Here's what I do and why, and I would like to know if anyone else does it this way or if there are any drawbacks I haven't thought about.
I hang a small bag of 0.9 as primary--100 ml, if we have it in stock, or 250 ml, most often in stock. I prime the primary line with the 0.9, then backprime the secondary line abx. I then set the pump for the primary at 50 ml/hr with 18 ml VTBI, because that is the volume that most most primary IV tubes hold based on my research. I set the secondary to the rate recommended for the abx.
Here's my rationale. For 50 ml bags of abx, using a primary line to infuse means that the patient will get only 32 ml per infusion! Even if we set the VTBI at 50 ml, the pump will stop because of air in the line at the point of the pump. By the end of three doses, infusing the abx as primary means that we will have "skipped" an entire dose!
I also like the fact that you can backprime for each new dose in the same tubing or for new abx with new secondary tubing. Much, much faster.
Am I missing something here? I can't figure out why my coworkers always hang the abx as primary. I've asked, but I just get a shoulder shrug as a response.