it seems clear to me that you don’t really want it at ISO Class 9 (outside world) either. I refer many of my clients to a firm that has mastered this strategy and these rooms typically outperform others.
I’m very weary of seeing my hospital clients waste their precious capital budgets on poorly-designed cleanrooms that are obsolete before they’re even opened because their architects and construction partners don’t understand the multitude of intricate requirements.
All of the above are my personal interpretations. If I stated anything incorrectly, I urge you to reply, as it’s the only way for us all to learn. These are complex concepts and I try to improve my understanding every day.