Sterile filtration is something done to a batch. A sterility test is something done to a sample of it afterwards. They appear near each other on certificates and are frequently read as the same assurance, which they are not.
What sterile filtration does
A solution is passed through a membrane with a nominal pore size of 0.22 micrometres, small enough to retain bacteria while allowing the dissolved product through. It is a process step, performed immediately before filling, and it removes organisms present in the solution at that moment.
Two limitations follow directly from the mechanism. The filter retains organisms by size, so anything smaller passes, including viruses and mycoplasma. And it protects only the fluid that passes through it; every operation downstream, the filling, the stoppering, the sealing, must be conducted aseptically or contamination is reintroduced after the filter has done its work.
What the sterility test does
The compendial sterility test, described in United States Pharmacopeia general chapter ⟨71⟩, incubates sample units in growth media and looks for turbidity indicating microbial growth. Two approaches are recognised:
- Membrane filtration, in which the sample is filtered, the membrane rinsed and then transferred to media. This is preferred where the product itself might inhibit growth, since rinsing removes it.
- Direct inoculation, in which the sample is added straight to the media. Simpler, but requires demonstrating the product does not suppress the growth it is meant to reveal.
Two media are used in parallel to cover different organism classes, and incubation runs for 14 days. That duration is why sterility is often the line that gates a lot's release, and why certificates sometimes issue with sterility pending.
| Sterile filtration | Sterility test | |
|---|---|---|
| What it is | A process step on the batch | A test on a sample afterwards |
| Applies to | Everything that passes the membrane | Only the units pulled for testing |
| Mechanism | Retention by size at 0.22 micrometres | Growth in culture over an incubation |
| Misses | Anything smaller, including viruses and mycoplasma | Contamination outside the sampled units |
| Proves | The fluid was filtered | No growth was recovered from that sample |
| Says nothing about | What happened after filtration | Endotoxin, which is a separate test |
One is a process applied to the batch, the other a test applied to a sample of it. Neither substitutes for the other.
The statistical limit nobody can engineer around
A sterility test examines a sample, not the batch. If contamination is present in a small fraction of units, the probability that a modest sample catches it is low. The test can demonstrate the presence of contamination; it cannot demonstrate its absence across a lot.
This is not a criticism of the method, it is a property of sampling, and it is the reason regulatory thinking places the weight on process control rather than on end-product testing. A validated aseptic process with environmental monitoring, operator qualification and media fills provides far more assurance than a passing sterility test on a handful of vials.
Growth promotion and the suitability test
Two controls determine whether a sterility result means anything.
- Growth promotion confirms the media will actually support growth of specified organisms. Media that cannot grow anything will reliably report no growth.
- Method suitability, sometimes called bacteriostasis and fungistasis testing, confirms the product does not inhibit growth in the media at the concentration used. A compound with antimicrobial character can suppress the very signal the test is looking for.
A certificate stating only "sterile: pass" has not told you whether either control was performed.
How this reads on a lyophilised research product
For material supplied as a lyophilised powder, the relevant history is the sterility of the solution before filling and the asepsis of the fill itself. The dry state does not sterilise anything; it suppresses growth by removing available water, which is a different property. Organisms can survive lyophilisation and resume growth on reconstitution.
The practical implication is that reconstitution is the point at which handling discipline matters most, because it restores the conditions under which anything present can multiply.
What to ask for
- Was the solution sterile filtered, at what pore size, and was filter integrity tested after use?
- Was a compendial sterility test performed on the finished lot, by which method, and over what incubation period?
- Were growth promotion and method suitability established for this product?
- Is the fill aseptic, and is there environmental monitoring data behind it?
This sits inside the wider picture of what gets tested and why, which why peptide testing matters sets out across the whole analytical panel.