The 0.22 micron filter does the work, and it is the last line

There are two routes to a sterile injectable. Terminal sterilisation subjects the sealed, filled container to a lethal process — most commonly moist heat — and its great virtue is that the thing being sterilised is the thing being sold. Aseptic processing sterilises the components separately, filters the solution, and assembles them under conditions intended to exclude contamination. Peptides in solution generally cannot survive an autoclave cycle, and lyophilised products are filled as solutions before drying. The consequence is that essentially every product in this market, legitimate or otherwise, is aseptically processed rather than terminally sterilised.

Why nothing in this market is terminally sterilised

Where a product will tolerate it, moist-heat terminal sterilisation is the preferred route by a wide margin, for the simple reason that the object subjected to the lethal process is the sealed, filled, finished container. There is no subsequent opportunity for contamination, and the lethality delivered can be measured directly through the load.

Peptides make poor candidates. Autoclave conditions accelerate hydrolysis of the amide backbone, deamidation of asparagine and glutamine residues, and aggregation, and a cycle sufficient to guarantee microbial lethality will typically destroy a measurable fraction of the active substance. Lyophilised presentations do not escape the problem, because the solution is filled before it is dried, and the sterility of the finished cake is inherited from the sterility of that solution and the environment of the fill.

The consequence is that every product in this market is aseptically processed. That places the entire burden of sterility assurance on filtration, environmental control, component preparation and operator technique — precisely the four things about which the trade publishes nothing. The Journal states this as a structural observation, not as an accusation: the same is true of many legitimate aseptically filled products, and the difference lies in whether an inspectorate has looked.¹

Bioburden, and why the number before the filter matters

A sterilising filter is not an unlimited barrier. Its qualification is expressed as a retention capability under a defined challenge — conventionally a high titre of a small bacterial species per square centimetre of membrane — and its performance in use depends on the load presented to it. A bulk solution carrying a heavy microbial burden presents a filter with a harder problem than one carrying a light burden, and it presents a second problem the filter cannot address at all: the endotoxin released by organisms that die upstream passes through the membrane unimpeded.

Regulated manufacture therefore specifies a pre-filtration bioburden limit, tests against it on every batch, and treats an excursion as an investigation rather than a curiosity. The specification is usually expressed in colony-forming units per hundred millilitres, and a well-controlled process runs far below it.

Of the twenty companies the Journal wrote to, four stated that pre-filtration bioburden is determined on every batch, three stated that it is determined periodically, and the remainder did not answer the question. We regard that distribution as the single most informative result of the correspondence, because bioburden testing is inexpensive, is performed on the bulk rather than the finished container, and is the earliest point at which a problem is cheap to fix.

The filter, and the test that proves it survived

Sterilising-grade filtration through a membrane of 0.22 micron nominal rating is the operation on which aseptic processing rests. The membrane is qualified by challenge with a small bacterium at high concentration under the process conditions, and a filter that permits no passage under that challenge is accepted as sterilising for that fluid.

The critical practice is not the filtration but the integrity test that follows it. A membrane can be damaged during installation, during sterilisation, or by pressure excursions in use, and a damaged membrane looks exactly like an intact one. Bubble point, diffusive flow and pressure hold tests each detect a breach by measuring gas behaviour across a wetted membrane, and a post-use test is the only evidence that the filter was intact while the product was passing through it. A pre-use test alone establishes nothing about the state of the membrane at the end of the run.

This is the question the Journal has found most useful when assessing whether a fill operation is a real one. It is specific, it has a yes or no answer, the answer is recorded in the batch documentation as a matter of course, and it cannot be answered plausibly by anybody who is not actually running the process.

Media fills, and proving that a rare thing is rare

An aseptic process simulation, generally called a media fill, replaces the product with a sterile growth medium and runs the line exactly as it would run for a real batch: same components, same interventions, same operators, same duration. Every filled unit is then incubated and examined for growth. The purpose is to estimate the contamination rate of the process itself.

The arithmetic constrains the design. To support a claim that the contamination rate is below one unit in a thousand with reasonable statistical confidence, several thousand units must be filled with no contaminated unit recovered. Contemporary practice runs simulations of five to ten thousand units and treats a single positive as a signal requiring investigation rather than as an acceptable rate. The simulations are repeated at defined intervals and after any significant change to the line, and they are the closest thing in manufacturing to a direct measurement of aseptic capability.

One company in the Journal’s correspondence provided media fill records. They covered three simulations across eighteen months, at scales between four and six thousand units, with no recovered contamination. We report that because it is the only such record we have seen from this trade, and because it demonstrates that the documentation exists and can be shared when a company chooses to.

The compendial sterility test, described plainly

The compendial test proceeds by one of two routes. In membrane filtration, the entire contents of the sampled containers are passed through a retentive membrane which is then divided between two growth media. In direct inoculation, the contents are transferred into the media directly. The media are a fluid thioglycollate medium incubated at thirty to thirty-five degrees for anaerobes and aerobes, and a soybean-casein digest medium incubated at twenty to twenty-five degrees for fungi and aerobes. Incubation runs for fourteen days with periodic examination for visible growth.

The number of containers sampled depends on batch size, and for a parenteral batch above five hundred containers the requirement is twenty. Every one of those twenty is destroyed. Method suitability must be demonstrated separately, because a preserved formulation or an antimicrobial residue can inhibit the very growth the test is looking for, and the bacteriostatic and fungistatic properties of the article have to be neutralised or shown absent before a negative result means anything.²

A reader who takes nothing else from this section should take the sample size. Twenty containers, fourteen days, destroyed. That is the entire empirical basis of the finished-product sterility claim, and it is why the process argument carries the weight.

The statistics, worked in one paragraph

Suppose a batch of ten thousand vials in which one vial in a thousand is contaminated — a rate that would be a serious finding in any regulated operation and is invisible to any buyer. The probability that a single randomly chosen vial is clean is 0.999. The probability that all twenty sampled vials are clean is 0.999 raised to the twentieth power, which is approximately 0.980. The sterility test therefore passes this batch about ninety-eight times in a hundred.

Push the contamination rate up tenfold, to one vial in a hundred, and the test still passes the batch roughly eighty-two times in a hundred. To reach an even chance of detection at a one per cent contamination rate you would need to sample about seventy containers; to have a reasonable prospect of catching a one-in-a-thousand rate you would need to sample several hundred, which for most batches means testing a substantial fraction of the product.

This is not a criticism of the compendial test, which is designed as a final check against gross failure and performs that function. It is the reason no serious manufacturer treats a passed sterility test as the basis of the sterility claim, and the reason that a research supplier offering to have a vial sterility-tested on request is offering something considerably weaker than it sounds.

Depyrogenation, and the step that gets skipped

Because endotoxin survives sterilisation, glass components require a separate treatment. The standard is dry heat: a tunnel or oven cycle at two hundred and fifty degrees or above, validated to achieve at least a three-log reduction in a deliberately applied endotoxin challenge. Vials emerging from a qualified depyrogenation tunnel are both sterile and pyrogen-free, and they remain so only if the subsequent handling maintains it.

Elastomeric closures cannot take that treatment and are instead washed, siliconised and steam-sterilised by a validated cycle, with endotoxin control achieved by the washing step and specified as a limit per stopper. Water for injection is controlled at source, with a compendial endotoxin specification, and a water system is the commonest origin of a pyrogen problem in an otherwise competent facility.

Depyrogenation is also, in the Journal’s experience of asking, the step most frequently absent from descriptions of small fill operations. Autoclaved vials are sterile. Autoclaved vials are not depyrogenated, and an operation that describes its glass preparation solely in terms of autoclaving has told you something specific about what it has not done. We put this point to eleven correspondents; two said their glass is depyrogenated by dry heat with a validated cycle, and one asked us what depyrogenation was, which we took as a straight answer and a useful one.

The sterility half is not solvable within the current economics, and it would be dishonest to pretend otherwise. Fourteen days, two media, a destroyed container and a statistical yield that passes most contaminated batches: no commercial testing service can build a product out of that for private buyers, and none has tried. The honest position is that this attribute is not verifiable downstream and must be assured upstream or not at all.