Terminal Sterilization vs. Aseptic Processing

terminal sterilization





In a previous post I briefly identified the two primary methods (terminal sterilization and aseptic processing) for producing a sterile pharmaceutical product, such as a vaccine injection. In this post I will contrast the two methods.

A sterile product contains no living organisms. This means the sterilization process must kill any present organisms, including disease-causing microorganisms called pathogens. The only two methods used to manufacture sterile products are terminal sterilization and aseptic processing.

Terminal Sterilization:

Terminal sterilization is a traditional method going back to the 19th century canning industry. For this method, most drug products are produced by mixing the ingredients to form the bulk drug product solution. Which is then:

filled into a container such as a vial or syringe;

tightly sealed with a rubber-type stopper or syringe plunger; and

then sterilized the sealed container, usually with high heat in a chamber called a steam autoclave.

The sterilization process is very well controlled through well-defined conditions of time, pressure, and temperature. The items in the chamber are arranged to ensure sterilization of each.

Terminal sterilization is widely used in other industries. Pasteurization of milk is a similar but lower temperature process to kill pathogens. Autoclaves are widely used to sterilize materials in hospitals, dentists’ offices, and tattoo parlors.

Other types of terminal sterilization include: ethylene oxide gas for surgical garments; hydrogen peroxide vapor for items not suitable for high heat; or gamma-ray irradiation.

The first two only sterilize the outside surface of the container. Only radiation and steam (indirectly) can sterilize the interior.

Terminal sterilization has several advantages that make its use the default method for pharma:

The sterilization cycle can be tightly controlled with typical temperatures varying by less than 1 °C and timed to the second. Steam is ideal as it disperses throughout the camber quickly while pressure controls temperature. It also holds a unique chemistry that makes it more lethal to living cells at lower temperatures than dry heat alone such as in a hot-air oven.

Thus, the sterilization process is highly predictable and repeatable and provides a very high level of statistical assurance that the filled containers are sterile. Although there is never a 100% assurance of sterility, the minimum standard is typically that the chance of a non-sterile unit is less than one in a million.

Since the container is fully sealed prior to terminal sterilization, there is very little risk of contamination afterwards.

Because terminal sterilization occurs after the formulation and filling process, these manufacturing steps can occur in more relaxed environment. This lowers the cost and complexity of maintaining and operating such a manufacturing space.

As a general rule, regulatory agencies such as the FDA regard terminal sterilization as the preferred method of sterilization, due to the high level of sterility assurance it provides. So, why would drug manufacturers use any other method?

Unfortunately, less robust medications, including many newer biologic drugs such as proteins, mRNA, hormones, and monoclonal antibodies will quickly deteriorate at the extreme conditions of terminal sterilization. While the end product might be sterile, but the drug will not work!

Luckily, most products can be sterilized by filtering the clear bulk solution through filters so fine that even the smallest microbes cannot pass. But that leaves the problem of how to get the now sterile solution into the vial and sealed without recontamination.  This is where aseptic processing is required.

Aseptic Processing:

Autoclave Used In Terminal Sterilization

When terminal sterilization is not an option, aseptic processing is used. Each component (drug, container, closure, etc.) is individually sterilized first, then carefully assembled in a clean room to make the finished drug product in a manner that prevents contamination.

Aseptic processing cannot provide the same quantitative level of sterility assurance as terminal sterilization, but it features several layers of control to minimize the risk of contamination:

Containers, closures, and filling materials go through their own validated sterilization cycles.ake it stand out

The filtration of the formulated drug product is also validated – manufacturers will demonstrate repeatedly that passing the product through the selected filter will remove a particular microorganism. For this validation a bacterium of very small size (even compared to other bacteria), which is therefore more likely to pass through a filter, is used.

The clean room environment in which the filling takes place is much more strictly controlled than the environment for non-aseptic processing.

Operator gowning in an aseptic processing area typically consists of full coverage – no skin or hair is exposed – with sterilized coveralls, hoods, boots, goggles, and gloves. Operators are highly trained from how to put these gowns on to using proper aseptic techniques to avoid contamination. Each operator must pass certification testing before they are allowed to participate in aseptic activities.

The entire aseptic process is routinely simulated in a challenge is designed to detect the potential for contamination in the finished drug product. A single unit positive for growth is a sign that somewhere the process has failed and would be thoroughly investigated – zero positives is the norm for this challenge.

There are several variations on aseptic processing. In addition to the traditional clean room, some facilities use barriers to separate operator from the actual filling area. These can be full isolators completely closed to the surrounding environment, or less enclosed systems such as RABS (“remote access barrier systems”). Both require operators to interact with the filling area only through special glove ports, further minimizing the potential for contamination of the drug product.

Whether a product is sterilized via terminal sterilization or aseptic processing, it is very important to prove that there is no degradation of the product over time. This is shown through extensive stability testing, which is a long-term evaluation of the container/closure system and the product itself.

When possible, terminal sterilization is the expected method of sterilization for drug products. Since terminal sterilization is destructive to some products, aseptic processing is used as a proven safe and effective method to sterilize medications. As new products are developed and manufactured, both methods will continue to be used to ensure that they are safe for patients.

“Aseptic Filling, Weird Stuff”

Or at least the CNBC Anchors think so …


“The Fill Finish process of manufacturing has to be done under aseptic conditions to ensure there is no contamination. That capacity is at a premium. Weird right?” from CNBC on 01 February 2021

Why should it matter to you?

Admittedly, it is not common knowledge but certainly not weird stuff. If you have ever had medication injected via a syringe (think COVID-19 vaccine) chances are it was the product of aseptic fill. I don’t know about you but I’m not interested in a side serving of bacteria injected along with my vaccine. Neither should you be.

What is Aseptic Processing?

A critical feature of injectable drugs is that they are completely free of disease-causing microorganisms, such as bacteria. Unlike other forms of medication (such as pills, capsules, ointments, etc.), injections bypass most of the body’s natural barriers, and therefore sterility, or the complete absence of microorganisms, is crucial.


Generally, there are two methods of producing sterile product. The first is to assemble all the ingredients into the finished dose container (for example, vials or pre-filled syringes), and then sterilize the product, usually using high temperatures or radiation. This method has traditionally been the most common method used. However, many newer products such as biologically-derived materials such as mRNA and proteins, cannot withstand these sterilization conditions and would be irreversibly damaged. Think of how poaching an egg changes its structure. For these fragile materials, aseptic processing is used.

“Aseptic” means free of pathogenic (i.e., disease-causing) microorganisms. For this method, the different components of the finished drug form are each sterilized separately, then brought together and assembled under conditions designed to prevent the potential re-introduction of microorganisms. For example, the containers and closures (e.g., vials, syringes, cartridges, stoppers, caps) may be sterilized ahead of time using heat (either dry or steam heat). Product-contact materials such as bulk containers (carboys), filters, fill tubing, and fill nozzles, can similarly be heat sterilized. The actual drug ingredients, either individually or in a pre-formulated state, are usually sterilized through very fine specialized filters that have been proven to remove bacteria down to 0.22 microns from the product stream as it passes through the filtration media.

These components, separately sterilized, are brought together in a clean room. These rooms have controls such as HEPA (High Efficiency Particulate-free Air)-filtered air to remove bacteria in the air, higher pressures to keep outside air out, and strict controls on operator dress and behavior, that together help to ensure that no microorganisms can be introduced into the product by the operator. In the clean room, the sterile containers are filled with the correct amount of product and then sealed to protect them from future contamination.

A number of additional controls go into ensuring that clean rooms stay “clean”. These rooms are routinely certified to demonstrate that they are performing according to their design. Environmental monitoring is performed during every product batch to make sure that the proper environment is maintained. Operators entering the clean room are highly trained in proper gowning and behavior, and must pass a test before initial entry and routinely thereafter. The aseptic filling method is challenged periodically to ensure that there is nothing within the process that could result in a sterility failure. The entire process is simulated during these challenges, and a special growth media designed to show the presence of any microorganisms within the product container is used. Samples are taken from the operator’s hands and gown in search of microbes. A zero “positive” result is the expected result.

Aseptic processing is an established and well-controlled method for producing quality, sterile drug products. Manufacturers conform to strict guidelines and requirements set forth by regulatory agencies such as the FDA. This method is crucial for manufacturing many of the most critical medicines used to treat diseases in the modern world.


How Pharma Uses “Smoke Studies”​ To Help Keep Employees Safe During The Pandemic

In early 2020 many industries opted for remote working. Afton Scientific knew that this was not an option. We needed to remain open to produce life-saving drugs. The challenge was to prevent community spread should one employee become infected.

Early on we implemented standard safety protocols, such as:


  • Twice daily temperature checks

  • Mask protocols

  • Hand sanitizing stations

  • Employee education

  • Outdoor morning meetings

The nature of pharmaceutical manufacturing often requires employees to work in close proximity for long periods. We asked ourselves how we could minimize the virus spreading in the air to other masked employees.

Air flow patterns in cleanrooms are designed to assure sterility by observing the sweeping of airborne micros and particles away from important areas with controlled air flow. Smoke studies are conducted to observe those air flow patterns.

We applied the same air flow analyses in non-sterile work areas to determine:

smoke study.png

  • Where targeted airflow would be helpful

  • Where to add plexiglass panels.


For example, in our inspection rooms operators work in close proximity. Based on the path of the non-toxic smoke we added ductwork to give each workstation its own air supply. Then we followed the air to a safe capture location.


Thick plexiglass panels were also added between each station and operators are masked at all times.

While a few of our staff have become infected with COVID outside of work, each was an isolated incident and there has been no community spread among any employees.

According to CDC and Virginia guidelines, with each positive result (less than .5%) we conducted internal contact tracing and determined which employees required testing and quarantining. In all instances tests were negative and employees were able to return safely to work.

As an essential business we are now well on our way to having our staff vaccinated. We will continue to mask and observe best-practices while looking forward to the next new normal.