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filtering left overs, YUCK

7 replies6 peopleOct 24, 2026☆ Follow
Summary

what happens when a filter clogs during multi-vial sterile filtering and should the leftover solution be used

The original poster saw a cloud in the syringe after back-pressure during filtering of three clear vials into one new vial and checked the filter afterward. Replies agree the cloud was trapped material and the filter likely clogged, with some warning against reusing the membrane or the remaining liquid because back-pressure can damage it and filtration must stay one-directional. Others suggest switching to larger filters or adding a vent needle to avoid the issue. Everyone accepts that the filter was working but opinions split on whether the small leftover volume is worth keeping.

What this discussion establishesWhere people disagree

whether any leftover solution after a clogged filter should ever be pushed through a fresh filter or simply discarded

Still open

whether the observed cloud was only accumulated debris or also indicated filter damage from pressure reversal

Nothing here is advice.

7 replies · 6 people
PlainQuill53archiveopening post1d

I combined three T30 vials by drawing 1 ml from each into one syringe then pushed everything through a filter into a fresh 5 ml sterile vial. This time extra resistance built up and when I relaxed the plunger some air came back through the filter into the syringe. I had already moved about 2.25 ml of the 3 ml total and saw a cloudy streak in what remained in the syringe, but I kept going and skipped the usual flush so I could look at it later. After I drew a little air back through the filter and put the needle into one of the original vials this is what showed up. Could that just be everything the filter caught from the three vials? All three started clear and still look clear on the right after filtering. I normally flush and throw the filter away without checking, but this shows it was actually catching stuff.

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GreyKettle68archive1d

The back-pressure you felt probably means the filter got blocked. If you want you could push the rest through a fresh filter, but I would not rely on the one that already resisted. The pressure that caused the blowback might have harmed the membrane. Still, what you saw is most likely the material collected from running three vials through a single filter. At least this shows filtering is actually removing something instead of being wasted effort.

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CopperPebble16archive1d

Do not keep or re-filter that leftover material. The body is not a garbage disposal. Filtration only works in one direction; never reverse the flow. This is not a paper coffee filter but a precision membrane whose pores are smaller than bacteria.

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PlainQuill53archive18h
↳ replying to @CopperPebble16

I was not going to use it anyway. Most of the time the new vial has enough vacuum that I do not run into back-pressure and the process stays smooth. The little resistance I hit this time was not enough to send air all the way into the syringe barrel, but it was enough to push some liquid back up the needle and loosen a bit of the trapped material. I did finish pushing the remaining 0.75 ml through the same filter, but I skipped the bac-water flush so I could examine what happened. It was not ideal, yet I still recovered that amount and the vials only last five or six weeks so it should be fine.

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SteadySparrow75archive17h

so no 'vent needle'?

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PlainAlder11archive15h

Discard that small amount of debris and add a vent filter as already suggested. I end up doing that often.

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CopperThistle37archive9h
↳ replying to @GreyKettle68

I wonder whether a 4 mm or a 13 mm filter was used. The smaller 4 mm size is often suggested, yet it seems like an unnecessary risk because a clog could develop without any clear warning.

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GreyKettle68archive8h

Those 4 mm filters are meant for 1 ml or smaller volumes. They can manage up to 3-4 ml when the liquid is fairly clean, but they have far less surface area and clog more easily. A 13 mm filter works better for 1-10 ml batches. We do not keep either size at the hospital; we use 30 mm filters instead. The amount left behind in the filter is usually small unless the doses are tiny to begin with.

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