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Sterilization: Instrumental in patient safety

Are your instruments sterile before you use them on patients? This article outlines the sterilization process, different indicators, regulations on testing office sterilizers, sterilization failure and what to do, and monitoring the overall process.

By Chris H. Miller, PhD

Have these situations ever happened to you?

• You're at chairside ready to begin with your first patient of the day and the instruments are arranged on the bracket table. The patient challenges, "How do you know those instruments are safe to use?"

• That afternoon your assistant has to leave for a family emergency, so you have to obtain the instrument set-ups. When you enter the sterilizing room you wonder which packaged instruments are ready to use and which have not yet been processed through the sterilizer.

• Your state department of health or the state dental board contacts you asking for your sterilizer spore-testing records for the last six months.

Although there is no way to absolutely guarantee the sterility of every instrument you process, there are methods to monitor the effectiveness of the sterilization process and to identify instrument packages that have been processed through the sterilizer. These sterilization monitoring procedures help ensure patient safety. Recommended by the Centers for Disease Control and Prevention (CDC) and the American Dental Association (ADA), they are mandated in some states.

Types of sterilization monitoring

Sterilization is a process designed to kill all microbes. Because we can't actually test whether all microbes are killed during the sterilization process, the next best thing is to determine whether the process kills the most resistant microbe. If so, we may assume all others have been killed as well.

Bacterial spores have been selected as the most resistant type of microbe, and provide the foundation for the process of sterilization. Although the Creutzfeldt-Jakob Disease (CJD) prion protein apparently is more resistant to inactivation than bacterial spores, the bacterial spores traditionally have been earmarked as the hallmark of resistance for three reasons:

• Bacterial spores are more resistant than all other disease-producing microbes;

• The bacterial spores used are considered non-pathogenic;

• Bacterial spores are more easily handled than CJD prion protein.

Thus, demonstrating the death of bacterial spores provides the main guarantee of sterilization, because it assesses the process directly using live resistant microorganisms. Using bacterial spores to monitor the sterilization process is referred to as biologic monitoring (or spore-testing), and the bacterial spores used for monitoring the sterilization process are referred to as biologic indicators (BIs).

The sterilization process also can be monitored by using chemical indicators (CIs) that change color or form when exposed to specific high temperatures or to the sterilizing conditions within a sterilizer. This is referred to as chemical monitoring (or process monitoring).

Mechanical monitoring involves observing and recording the physical aspects (e.g., temperature, pressure, or time) of the cycle when the sterilizer is being operated.

Regulations and recommendations

Several state dental boards or state departments of health require periodic spore-testing of dental office sterilizers. These agencies include those in California, Florida, Indiana, Nebraska, Ohio, Oregon, and Washington. Check with your state dental association to see whether your state has a sterilization monitoring requirement. In addition, the CDC recommends spore-testing each dental office sterilizer at least weekly using a matching lot control BI.1 Likewise, the ADA also recommends weekly spore-testing of all dental office sterilizers.2 The CDC also recommends performing a mechanical monitoring of every sterilizer load (recording the time, temperature, and pressure) and placing a chemical indicator on the inside of every package in the load.1(p36) If this internal chemical indicator is not readily visible from outside the package, then an external indicator should be placed on the package exterior.

Failure to sterilize

Heat sterilization failure results when direct contact between the sterilizing agent (e.g., steam, dry heat, or hot chemical vapors) and all surfaces of the items being processed does not occur for the appropriate length of time. Some common causes of sterilization failures are listed in the sidebar, "Reasons behind sterilization failure," below left. For example, improper instrument cleaning may leave debris on the instruments that can insulate underlying microbes from the heat. This inadequate cleaning cannot be detected by sterilization monitoring; therefore, cleaning must be performed very thoroughly. Sterilization monitoring usually can detect improper packaging, loading, timing, and temperature.

Biologic monitoring

Types of BIs: BIs contain the bacterial spores used for monitoring. The spores used are Geobacillus stearothermophilus (for testing steam or chemical vapor sterilization) or Bacillus subtilis (for testing dry-heat or ethylene oxide gas sterilization). Currently, no BIs are available for routine testing of liquid chemical sterilants or disinfectants during use in the office.

Spore strips. BIs are packaged in different varieties. Spore strips are one-inch paper strips that contain one or both types of monitoring spores (Figure 1, i "Reasons behind sterilization failure" in related links below). The spore strip is enclosed in a protective glassine envelope. After processing through the sterilizer, the strip is aseptically removed from the envelope and placed in a tube of appropriate Tryptic-soy broth medium that in turn is incubated for two to seven days at 55°C/131°F (for G. stearothermophilus) or at 37°C/98.6°F (for B. subtilis). If live bacterial spores still are present, they will grow and produce cloudiness and/or change the color of the growth medium, indicating sterilization failure. Spore strip BIs can be used to monitor all forms of heat sterilization.

Self-contained vial. Another type of BI is a self-contained vial. This comprises both a spore strip or disk and an ampule filled with growth medium, contained in a plastic vial with a vented cap to permit entrance of the sterilizing agent into the vial (Figure 1, page 74). After processing through the sterilizer, either the vial is squeezed or the cap is pushed down to break the internal ampule, which mixes the growth medium with the spores. The vial is then incubated at 55°C/131°F, and if live bacterial spores still are present, they will grow and change the color of the growth medium, indicating sterilization failure.

Using a BI: Biologic monitoring of dental office sterilizers involves following the sterilizer manufacturer's directions for spore-testing. If these are not available, follow the procedures given in the sidebar, "Biologic monitoring," in related links below. Place a BI inside a pack, bag, or cassette, and then place that package in the center of the load to be sterilized. Operate a normal cycle. Retrieve the BI and analyze it for live spores. Be sure to include a control BI—one that is not processed through the sterilizer but otherwise is handled the same way as the test BI. The control should be analyzed along with the test BI that is processed through the sterilizer. The control BI also should yield growth of the spores, thereby confirming that any live bacterial spores present could grow and would be detected.

Analyzing a BI: BIs can be analyzed in the office with the purchase of an incubator and the appropriate self-contained BIs, or by using a spore strips and broth culture system. Alternatively, the office can subscribe to a sterilization monitoring service that is available commercially and at some dental schools. These services will send you the proper BIs, instructions for use, a return envelope for mailing results back to the lab, and a report of the results. The service also will call your office if a sterilization failure is detected, and should help you identify the problem causing the failure. Many services can confirm a sterilization failure by performing certain microbiological procedures regarded as too difficult to perform in the typical dental office (such as a gram stain and microscopic observation of the bacterial cells grown from a test BI). Some monitoring services also provide a newsletter and serve as a source for general information about sterilization methods or other aspects of infection control.

Chemical monitoring

CI types and uses: One type of chemical monitoring involves the use of CIs that change color or physical form when exposed to certain temperatures. Examples would be autoclave tape and special markings on sterilization pouches and bags (Figure 2, found in "Biologic monitoring in related links below). This type of indicator is commonly used as an external indicator on the outside of every pack, pouch, or cassette to indicate that the item has at least been processed through a heat sterilizer. This distinguishes the items that have been heat-processed from those that have not; otherwise, the two types may appear identical. In turn, this helps prevent the accidental clinical use of unprocessed items.

However, note that these indicators do not show whether sterilization has been achieved or even that a complete sterilization cycle has occurred. A sterilizer could heat up to the proper temperature, thereby changing the chemical indicator, and then immediately malfunction. Although this would prevent sterilization, the indicator already would have changed color. These indicators, such as autoclave tape and special markings on pouches and bags, demonstrate only that an item has been processed through a sterilizer (i.e., the item has been exposed to a certain temperature for some unknown length of time), not that sterilization has been successful. Thus, these CIs are commonly called process indicators.

A second type of CI, an integrated indicator, changes color or form more slowly, and responds to a combination of time and temperature or to a process involving time, temperature, and the presence of steam. Integrated indicators commonly are used on the inside of every pack, pouch, or cassette to ensure that the instruments inside the packaging material all have been exposed to sterilizing conditions. If these CIs placed inside paper/plastic peel pouches are visible from the outside, there is no need to use an external CI.

Analysis of CIs: The proper use of CIs is described in the sidebar, "Chemical monitoring," at left. One helpful aspect of chemical monitoring is that the results are available immediately, either as soon as the package is removed from the sterilizer or right after the package is opened (Figure 3, in Chemical monitoring in related links below). This is unlike biologic monitoring, which requires two to seven days' incubation before results are known.

Mechanical monitoring

Be watchful and wary during sterilizer operation. Observe the machine's dials and gauges and listen as it functions to make sure it sounds normal. For example, a steam sterilizer that makes a hissing noise or has difficulty getting up to pressure may need a new door gasket. Although some sterilizers have recording devices that document these physical parameters of time and temperature for each cycle, handwritten records of the date, run number, time, temperature, and pressure will suffice.

Sterilization failure: Now what?

Be prepared for sterilization failure. Investigating such a failure takes time, so a have a back-up sterilizer ready to run while the office analyzes the failed process.

In the case of a positive spore test (i.e., if both the test and the control BIs show growth of spores), or if the CIs did not properly change color, take the sterilizer out of service and review your office procedures in packaging, loading, and sterilizer operation. Determine whether operator error could be responsible.

If the sterilizer itself is deemed faulty, be aware that any items processed through the sterilizer since the last spore test was conducted may not be sterile, so review chemical and mechanical monitoring results since the last spore test to help determine when the problem may have happened. Going as far back as possible, retrieve any items from suspect loads, and re-package and re-sterilize these items in a properly working sterilizer. As soon as you can, repeat the spore test in the suspect sterilizer, using proper sterilization and monitoring procedures; be sure to include chemical indicators in the repeat run. If this spore test's results read negative and the CIs change appropriately, you may return the sterilizer to service. However, if the repeat spore test is positive or the CIs do not properly change, and you have confirmed that the packaging, loading, and operating procedures were performed correctly, contact your sterilizer repair service for unit repair or replacement.

Keep in mind that before either a repaired or a new sterilizer is placed into service, the unit should be spore-tested under normal operating procedures and achieve a negative (spore-free) result.

Ensure safety

When it comes to instrument processing and sterilization, use all three methods of monitoring—mechanical, chemical, and biologic—to help ensure patient safety. Mechanically monitor each sterilizer load and chemically monitor each package in each load. Biologically monitor every sterilizer at least once a week. Be sure to keep accurate records on all monitoring. Of the three methods, biologic monitoring is regarded as the most valid for monitoring the sterilization process, for it uses live, highly resistant bacterial spores. Chemical monitoring uses heat-sensitive or heat-time-steam-sensitive chemicals that change color and may help to immediately detect certain sterilizer malfunctions and help to earmark procedural problems. In addition, use of mechanical monitoring by observing the time, temperature and pressure of sterilizer cycles may provide early detection of a mechanical problem. Keep in mind that effective sterilization helps you reach the ultimate goal of proper instrument processing—delivering sterile instruments to chairside.


1. CDC. Draft. Recommended infection control practices for dentistry, 2003:36. Available at: "" Accessed June 17, 2003.

2. ADA Councils on Scientific Affairs and on Dental Practice: Infection control recommendations for the dental office and the dental laboratory. J Amer Dent Assoc 1996;127:972.

3. Miller CH, Palenik CJ. Instrument processing. (Table 11-2) Some causes of sterilization failure. From: Infection Control and Management of Hazardous Materials for the Dental Team, 2nd Ed. St. Louis: Mosby Co.; 1998:157.

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