Exploring sterilizer performance through external biological indicator testing: a retrospective study

Abstract

Background

Quality assurance policies mitigate the risk of nosocomial infections from dental office instrument sterilization by assessing sterilizer performance through biological indicator (BIs) testing. This study aimed to evaluate the prevalence of failed sterilization cycles and their causes of failure for a period of eight years through database analysis of a quality assurance laboratory in the province of Saskatchewan, Canada.

Methods

A database of BIs (n = 198,771) performed by an independent quality assurance laboratory from 2015 to 2022 was analyzed. Dental offices (n = 362) inserted Sporview^® Biological Sterility Indicators strips in full sterilizer loads and mailed the processed BI tests to an external quality assurance laboratory for analysis. Samples were assessed based on a colorimetric method checking for changes in color and turbidity. Data was collected and statistical analyses were performed using IBM SPSS 28.0.

Results

The overall failure rate throughout the study was 0.20%, and it decreased gradually from 0.51% (2015) to 0.15% (2022). On average, retests were conducted within 2 days of failure notification. The preferred method of processing was steam sterilization (98%), which had a steadily increasing utilization over time and displays a statistically lower failure rate (0.20%) as opposed to dry heat (1.30%) and chemical vapour (1.40%) sterilizers. Most BI failures were attributable to human error (91.80%), and equipment failures were significantly more likely to occur with dry heat or chemical vapour sterilizers (p < .001).

Conclusion

This study significantly contributes to the understanding of dental sterilizer performance in Canada. The low and decreasing sterilizer failure rates over the study period indicate safe dental office procedures and reduced potential for disease transmissions. The study highlights the effectiveness of steam sterilizers with remarkably low failure rates, while human error remains the primary cause of failures. Further research should focus on identifying factors leading to human error and interventions to minimize sterilization failures in dental settings.

Background

In dental settings, close contact between oral healthcare professionals (OHCPs) and patients can lead to the transmission of infectious diseases [1]. During appointments, instruments and equipment are contaminated with patients’ blood and saliva and can be a source of infection through indirect transmission [2]. Blood-borne pathogens of primary concern include hepatitis B virus, hepatitis C virus, and human immunodeficiency virus [3]. Patient-to-OHCP and patient-to-patient transmissions have been documented [4, 5], emphasizing the importance of infection prevention and control (IPAC) guidelines to mitigate the transmission of infectious agents through contaminated dental instruments and equipment [5, 6].

The sterilization process eliminates all living organisms from dental instruments, including spores, thereby reducing the risk of infection and ensuring patient safety [7]. Steam and dry heat sterilization are the two most common methods used in dentistry. Less common techniques also exist, such as chemical vapour pressure sterilization [7]. Sterilizer monitoring is a quality control measure that ensures instrument loads processed in sterilizers are sterile. Biological indicators (BIs), also known as spore tests, are the ideal monitoring method [8] because they directly evaluate the effectiveness and lethality of the sterilization process by challenging cycles with strips containing highly resistant bacterial spores [8]. Canadian regulations mandate that after a failed BI, the sterilizer must be immediately removed from service until it passes a repeat full-load BI or is inspected, repaired and passes three consecutive empty-chamber BIs [9–18].

The Canadian Dental Association mandates the inclusion of a BI on each sterilizer in use at least daily [19]. In Canada, BI monitoring frequency regulations vary by province; for example, Saskatchewan conforms to daily in-house BIs and weekly BIs processed by an external laboratory for each sterilizer [9]. Ontario and Alberta mandate daily BI tests for all sterilizers, without specifying in-house or external testing [10, 16]. European countries such as Scotland and England recommend periodic equipment and chemical assessments such as steam penetration tests and air leakage tests, but have no mandates on using BIs [20, 21]. In many developing countries, quality assurance requirements for sterilizers continue to be poorly regulated [22, 23].

Sterilizer monitoring practices and regulations regarding the use of BIs vary considerably by region, and there is little data on testing frequencies and BI failure rates in regional studies. The aim of this study is to evaluate the rate of sterilization failures, factors associated with failure, and reasons for failure in a convenience sample of dental offices in Saskatchewan over the past eight years.

Methods

Sterilizer testing

A database of BIs processed between January 2015 and December 2022 was accessed from the Sterilizer and Waterline Monitoring Services (SWMS) quality assurance laboratory, located at the College of Dentistry, University of Saskatchewan. First, dental office staff inserted a BI test from SporView^® Biological Sterility Indicators (Crosstex, International inc., Hauppage, NY, USA), containing G. stearothermophilus and B. atrophaeus, in the most challenging area to sterilize along with the rest of the load. Then, processed BI tests were mailed to the SWMS laboratory via courier then and incubated with Tryptic Soy Broth with Bromocresol Purple (MesaLabs, Lakewood, CO, USA) at 55–60 °C for 24 h for steam sterilizer, 55–60 °C for 3 days for chemiclaves, and 37 °C for 7 days for dry heat [24]. All BI tests were incubated along with one unprocessed BI which served as a positive control to verify incubator functioning and spore viability. A passing sterilization cycle was indicated by a BI test with no signs of turbidity and color stability whereas a failed sterilization cycle was evidenced by a BI test with turbidity and/or a color change from purple to yellow. For results to be considered valid, the corresponding positive control must have exhibited a color change and/or demonstrated turbidity. Test results were emailed to clients. For failed tests, dental offices were contacted by phone and were requested to perform a re-test immediately. Simultaneously, the local regulatory body was notified of the failed test via email. If a sterilizer failed a second test, immediate service to the sterilizer was mandated.

Determination of the cause of sterilization failure

To differentiate between sterilization failure due to human error and equipment failure, the following criteria were established: if two or more failures occurred on the same sterilizer within a four-week period, they were categorized as equipment failures. This was based on the understanding that equipment failures would lead to multiple BI failures in a brief timeframe, while human error-related failures would be more sporadic. A four-week timeframe was chosen, aligning with weekly test strip evaluation at the SWMS Lab, which provided sufficient time to detect consecutive failures and minimize unrelated events. Other failed tests were attributed to human error.

Statistical analysis

Data analysis was conducted using IBM^® SPSS^® Statistics for Mac version 28.0 (IBM Corp., Armonk, N.Y., USA). Descriptive statistics were computed to determine the overall frequencies of BI failures during the study period. A Chi-Square test of independence was conducted to compare failure rates across different sterilizer types (steam, dry heat, and chemical vapour). To identify specific differences, post hoc pairwise Z-tests with Bonferroni corrections were applied to control for the increased Type I error risk due to multiple comparisons. In cases where minimum cell count assumptions for the Chi-Square test were not met, pairwise Fisher’s exact tests with Bonferroni corrections were used to analyze the percentage of failures attributed to equipment issues across sterilizer types. A p-value less than 0.05 indicated statistical significance.

Results

This study analyzed a total of 198,771 BIs conducted between January 2015 and December 2022, involving sterilizers from 362 dental offices across Saskatchewan. An average of 24,846 tests were processed each year throughout the study period (Fig. 1A). The number of participating dental offices per year was lowest in 2016 (218) and highest in 2022 (297) (Fig. 1B). Participating dental offices were dispersed throughout 94 locations in Saskatchewan, with the majority being urban centres [25].

Fig. 1.

(A) Count of BI tests performed at the Sterilizer and Waterline Monitoring Services Lab from January 2015 through December 2022. A total of 198,771 tests were analyzed during this period. (B) Count of dental offices that submitted BI tests to the Sterilizer and Waterline Monitoring Service Lab from January 2015 through December 2022. A total of 362 dental offices were included in this study

The total number of failed BIs was 486, with an overall failure rate of 0.2% throughout the study period. Failure rates showed a significant decrease over time, from 0.51% in 2015 to 0.15% in 2022 (Fig. 2). The mean time between offices being notified of a failed test and their submission of a sample for retesting was two days, 95% CI [1.38, 2.34]. The mean turnaround time between when the BI was conducted and the date the office was notified of the results was 7 days, 95% CI [6.95, 6.98]. Most tests (98%) were performed on steam sterilizers, with dry heat and chemical vapour sterilizers making up only 1% and 0.6% of the total tests, respectively. Throughout the study, the annual test counts for steam sterilizers consistently increased. In contrast, tests conducted on both dry heat and chemical vapour sterilizers showed a declining frequency over the same period (Table 1). Sterilizer failure rates were analyzed for each type of sterilizer (Fig. 3). A Chi-square test of independence was performed to assess the relationship between sterilizer type and failure rate, revealing a significant relationship between the two variables, χ2(2, N = 198,771) = 161.2, p < .001. Post hoc comparisons using pairwise Z-tests with Bonferroni correction revealed that the failure rate for steam sterilizers (0.2%) was significantly lower than that of dry heat sterilizers (1.3%; p < .001) and chemical vapour sterilizers (1.4%; p < .001). No significant difference in failure rates was observed between chemical vapour and dry heat sterilizers (p = .848).

Fig. 2.

Failure rates of BI tests from January 2015 to December 2022, showing a decline in BI failure throughout the study period, with an overall failure rate of 0.2%

Table 1.

Number and percentage of BI tests conducted on each type of sterilizer from 2015 to 2022. The percentage of tests conducted on each type of sterilizer are shown for each year, relative to the total number of tests conducted that year

Year	Steam			Dry heat			Chemical vapour
	N	%		N	%		N	%
2015	19,745	96.6		365	1.8		341	1.7
2016	20,187	96.7		403	1.9		285	1.4
2017	20,680	97.2		391	1.8		206	1.0
2018	21,103	97.4		428	2.0		129	0.6
2019	26,340	98.5		331	1.2		68	0.3
2020	25,782	99.7		44	0.2		43	0.2
2021	30,398	99.7		50	0.2		49	0.2
2022	31,334	99.8		43	0.1		26	0.1

Fig. 3.

Percentage of failed BI tests across different types of sterilizers. Among the 195,569 tests performed on steam sterilizers, 443 (0.2%) resulted in failure. For the 1147 tests conducted on chemical vapour sterilizers 16 (1.4%) were failures. In the case of dry heat sterilizers, 27 out of 2055 (1.3%) tests were failures

Of the 486 total failures, 446 (91.8%) were due to human error, while 40 (8.2%) were attributed to equipment failure (Fig. 4). Equipment failure rates varied by sterilizer type, with chemical vapour sterilizers having the highest rate (43.8%), followed by dry heat (14.3%) and steam sterilizers (6.5%). Pairwise Fisher’s exact tests with Bonferroni correction indicated a significantly higher rate of equipment failures in chemical vapour compared to steam sterilizers (p < .001). Although the comparison between dry heat and chemical vapour sterilizers was initially significant (p = .039, α = 0.05), it was no longer significant after Bonferroni correction. No significant difference in equipment failure rates was found between steam and dry heat sterilizers (p = .254).

Fig. 4.

Cause of BI test failure from 2015 to 2022. From the 486 total failed tests, 446 (91.8%) can be attributed to human error, while 40 (8.2%) can be attributed to equipment failure of the sterilizer

Discussion

This pioneer study represents the first dental sterilizer performance assessment in Saskatchewan, Canada. The overall failure rate of 0.2% observed in our study is considerably lower than previously reported BI failure rates for sterilizers in countries such as Mexico (7.4% [26] and 26% [27]), Brazil (7.4%) [28] and Lebanon (7.5%) [23]. Recent data on sterilizer failure rates in developed countries is limited. In 1998, a study in the United Kingdom revealed a 2.0% BI failure rate [29]. Another study in Canada from 1992 indicated a BI failure rate of 4.4% [30]. The reduction in sterilizer failure rates in Saskatchewan over the past eight years, from 0.51% in 2015 to 0.15% in 2022, may be linked to improved training, stricter quality control, better adherence to protocols among OHCPs, changes in types of sterilizers used by dental offices and updated standards from provincial IPAC committees based on the latest scientific knowledge, collectively resulting in safer dental healthcare practices.

Steam sterilizers consistently exhibited lower failure rates compared to dry heat and chemical vapour methods, aligning with previous studies [23, 30, 31]. This could be attributed to their mechanism of action—steam sterilization excels in penetrating loads due to the effective permeation of moist heat, reaching intricate crevices more effectively than other methods [32]. In contrast, dry heat sterilizers rely on conduction to transfer heat, leading to prolonged cycle times, reduced efficiency, and potential utilization issues [33]. Common causes of higher failure rates among dry heat sterilizers include operator error and inadequate cycle warm-up periods [34]. This is corroborated by the high failure rate due to human error (86%) we observed among BIs conducted on dry heat sterilizers. Our study underscores the superior effectiveness of steam sterilizers, evidenced by the lower failure rates observed, thereby corroborating existing findings in the literature.

Human error was as the primary cause of failure, contributing to 91.8% of all failures. While our study did not specifically assess the causes of human error, the prevailing reasons for sterilizer failure in existing literature include overloading the sterilizer, improper or excessive packaging, and insufficient exposure time [27, 35]. Overloading the sterilizer compromises its effectiveness, as steam or vapour can only penetrate the paper side of packaging. When overloaded, paper sides may press together, reducing the permeable surface area essential for thorough sterilization. Rushing the process shortens exposure time, hindering complete sterilization, while omitting the drying cycle weakens packaging integrity and compromises the sterile seal. Operators should allow the drying cycle to complete fully and position packages paper-side up to facilitate moisture escape, as residual moisture can dilute the concentration of chemical vapour [36]. Addressing common operator errors can improve sterilization outcomes.

A key finding was the significantly higher equipment failure rate in chemical vapour sterilizers (43.8%) compared to dry heat (14.3%) and steam sterilizers (6.5%), highlighting the critical need for more reliable sterilization methods that adhere to current IPAC standards. Chemical vapour sterilizers operate at lower temperatures for shorter durations and lack a drying phase, in contrast to steam sterilizers, which use higher temperatures for extended periods and incorporate drying. Furthermore, chemical vapour sterilizers rely on a chemical solution, making precise solution dispensing essential, as temperature and exposure time alone are insufficient to eliminate spores [37]. Additionally, the limited shelf life of chemical solutions and difficulties in achieving consistent load penetration may contribute to higher failure rates for chemical vapor sterilizers. Even if the chemical vapor loads meet al.l time, pressure, and temperature parameters, increased equipment failures could stem from challenges in distributing the vapor evenly throughout the chamber at lethal concentrations. This issue could be further linked to the need for more frequent maintenance and cleaning compared to steam and dry heat sterilizers [38].

The submission of samples for retesting within an average of two days after a failed test displays a proactive approach by offices toward rectifying issues. The average turnaround time of seven days for receiving test results is consistent with previous findings [39]. Failed BI tests were immediately reported to the regulatory body, and offices could not process instruments in a failed sterilizer until BI tests were cleared.

Our comprehensive analysis of sterilizer performance over an eight-year period, including nearly 198,771 BIs from 362 dental office locations, enhances the study’s generalizability, yet it has limitations. The convenience sample from a single Canadian province limits the external validity, potentially affecting the generalizability to other regions or countries with different sterilizing regulations. The retrospective data collection prevents the study from establishing causal relationships between specific variables and sterilizer failures. The study does not examine reasons for human and equipment error, lacking a comprehensive understanding of the factors contributing to these failures. Despite these limitations, this study offers a valuable contribution to the understanding of sterilizer performance in dental settings and highlights areas for further research and quality improvement.

Future qualitative research into dental practitioners’ behaviors and attitudes toward sterilization practices could offer valuable insights into the factors affecting compliance and the frequency of human errors in these protocols. Qualitative studies can uncover details that quantitative research might miss, such as individual beliefs about sterilization effectiveness and perceived obstacles to compliance. Findings from these studies can guide the development of educational interventions tailored to specific challenges. For instance, if OHCPs report uncertainty regarding certain sterilization techniques or equipment, targeted training sessions can be implemented to enhance their knowledge and confidence. To improve error detection, future quality assurance studies should evaluate mechanical and chemical indicators alongside biological indicators in failed cycles, clarifying whether failures stem from unmet parameters or package penetration issues. Given the dental field’s preference for steam sterilization due to its efficacy, a comparative study on steam sterilization methods, like gravity displacement versus dynamic air removal, would also be valuable.

Conclusion

Our analysis presents a significant contribution to understanding dental sterilizer performance in Saskatchewan, and open new avenues for additional investigation in other Canadian provinces and abroad for quality assurance purposes and policy revisions. Sterilizer failure rates were extremely low and declined over the study period, indicating that potential disease transmissions are unlikely. Our findings highlight the superiority of steam sterilizers, which had an impressively low failure rate. Human error emerged as the primary cause of failure. The study also revealed efficient response times, with dental offices taking approximately two days to submit samples for retesting after a failed test. Overall, this study underscores the importance of adhering to up-to-date sterilization protocols. By doing so, OHCPs can help ensure patient safety and minimize the risk of infectious disease transmission. Further research in this area is warranted, particularly investigating specific factors contributing to human error and potential interventions to reduce sterilization failures in dental settings.

Abbreviations

BI

Biological indicator

IPAC

Infection prevention and control

OHCP

Oral health care provider

SWMS

Sterilizer and Waterline Monitoring Services

Author contributions

BV contributed to data analysis and interpretation; drafted and critically revised the manuscript, JMB contributed to data analysis and interpretation; drafted and critically revised the manuscript. MFS contributed to the conception, design, data acquisition, and interpretation and critically revised the manuscript. All authors gave their final approval and agreed to be accountable for all aspects of their work.

Funding

This research was supported by the University of Saskatchewan fund 45410 (2021). Bahar Vatanparast is a recipient of the College of Dentistry, University of Saskatchewan Summer Program Scholarship, and Juan M. Buitrago is a recipient of the College of Dentistry, University of Saskatchewan B.Sc. Scholarship.

Data availability

The datasets used and/or analyzed during the current study are available from the corresponding author upon reasonable request.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.