Sterilizer Monitoring: Rationale for Using Biological Indicators

Dentistry Today


In late 2003, the Centers for Disease Control and Prevention (CDC) released revised Guidelines for Infection Control in Dental Healthcare Settings.1 These guidelines expand on those published in 1993 by the CDC.2 The report consolidates recommendations for preventing and controlling infectious diseases and for managing personnel health and safety concerns related to infection control in dental settings. One of the first questions asked regarding the new guidelines is, “Are these procedures mandatory or required?” The simple answer is no, because the CDC is not a regulatory agency and therefore has no enforcement authority. The guidelines are just that—a guide to better clinical practices.

A dental office that adheres to the recommendations will be safer for both patients and staff. However, in several states, many of the guidelines do carry the weight of law through adoption of the recommendations by state-level occupational safety and health offices that do have enforcement authority. Federal regulatory agencies that apply to dentistry include the US Food and Drug Administration (FDA), which has regulatory authority over the design and manufacture of dental instruments and materials for their maintenance, including disinfecting solutions used specifically for the instrumentation. Further, the Environmental Protection Agency (EPA) retains regulatory control over disinfectants used throughout the clinical environment. The federal Occupational Safety and Health Administration (OSHA) regulates infection control procedures applicable to all healthcare facilities.

For the majority of dental practices, preparation of patient care items is one of the most essential procedures for controlling transmission of infectious agents to the clinical staff and/or patients. Maintenance of quality control over the processing of reusable instrumentation relies heavily on the attention given to the task by support staff. Sterilization of instruments is common to all practices, but it is important to define the controls in place to ensure that the sterilizer is operating properly and that items placed in it were exposed to sterilizing conditions. According to the guidelines, 3 distinct methods can be used to monitor the dental clinic sterilization procedures3: mechanical monitoring; chemical indicators; and biological indicators. Use of these different monitors greatly reduces the potential variability inherent in staff performance.


Sterilizer Type 1991 % nonsterile 2002 % nonsterile
Autoclave 4.08 1.67
Dry Heat 3.10  1.06 
Chemiclave  11.02  1.90 
Average  6.07  1.54 
Table. Comparison of 1991 and 2002 Nonsterile Samples for all Sterilizers Tested.  



Patient care items used in dental practice have different levels of infection control associated with them, and this is dependent on their use. Spaulding4 described 3 categories for the processing of medical/dental instruments:

(1) Critical Instruments: must be sterilized by heat. These instruments would be used to penetrate soft tissue, contact bone, and have contact with blood or other tissues that are normally sterile. Typically, these include items such as surgical burs, surgical instruments, periodontal scalers/probes, and scalpels. The one exception is the dental handpiece. While they do not usually penetrate tissue, handpieces are continually exposed to aerosols laden with potentially infectious organisms. These organisms can be harbored in small, hard-to-reach spaces on and in the handpiece.

(2) Semicritical Instruments and Devices: should be heat sterilized if possible or, at a minimum, disinfected with an EPA-registered, high-level disinfectant. Semicritical items touch mucous membranes and saliva but have a lower risk of transmitting disease due to the innate host immunity. Typical items include mouth mirrors, amalgam condensers, impression trays, and intraoral film holders. All of these dental instruments are available in a form that can be heat sterilized.

(3) Noncritical Devices: may be cleaned and disinfected with an EPA-registered, hospital-level disinfectant. Noncritical devices contact healthy, intact skin in which the potential for transfer of opportunistic pathogens is negligible. Examples include radiograph head/cones, facebows, and blood pressure cuffs.



There are 2 general approaches to sterilization: chemical and physical/heat. Both are based on the assumption that if the process kills spores (the most resistant life form known), then all other life forms in similar amounts and conditions also will be killed. While most consider the term sterile to be an absolute, in reality it is a probability based on chemical kinetics, and microbial death follows a logarithmic regression5 where a percentage of cells survive a given time of exposure. The cleaning and sterilization cycles that are in general use have a significant level of “overkill” for bacterial spores, resulting in an extremely low percentage or probability of any infectious organism surviving, thus assuring a high degree of staff and patient safety.6

(1) Chemical: includes glutaraldehydes, some peroxides, and peracetic acid. Chemical sterilants are capable of killing bacterial spores based on the sporicidal tests specified by the FDA7; however, a number of variables and conditions must be considered. The first is the time factor. Liquid sterilants often take 10 hours or more for inactivation of a moderate spore suspension. This extended time period is usually not practical for dental practices. The second is penetration of the sterilant. For extended contact time, the instrument must be submerged in the liquid; however, the liquid may not have access to the interior of all items, particularly mated surfaces, eg, the tight-fitting joints of extraction forceps. In addition, instruments “sterilized” by chemical immersion must be rinsed using sterile water and placed in sterile containers. Finally, there are no recognized procedures for biologically monitoring the effectiveness of liquid sterilants. Because of these limitations, the ADA does not recognize any liquid disinfectant as an effective sterilant. Those labeled “liquid sterilant/high-level disinfectant” should be considered only high-level disinfectants when used for dental instruments.8

(2) Physical/Heat: includes autoclaves, dry heat, and unsaturated chemical vapor units (chemiclaves). Autoclaves fill the sterilizing chamber with saturated steam (relative humidity of 100%), which is a very good medium for heat transfer within the chamber. Most autoclaves operate at 121ºC, although 134ºC is used in rapid sterilizers. The saturated steam promotes irreversible protein denaturation, which will cause microbial death. Chemical vapor units rely on a mixture of heated alcohol and formaldehyde to denature microbial proteins. The net effect of dry-heat sterilizers is oxidation of the biological material; however, for this to occur, higher temperatures (160ºC to 170ºC) must be used. Each of these provides advantages depending on the instrument or material to be sterilized; however, the autoclave is the most widely used sterilizer in dentistry.


A major recommendation in the CDC guidelines and supported by the ADA is that dental sterilizers need to be monitored for effectiveness using 3 independent methods.

(1) Mechanical monitoring of the sterilizer should occur for every cycle of operation and includes observation of the sterilizer’s temperature and duration of the cycle. Ideally, these are reported in a printout, but visual verification of the gauges is required at a minimum.

(2) Chemical indicators are heat-sensitive monitors that assess the physical conditions the instruments were exposed to during the cycle. These are often chemicals incorporated into markers on instrument bags or chemicals added to tape (autoclave tape) that change color when processed. Chemical indicators should be used with every batch and they serve 2 purposes: verification that the item was exposed to elevated temperatures and potentially sterilizing conditions, and the very practical side of visually being able to distinguish items that have been processed through the sterilizer versus those that are waiting for sterilization. While valuable as an indicator, chemicals do not verify that sterilization conditions were achieved.

(3) Biological indicators are based on the inactivation of resistant bacterial spores. Biological indicators actually verify the machine’s ability to sterilize, as the spores are killed, leaving no doubt about the capabilities of the sterilizer. The standards for biological monitors were developed by the Association for the Advancement of Medical Instrumentation (AAMI), a voluntary group of healthcare scientists whose goal is to increase the understanding and beneficial use of medical instrumentation.9 Biological indicators should be used on a weekly basis, and neither the CDC, ADA, nor the AAMI recognize monthly sampling as adequate used in biological indicators. They were chosen for their resistance to the different types of killing mechanisms and unique growth characteristics.9 For moist-heat units including steam-based autoclaves and unsaturated chemical vapor units, the spore of choice is from Geobacillus (formerly Bacillus) stearothermophilus. The indicator spore of choice for dry heat sterilizers is from Bacillus atropaeus (formerly B. subtilis var niger). Generally, the spores are supplied either on a filter paper strip wrapped in a glassine envelope or as a liquid in a sealed container. The monitor is used by simply assuming that it is an instrument and packaged accordingly. The preferred location of the monitoring sample package is the least effective area of the sterilizer, often near the front of the unit or in the middle of the load. A biological indicator should never be run by itself because a major concern in the performance of a sterilizer is the mass and configuration of the load within the chamber.9



The biological indicators must be processed following “sterilization.” If the sterilizer is functioning properly, then there will be no viable spores in the sample and consequently no growth when placed in growth-promoting conditions. For autoclave and chemiclave samples, the strips are placed into a nutrient medium and incubated, or the sealed containers are placed directly into an incubator according to the instructions supplied by the manufacturer of the biological indicator. The strips must be handled aseptically; however, environmentally contaminated samples are exceedingly rare because the Geobacillus spores germinate and grow at temperatures of 55ºC (131ºF) or higher, well above the optimal temperature of 37ºC (98ºF) required by most bacteria. Culture and cellular characteristics are used to verify that the growth from the monitoring strip is typical for the test spores. The spore strips used for dry heat sterilizers are also placed into a nutrient medium but are incubated at 37ºC. Growth due to contamination does occur more frequently with these cultures, and positive or nonsterile results are further tested by streaking the growth onto nutrient agar medium and verifying that the microbe from the positive tube produced a brown, pigmented colony with rough edges and that the cells were Gram-positive rods. All samples are incubated for a minimum of 7 days, although all controls and most nonsterile samples demonstrate growth following 1 to 2 days of incubation.



The University of Louisville Monitoring Program has been in operation since 1988. There are many anecdotal reports of how the program identified sterilization units that were not working as they should, and in these cases the benefit is obvious. However, data collected over an 11-year period demonstrates an important but a more subtle benefit. The Table shown is a comparison of the nonsterile rate (percentage) for all samples submitted to the program in 1991 and samples processed in 2002. The rate of nonsterile samples is lower for all 3 sterilizer types in 2002.

There are numerous possible explanations for these results, including improved equipment and perhaps that only concerned clinicians continued to participate in the monitoring program. But many of the machines used in 1991 were still in use in 2002, and many of the same dentists were participants. It is likely that weekly sterilizer monitoring is valuable not only as a check on the function of machines as recommended by the CDC, but is equally important as an educational reinforcement of the infection control procedures for the clinical staff.

To support this statement, the University of Louisville Monitoring Program found that in 1991 approximately 40% of the enrolled practices had at least one nonsterile sample during that year. In contrast, in 2002 only 20% of the practices had at least one nonsterile sample. This could be due to better machine maintenance, reduced incidence of human error, or both.

It is very difficult to determine the cause of a failed sterilization cycle when human error is involved. Human error can consist of such actions as machine overloading, stacking packages horizontally rather than vertically, cutting the run short, or simply not putting the spore strip through the sterilizer. Single event or isolated nonsterile samples most likely represent some type of human error. This is based on the information from the various practices, where in nearly all cases of an initial nonsterile report a second sample processed by the clinic staff was sterilized. No repair was done to the machine prior to processing the second sample because the routine weekly monitoring often overlapped the notification by the monitoring laboratory of the nonsterile finding. (Typically a clinic will run the monitoring sample on Monday or Tuesday, place it in the next day’s mail, and the laboratory processes it on Thursday or Friday with results reported by telephone Monday or Tuesday.) Machine malfunctions are usually indicated by consecutive nonsterile results. The relatively rapid reporting of data from sequential samples can occur only with weekly (or even more frequent) sampling. Single or isolated nonsterile samples (presumably due to human error) has remained steady at about 70% of the sterilization failures for all of the years of this study.

The fact that the percentage of practices with at least one nonsterile sample annually has been reduced by half and the finding that the distribution of single verses multiple nonsterile samples has remained relatively constant suggest that the staff is paying closer attention to the sterilization process. Notification by a biological monitoring laboratory of a nonsterile sample is a very dramatic reminder of a process failure, which in turn is likely to translate into a proactive effort to enhance infection control awareness in the dental office.



Proper functioning of clinic sterilizers is vital to all dental practices, and biological indicators are used to verify that sterilization occurred during a given cycle. Weekly monitoring provides sufficient information to characterize nonsterile reports as either human error (which accounts for about 70% of the nonsterile results) or machine failures (accounting for the other 30%). Regard-less of the cause, a nonsterile report can be used effectively to increase the awareness and attention given to infection control procedures by the staff.



1. Kohn WG, Collins AS, Cleveland JL, et al; Centers for Disease Control and Prevention. Guidelines for infection control in dental health-care settings—2003. MMWR Recomm Rep. 2003;52(RR-17):1-61.

2. Recommended infection-control practices for dentistry, 1993. Centers for Disease Control and Prevention. MMWR Recomm Rep. 1993;42(RR-8):1-12.

3. Kohn WG, Collins AS, Cleveland JL, et al; Centers for Disease Control and Prevention. Guidelines for infection control in dental health-care settings—2003. MMWR Recomm Rep. 2003;52(RR-17):24-25.

4. Spaulding EH. Chemical disinfection and antisepsis in the hospital. J Hosp Res. 1972;9:5-31.

5. Weavers LK, Wickramanayake GB. Kinetics of the inactivation of microorganisms. In: Block SS, ed. Disinfection, Sterilization, and Preservation. 5th ed. Philadelphia, Pa: Lippincott Williams and Wilkins; 2001:31-56.

6. Favero MS. Patient infections: the relevance of sterility assurance levels. In: Sterilization of Medical Products: Volume VI. Morin Heights, Quebec, Can: Polyscience Publications; 1993:156-163.

7. Association of Official Analytical Chemists. Official Method 966.04. Sporicidal Activity of Disinfectants. In: Cunniff P, ed. Official Methods of Analysis, 6.3.05. Arlington, Va: AOAC; 1995:12-13.

8. Infection control recommendations for the dental office and dental laboratory. ADA Council on Scientific Affairs and ADA Council on Dental Practice. J Am Dent Assoc. 1996;127:672-680.

9. Association for the Advancement of Medical Instrumentation. AAMI Standards and Recommended Practices: Sterilization, Part 1. Sterilization in Health Care Facilities. Arlington, Va: AMMI; 2001:89-168.


 Dr. Staat is a professor of microbiology and a director of the Sterilizer Monitoring Program at the University of Louisville’s School of Dentistry. He can be reached at (502) 852-1296.

Disclosure: The Sterilizer Monitoring Program is a service of the University of Louisville, and while an employee of the university, Dr. Staat receives no direct financial benefit through this program.