I saw this coming, and intentionally made the comment about an "imperfect" process, and that I had eliminated backfilling entirely, via a couple of incrementally better approaches. Add a 31g needle to the filtered syringe, if you're confident you can control it well enough to not inadvertently overdose, and any concern about backfilling is eliminated.
Still, after some practice, I can do this very quickly now, holding the plunger in my hand, leaving the insulin syringe exposed for only a few seconds.
"OMG an exposed syringe opening you don't care about sterility."
EVERY SINGLE LUER LOCK. SYRINGE AND NEEDLE ARE OPEN AND EXPOSED TO A NON-STERILE ENVIRONMENT ONCE REMOVED FROM THE PACKAGE.
Look at these gaping holes.
View attachment 314920
Is it ideal to have an open syringe? No.
Yet luer needles and syringes are packaged separately, exposing both to a non-sterile environment once opened. In fact syringes are often filled with one needle (large gauge, blunt, or filter) which is then removed, again exposing an OPEN syringe and now the contents to a non sterile environment, and another needle is removed from its packaging, exposing its open end to a non-sterile enviornment, and then attached.
How about ampules, commonly used for injectable drugs. Are they exposed to a non-sterile environment briefly?
View attachment 314921
The risk is low enough ampules and luer syringes are routinely exposed in clinical environments, even hospitals. Is that ideal? No, but then neither is you filing vials in a non-sterile environment, without a sterile flow hood, that you supply to other people, putting their health at risk.
Acting intelligently means understanding that while perfection is the goal, striving for it shouldn't be an impediment to incremental improvements where and when they can be implemented.
On balance, filtering removes existing bacteria, which Jano has found in peptides, particulates that are almost certainly present, and aggregates, the latter two are most dangerous with cumulative exposure. Any time exposed to a non-sterile environment risks the possibility of new post filtering bacterial contamination, but as a practical matter, based on the routine use of luer syringes and ampules, the risk is very low , and potential worst case outcome, unless you're somewhere with uniquely dangerous bacteria, is minor.