A Bowie-Dick test is a daily quality check performed on vacuum-assisted steam sterilizers (autoclaves) to verify that the machine is properly removing air from the chamber before sterilization begins. If air remains trapped inside, steam can’t reach every surface of the instruments being sterilized, which means some items may not actually be sterile when the cycle ends. The test catches this problem before any patient instruments are processed.
Why Air Removal Matters
Steam sterilizers work by exposing instruments to pressurized steam at high temperatures, typically around 134°C (273°F). For sterilization to work, steam must make direct contact with every surface. Pockets of trapped air act as barriers, preventing steam from reaching certain areas. This is especially problematic in pre-vacuum sterilizers, which use a vacuum pump to pull air out of the chamber before injecting steam. If that vacuum system has a leak, a worn seal, or a malfunctioning pump, air stays behind and creates dead zones where bacteria can survive.
The Bowie-Dick test exists specifically to detect these failures. It doesn’t test whether the sterilizer can kill microorganisms (that’s the job of a biological indicator). Instead, it answers a more fundamental question: is the machine physically capable of removing air and getting steam where it needs to go?
How the Test Works
The original version of the test, developed in the early 1960s by J.H. Bowie and J. Dick, used a stack of folded 100% cotton surgical towels with a chemical indicator sheet placed in the center. The towels create a dense pack that’s difficult for steam to penetrate, essentially simulating a worst-case scenario. If steam can reach the indicator sheet buried in the middle of that pack, it can reach instruments in a normal load.
Today, most facilities use commercially made disposable test packs that serve the same purpose. These are compact devices designed to simulate the challenge of a full towel pack without the hassle of folding and conditioning cotton towels each morning.
The test pack is placed horizontally on the bottom rack of the sterilizer, near the front door and directly over the drain. This location is chosen because it represents the area most susceptible to air pockets. The cycle is then run at 134°C for 3.5 minutes in an otherwise empty chamber. Running it empty is important: the test evaluates the machine’s air removal capability in isolation, not mixed in with a load of instruments.
Reading the Results
After the cycle finishes, you open the test pack and examine the chemical indicator sheet inside. A passing result shows a uniform color change across the entire sheet. This means steam penetrated evenly through the pack, confirming that the vacuum system successfully removed air from the chamber.
A failing result shows an incomplete or uneven color change. You’ll typically see a lighter spot or an unchanged area somewhere on the sheet, usually toward the center. That spot marks where trapped air prevented steam from reaching the indicator. The pattern can sometimes hint at the type of problem. A distinct spot in the center often points to inadequate vacuum depth, while irregular patches may suggest air leaking back into the chamber during the cycle.
If the sterilizer fails, it should not be used to process any instruments. Maintenance personnel need to inspect the machine, identify the cause of the failure, and confirm that it passes a subsequent Bowie-Dick test before returning it to service.
When and How Often to Run It
The test is performed once per day, before the first processed load, on every vacuum-type steam sterilizer in use. The Association for the Advancement of Medical Instrumentation (AAMI) recommends running it at the same time each day to create a consistent baseline for comparison. Facilities that run sterilizers around the clock typically perform the test at the start of each shift or at a designated daily interval.
Gravity-displacement sterilizers, which rely on steam pushing air downward and out through a drain rather than pulling it out with a vacuum, do not require a Bowie-Dick test. The test is designed specifically for pre-vacuum (also called dynamic air removal) sterilizers.
Common Causes of Failure
When a sterilizer fails the Bowie-Dick test, the problem usually traces back to one of a few mechanical issues. A worn or damaged door gasket is one of the most frequent culprits, allowing air to seep back into the chamber after the vacuum phase. Vacuum pump problems, whether from wear, clogged filters, or failing valves, can prevent the chamber from reaching adequate vacuum depth. Poor steam quality, such as steam mixed with non-condensable gases from the facility’s boiler system, can also cause failures because those gases behave like trapped air and block steam contact.
Less obvious causes include a clogged drain line, which can leave condensate in the chamber and interfere with air removal, or a control system error that cuts the vacuum phase short. Seasonal changes in water supply or boiler maintenance schedules can sometimes trigger intermittent failures that are harder to diagnose.
Bowie-Dick Test vs. Other Sterilization Checks
The Bowie-Dick test is one piece of a larger sterilization monitoring program. It checks mechanical function, specifically air removal and steam penetration. Biological indicators, which contain live bacterial spores, test whether the sterilizer actually kills microorganisms. Chemical indicators placed inside instrument packs verify that each individual load was exposed to adequate conditions. Together, these three types of monitoring cover different failure points in the sterilization process.
Think of the Bowie-Dick test as the first line of defense each morning. It catches problems with the machine itself before any instruments go through it, preventing a scenario where an entire day’s worth of loads would need to be recalled and reprocessed because the sterilizer wasn’t functioning properly from the start.