Leftover gunk in the filter, gross

I make pins for my wife, my sister and me by pooling three T30s (1ml from each original vial) and pushing them through a filter into a fresh 5ml sterile vial. This time there was extra back pressure while filtering, so when I let up on the plunger, air pressure got pushed back into the syringe through the filter. I had already pushed about 2.25ml of the 3ml that was in the syringe, and I noticed something like a cloudy streak in the solution still inside the syringe, but I kept filtering anyway and this time skipped my usual filter flush so I could look into it. Here is what I see after drawing some air back through the filter, using only the needle and putting it back into one of the original vials.

Do you figure this is just the buildup of what the filter caught from three vials? All three looked nice and clear before filtering, and as you can see on the right they are still nice and clear after. I have never actually inspected a filter like this; I just flush it and toss it, assuming it did its job, but I suppose this is proof the filter really is doing its job.

Sounds to me like the filter got clogged. You could push whatever is left through a fresh filter if you're set on salvaging it, but I wouldn't put any faith in the old one. That kind of back pressure is enough to have likely compromised the membrane.

That said, yes — I'd figure what you're seeing is the buildup you collected from running 3 vials through a single filter. On the bright side, you've now shown yourself (and ideally a few folks reading along) that filtering is time and money well spent.

Skip that leftover entirely. Filtering it again isn't worth attempting — the human body isn't a trash can, for goodness sake.

Also keep in mind that filtration goes one way only. You should never pull back and then push forward again. This is not a coffee filter; it's a precisely engineered membrane whose pores are smaller than bacteria.