Detection of Superheated Steam during Sterilization Using Biological Indicators
Abstract
Saturated steam (SS) is used for sterilizing many medical devices. Exposure to SS for appropriate temperature/time combinations creates a microbicidal environment that renders product sterile. Superheated steam (SHS) has been heated beyond its saturation point and is less microbicidal, compromising process efficacy. Sterilization monitoring systems should detect SHS. One method is to use biological indicators (BIs; e.g., rapid-readout self-contained BIs [RRSCBIs]). The purpose of this study was to determine if RRSCBIs can detect SHS. Pressurizing the boiler to 4,700 mB, manifold to 4,000 mB, and chamber jacket to 3,600 mB and heating the viewing window to 150°C in a 10-L BI evaluation resistometer vessel allowed approximately 12°C and 4.5°C of superheat in a nominal 121.75 ± 0.25°C and 132.5 ± 0.25°C cycle, respectively, to be reproducibly achieved. Replicate tests using multiple RRSCBIs from different batches were exposed vertically (cap up), inverted (cap down), and horizontally to SS and SHS. RRSCBI viability was determined using a fluorescent readout method. RRSCBIs exposed to SS at 121.75 ± 0.25°C for 7 or 14 minutes were negative. A total of 135 type A RRSCBIs were exposed to SHS (12°C) at 121.75 ± 0.25°C for 14 minutes. Zero of 45 RRSCBIs mounted vertically showed a positive fluorescent result, 26 of 45 mounted inverted were positive, and 45 of 45 mounted horizontally were positive. A total of 135 type B RRSCBIs were exposed to SHS (12°C) at 121.75 ± 0.25°C for 7 minutes. Twenty-four of 45 mounted vertically were positive, 41 of 45 mounted inverted were positive, and 45 of 45 mounted horizontally were positive. RRSCBIs detected SHS, but this was orientation dependent. Further work is required to establish the application of these findings in healthcare facility settings.
Sterile medical devices must be sterilized by a validated sterilization process1,2 to have an acceptable sterility assurance level (SAL). The most common sterilization processes utilized in healthcare facilities for sterilizing reusable surgical instruments and other healthcare items use saturated steam (SS). Such SS sterilization cycles can operate at 121, 132, or 134°C for exposure times of 15, 4, and 3 minutes, respectively.3,4
The international standard ISO 17665-1:2006 describes the validation and routine control of sterilization processes that use moist heat (steam).4 Similarly, sterilizer standards include EN 285:2015+A1:20215 and EN 13060:2014+A1:2018.6 In SS sterilization processes, the steam entering the chamber is the sterilizing agent. SS sterilization cycles are divided into three phases: an air removal (conditioning) stage, a sterilization (exposure) stage, and a drying stage. The inactivation of contaminating microorganisms mostly occurs during the exposure stage, when the latent heat carried in SS rapidly heats the load and creates moist heat as it condenses on cold surfaces. When held at the correct temperature for a prescribed period (e.g., 132°C for 4 min), a sterile medical device results.
To have an efficient and efficacious process, the steam entering the chamber must be of suitable quality. International standards specify that the steam used should be pure (containing few inorganic or organic chemical contaminants) and be low in noncondensable gases and be saturated (containing neither high levels of liquid water nor be excessively superheated).4–6 To determine if the steam quality used during the sterilization cycle is satisfactory, demonstrating the absence of noncondensable gases and the presence of SS is necessary.5,6
Superheated Steam
SS is created in a steam generator by boiling water, the temperature of which is dependent on the pressure according to steam tables.7 Normally, the steam will have some liquid water present as fine droplets. The quantity of vapor present in the steam, compared with the liquid water, is called the dryness value and is expressed as a decimal fraction denoting the mass of the vapor fraction in the total mass of SS. Standards limit the dryness value to not less than 0.95.5 If any liquid water present evaporates or is removed using engineering methods, then the steam will be considered dry (no liquid water) and saturated (with energy). If dry SS is heated further while the pressure remains constant, the steam will become superheated.
Superheated steam (SHS) is less efficient at inactivating microorganisms. The presence of SHS in an SS sterilization process could cause process failures, resulting in nonsterile loads; therefore, the maximum level of SHS allowed is specified in standards.5,6 Thus, as an example, EN 285 allows not more than a 5°C overheat compared with the temperature measured at the reference measurement point (usually the chamber drain) during the first 60 seconds of the plateau period, which then must decrease to less than 2°C when determined under specific test conditions. Similarly, the amount of superheat measured when a sample of steam supplied to the sterilizer is vented to atmospheric pressure should not exceed 25°C.
In a well-designed steam sterilizer, the occurrence of SHS is uncommon. However, inadvertently causing SHS conditions is possible. If SS is supplied to the sterilizer at very high pressure (e.g., 10 bar), then as the steam enters the chamber, the high energy carried in the steam could cause superheat. Similarly, SS sterilizers usually have a jacket surrounding the chamber that, if overheated above the sterilization process temperature, will act as a heat source that can cause SHS in the chamber. Also, certain load items (e.g., cotton surgical towels), packaging materials (e.g., paper bags), or accessories (e.g., absorbent pads) containing natural fibers can become dehydrated during storage. When introduced into an SS sterilization process, such items will rehydrate, thereby releasing energy that can cause localized SHS and, as a result, reduced sterilization efficiency.
Monitoring Sterilization Processes
All sterilization processes should be routinely monitored using methods that provide assurance of sterility. Monitoring methods take one of three forms:
1.
Those that measure the physical characteristics of the process (e.g., time, temperature, pressure).
2.
Those that use a preparation of bacterial spores, which present a known resistance to, but are inactivated by, an efficacious process (biological indicators [BIs]).8
3.
Those that consist of chemical reagents, printed on a substrate, that respond to defined characteristics of the sterilization process, giving a visible change after exposure (chemical indicators [CIs]).9
Ideally, sterilization monitoring methods should detect the presence of SHS if occurring during the sterilization cycle. If SHS is present in the sterilizer, then a comparison between the measured temperature and that calculated from the measured pressure using steam table values7 can indicate the presence of SHS (i.e., if the measured temperature is higher). However, not all sterilizer monitoring systems provide this comparison. Some sterilizers are fitted with devices that measure steam quality in the chamber as the sterilization cycle proceeds. Such measurements will detect the bulk properties of the steam in the chamber but will not detect localized SHS conditions inside packs of medical devices. To be able to determine if the steam present within the load is adequate to ensure effective sterilization, the use of BIs and/or CIs usually is necessary.
Previous studies have shown that BIs containing bacterial spores can detect the presence of SHS. Savage10 examined the effect of SHS (range 100–135°C) and superheat levels (5 to >20°C) on the inactivation of spores in soil. The author found that SHS would reduce the sporicidal efficacy of steam; however, this was a temperature-dependent effect, with SHS having a greater impact at lower temperatures. Shull and Ernst11 examined the effect of SHS arising in fabric loads sterilized in high vacuum sterilizers. The rate of microbial kill was reduced in the presence of SHS (but this was a temperature dependent effect). In a more recent study, Spicher et al.12 demonstrated that exothermic rehydration of cellulose-based spore carriers would create localized SHS (~5°C), which increased the resistance of the BI. The undesirable effects of SHS (i.e., reducing microbial inactivation and sterility attainment) have been described previously.13
Purpose
The majority of previous studies examined the response of BIs impregnated onto carriers with viability evaluated using the ability of spores to germinate and grow in nutrient media. In modern sterile processing departments, self-contained BIs (often rapid-readout self-contained BIs [RRSCBIs]), which are widely available from various manufacturers, are used to monitor moist heat sterilization processes (i.e., rather than monitoring via spore strips).
The purpose of this work was to evaluate the response of two RRSCBIs containing spores of Geobacillus stearothermophilus toward SHS. In this study, the viability of spores within the RRSCBIs was determined, according to the manufacturer’s instructions for use (IFUs), by detecting the development of a fluorescent compound arising from metabolism of a substrate within the growth medium by viable spores.15 The presence or absence of the fluorescent compound was detected by specifically designed autoreaders (Table 2) which showed a positive (fluorescence detected) or negative (no fluorescence detected) result on the display.
The study was carried out in two stages. In the first stage, a method was developed for reproducibly creating sustainable SHS in a conventional BI evaluation resistometer (BIER) vessel.14 In the second stage, the two types of RRSCBIs were exposed to SHS to establish the response. This second stage included an evaluation of the impact of RRSCBI orientation when mounted vertically (cap up), inverted (cap down), and horizontally on the response of the indicators.
Materials and Methods
Steam Exposure Apparatus (BIER Vessel)
A horizontally mounted, steam-jacketed, 10-L cylindrical (~20 cm diameter and 29.5 cm length) BIER vessel and associated steam supply system (Lautenschläger, Köln, Germany) meeting the requirements of EN ISO 18472:201814 was used. The BIER vessel included a heated viewing port in the chamber door (15 cm diameter). SS was generated in a stainless steel boiler from degassed (heated to >70°C) purified water (5 μS), then transferred to an insulated steam manifold (header). SS then was transferred from the manifold to the BIER vessel chamber and jacket. The pressure within each component of the steam supply system and BIER vessel was separately adjustable and controlled by the control system. Table 1 shows the control parameters used during the study.
BIER Vessel Operating Cycle
The operating cycle of the BIER vessel was standardized and used the following steps:
1.
After loading, the chamber door was closed.
2.
The chamber was evacuated to less than 40 mB, the vacuum valve closed, and the condition held for 105 seconds to allow the jacket to pressurize to the set point, heat up, and equilibrate to the operating temperature.
3.
Steam was injected into the chamber to the set operating pressure within 10 seconds.
4.
The pressure was held at “steady state” for the set exposure period.
5.
The chamber was evacuated to less than 50 mB within 10 seconds.
6.
The chamber was equilibrated to atmospheric pressure within 30 seconds.
7.
The chamber door was opened and unloaded.
Measurement of Process Variables
Temperatures from within the sterilizer chamber and drain were measured using miniature (3 mm diameter) four-wire platinum resistance sensors (JUMO, Fulda, Germany) that were introduced into the chamber via steam-tight glands. The pressure from within the sterilizer chamber was measured using a precision pressure transducer with an accuracy of 0.15%, response time of 3 ms, and range of 0 to 4,000 mBA (JUMO). The pressure sensor was maintained at 55 ± 2.5°C using electrically heated jackets to minimize inaccuracies caused by temperature coefficient effects. The pressure sensor was mounted on a free-draining manifold that was in direct connection with the BIER vessel chamber via a 2.5-cm pipe and isolation valve. The temperature and pressure sensors were calibrated using instruments ultimately traceable to the German metrology standards (Physikalisch-Technische Bundesanstalt, Braunschweig, Germany).
Data Management
The various sensor systems were connected to a multichannel data management system (Delphin Technology, Bergisch Gladbach, Germany). Data were also analysed using standard office spreadsheet software (Excel; Microsoft, Redmond, WA).
During stage 1, various control settings for boiler, steam manifold, and jacket pressure and chamber viewing port temperature were tested to determine the optimum settings to create various levels of SHS in the chamber (Table 1). After the control parameters had been established, a number of repeat cycles were carried out in order to estimate reproducibility. Figures 2 through 4 show the temperature and pressure from a number of replicate operating cycles using the control settings shown in Table 1.
Estimating Level of Superheat in Chamber
During the exposure stage of the operating cycle, the theoretical steam temperature (Tth) in the chamber was calculated in real time by the data management system from steam table values using equation 1:
(equation 1)
where Tth is the theoretical steam temperature (in °C) and Pch is the measured chamber pressure (in MPa).
During the operating cycles, Tth, measured temperature (Tm), and Pch were graphically presented in real time, with the difference between Tm and Tth indicating the degree of superheat present within the chamber.
BI Samples and Autoreaders
Three batches of two types of RRSCBIs were used in the study (type A, Rapid Attest 1292, and type B, Super Rapid Attest 1492V; 3M, St. Paul, MN). Both types of RRSCBIs contained spores of G. stearothermophilus. Table 2 shows the characteristics of each batch of product used. Prior to use, samples were stored in their packaging in laboratory conditions at 20°C to 25°C and 30% to 55% relative humidity (RH), in order to avoid the localized SHS effects described by Spicher et al.12 Viability of the RRSCBIs was determined, according to the manufacturer’s IFU, using autoreaders designed for the purpose (Table 2).
Presentation of RRSCBIs to BIER Vessel and Evaluation
A sample holder was made from a wire mesh test tube rack appropriately modified to allow RRSCBI vials to be firmly mounted in vertical, inverted, or horizontal orientation (Figure 1). Before and between tests, the sample holder was preheated in a warm-up operating cycle or stored within the chamber of the BIER vessel. During tests, 15 vials from one batch of RRSCBIs were divided into three groups, then mounted on the sample holder in vertical, inverted, or horizontal orientations. The sample holder then was introduced into the center of the chamber, the temperature sensors arranged close to the RRSCBI samples, and the operating cycle initiated (see above). After processing, the RRSCBIs were removed from the sample holder, allowed to cool for five minutes, then activated and placed in the autoreader according to the IFU. If a positive result was indicated, the incubation time was recorded.
Determination of RRSCBI Kill Times in SS
Samples of five RRSCBIs were mounted on the sample holder in the vertical orientation and exposed to SS at 121.75 ± 0.25°C or 132.5 ± 0.25°C for increasing time intervals. Samples were incubated in the respective autoreader and the result (positive or negative) recorded. The exposure time when no positive results were observed in any of the samples was considered the kill time. A series of replicate tests (15 RRSCBIs × three batches × three replicates) was carried out to confirm kill times. The respective kill time then was used as the exposure time when RRSCBIs were exposed to SHS.
Exposure of RRSCBIs to SHS
Tests were carried out in two stages. In stage 1, the response of both types of RRSCBIs to 12°C of SHS with a nominal exposure temperature of 121.75 ± 0.25°C was determined. In stage 2, the response of type 2 RRSCBIs to 4.5°C of SHS with a nominal exposure temperature of 132.5 ± 0.25°C was determined. In both stages, the BIER vessel control settings were adjusted to create the level of SHS required (Table 1). Warm-up runs were carried out to allow all components, including the sample holder inside the chamber, to reach operating temperature. In each replicate test, five vials from one batch of RRSCBIs were mounted vertically, horizontally, or inverted on the sample holder (Figure 1).
Exposure of RRSCBIs to Dry Heat
As a control experiment, samples of RRSCBIs were exposed at 121 ± 2°C in a dry heat oven. The time for an RRSCBI to reach 121°C was determined by introducing a 1-mm diameter thermocouple into the RRSCBI and placing it in close proximity to the location of the spore carrier (base of the vial). The heat-up time was added to the kill time for each RRSCBI. Five RRSCBIs from each batch of type 1 and 2 were mounted (vertically) on the sample holder, placed in the preheated dry heat oven, and exposed for the heat-up plus exposure time. After heating, the RRSCBIs were allowed to cool for 5 minutes, activated, and incubated (see above).
Results
Reproducibility of Test Conditions Using SS and SHS
Figures 2 through 4 show the data recordings from the exposure phase (12.5 or 14 min SHS) of a series of SS and SHS test cycles. Figure 2 shows the different levels of SHS attained when the BIER vessel was operated using the settings shown in Table 1 during the initial experiments carried out to develop the method. Figure 3 shows the reproducibility of the SS cycles (five replicates), all of which operated within a tolerance of ±0.25°C. Figure 4 shows the reproducibility of the SHS cycles (nine replicates) with approximately 12°C of superheat, all of which operated within a tolerance of ±0.75°C.
All SHS temperature profiles showed a similar pattern. After a short lag period, the temperature recorded from the chamber probes began to increase compared with Tth. After a further lag period, the chamber drain temperature also began to rise, indicating the development of SHS in the chamber and drain. Processes operating at 121.75°C showed a shorter lag phase than those operating at 132.5°C, resulting in the proportion of the operating cycle when SHS was present being shorter in the higher temperature cycle.
RRSCBI Kill Time in SS
Table 3 shows the results from exposure of RRSCBIs to SS in order to determine kill times. The table indicates that the kill time at 121.75°C for type A was 14 minutes and for type B was 7 minutes. The kill time for type B at 132.5°C was 3.5 minutes.
To confirm the kill time, a total of 90 type A RRSCBIs were exposed to SS at 121.75°C for 14 minutes and 135 type B RRSCBIs for 7 minutes. None showed a positive result after incubation in the autoreaders.
Response of RRSCBIs to SHS
Table 4 shows the results when RRSCBIs were exposed to 12°C of SHS at a set exposure temperature of 121.75°C (Tth).
Of the 135 type A RRSCBIs exposed to SHS (12°C) at 121.75°C for 14 minutes, a positive result was observed for zero of 45 mounted vertically, 26 of 45 mounted inverted, and 45 of 45 mounted horizontally. Of the 135 type B RRSCBIs exposed to SHS (12°C) at 121.75°C for 7 minutes, a positive result was observed for 24 of 45 mounted vertically, 41 of 45 mounted inverted, and 45 of 45 mounted horizontally.
The time values of when a positive result was indicated by the autoreaders were pooled into groups representing RRSCBI orientation and type (A or B). Figure 5 shows box plots of the combined data. Statistical examination (Student’s t test) indicated that for type A, horizontally mounted vials showed a positive result significantly faster (60 min; P < 0.95) than RRSCBIs mounted in either the inverted (129 min) or vertical (all negative) orientations. For type B, horizontally mounted vials showed a positive result faster (22 min) than vials in either the inverted (22.4 min) or vertical (23 min) orientations. However, in the case of vials mounted inverted, the difference was not statistically significant. This suggests that in normal use, horizontally mounted RRSCBIs will confer optimum sensitivity toward SHS.
The response of type B RRSCBIs to 4.5°C of SHS in an operating cycle with a nominal exposure temperature of 132.5°C (Tth) was also investigated. Results indicated that when operating at 132.5°C, the time required for SHS to develop in the chamber was prolonged, resulting in the RRSCBIs being exposed to SS for a duration sufficient to enable microbial inactivation to take place despite superheat eventually developing in the chamber. Despite several attempts to reduce the thermal mass in the chamber by modification of the sample holder and reducing the number of RRSCBIs in the chamber, this problem could not be overcome. It was considered likely that greater levels of SHS could be created in a 132.5°C exposure cycle if the header and jacket pressures could have been increased further; however, such settings would have exceeded the safety tolerances for the equipment. Further method development is needed.
Response of RRSCBIs to Dry Heat
All RRSCBIs exposed to dry heat showed a positive result after incubation.
Discussion
SHS Method Development
Throughout the study, the presence and level of SHS in the chamber was assumed to be indicated by the difference between the measured chamber temperature (Tm) and that calculated from pressure according to steam table values using equation 1 (Tth). Normally, a BIER vessel will be operated to provide SS conditions. This will include controlling the steam manifold and chamber jacket pressures to give temperatures close to those required for exposure in the chamber. Similarly, if a heated viewing port is present, this will be heated to a temperature equal to the exposure temperature. However, by adjusting the steam manifold and jacket pressures and viewing port temperature to values much higher than normal, creating varying levels of SHS in the chamber was possible. The level of superheat created was reproducible and had close tolerances, making for a test method that would allow evaluation of the performance of both BIs and CIs.
Response of RRSCBIs to SHS
Previous studies have shown that BIs on carriers were able to detect the presence of SHS by virtue of the fact that they would survive a process that would cause inactivation in SS.10–13 The purpose of this study was to establish whether RRSCBIs could do likewise given that the spore carrier normally is located at the base of a long, thin polymeric tube, on top of which is located a glass vial containing 1 to 2 mL growth medium. The growth medium provides a thermal mass where condensation can occur, wetting the spore carrier and preventing detection of SHS.
The data presented in this work indicated that RRSCBIs were able to detect the presence of SHS at 121.75°C when viability was measured using the formation of a fluorescent compound in the growth medium as a result of the metabolic activity of an enzyme found in the spore. However, this appeared to be an orientation-related attribute. RRSCBIs mounted vertically either were unable to or had a much reduced ability to detect SHS compared with RRSCBIs mounted inverted or horizontally, with this latter orientation being the most sensitive— and how they would be expected to be mounted in healthcare facility use.
The lack of sensitivity of vertically mounted RRSCBIs was thought to be due to the fact that during testing, the cold surfaces of the media vial would cause condensation of steam. The condensate then would flow downwards and wet the spore carrier, causing sufficient moisture to give rise to microbial inactivation. When RRSCBIs were mounted inverted or horizontally, any such condensation would flow away from the spore carriers, allowing SHS to impinge on the carriers.
Why horizontally mounted RRSCBIs appeared to be more sensitive toward SHS compared with those mounted inverted remains unclear. It is also interesting to note that horizontally mounted RRSCBIs showed positive results statistically significantly quicker than vials mounted inverted. The reason for this difference was unclear, but it is possible that a greater quantity of the active enzyme, which gives rise to the fluorescent compound detected by the autoreaders, might be present. A greater quantity of active enzyme would give rise to a greater concentration of fluorescent compound sooner and therefore result in earlier detection by the autoreaders. This suggests the time to a positive result might be a semiquantitative method of assessing process efficacy. Experience shows that unexposed RRSCBIs will give a positive result within a few minutes of placement in the autoreaders. This phenomenon requires further investigation.
Tests carried out at an exposure temperature of 121.75°C were more successful than those carried out at 132.5°C. At an exposure temperature of 121.75°C, it was possible to create approximately 12°C of SHS when the BIER vessel was operated at its maximum allowed safety tolerances. When attempts were made to increase the exposure temperature to 132.5°C, SHS could be created (4.5°C) but the presence of the thermal mass of the sample holder and samples prevented rapid development, resulting in SS being present for a large proportion of the shorter operating cycle (3.5 min).
The results from this study indicated that SHS can be detected by RRSCBIs under controlled test conditions. Conducting further studies in a simulated healthcare facility setting to establish whether SHS could be similarly detected would be worth-while. Several operational conditions are known to potentially give rise to SHS in production sterilizers.
SHS and Wet Steam in Healthcare Facilities
Wet packs arising from SS sterilization processes are a persistent problem in healthcare facilities.16–22 In 125 facilities in four countries, 87% experienced this problem and took measures to prevent wet packs.16 A total of 73 subject matter experts from 19 countries concluded that wet pack use after the sterilization cycle is a multifactorial problem and depends on the equipment used.17
The methods used to overcome wet packs often can cause SHS problems. Practitioners frequently will raise the sterilizer jacket temperature in order to “dry” wet steam entering the chamber. This in turn can create SHS. Similarly, fabrics or wrappings made from natural fibers can become dehydrated if stored or processed in a very dry environment (<30% RH), such as might be found in an air-conditioned packing room. When these fibers are subjected to an SS sterilization process, the fibers exothermically rehydrate, giving rise to localized SHS. A similar problem can arise if overdried absorbent pads are placed on the bottom of the sterile barrier system to capture excess condensate. Although the SHS may prevent development of sterilizing conditions within the packs, the sterilizer’s temperature sensors are unlikely to detect the anomaly because they normally are located remotely in the drain.
The use of RRSCBIs (and CIs), which detect SHS, therefore would be of great value in detecting such process failures. Further studies are required to establish the response of RRSCBIs toward SHS when used in healthcare facility settings.
Conclusion
A method for creating SHS in a commercially produced BIER vessel was developed and shown to be stable, predictable, and reproducible. The method was used to establish the response of two types of RRSCBIs containing G. stearothermophilus spores, the viability of which was determined by a well-established fluorescent readout procedure, toward SS and SHS. Each type of RRSCBI gave fluorescent negative results when exposed to the determined kill times. When exposed to the kill times in the presence of 12°C of SHS at 121.75°C, RRSCBIs showed fluorescent positive results. However, the number of RRSCBIs showing positives was dependent of the orientation of the vials, with horizontally mounted vials exhibiting the greatest number of positives and therefore greatest sensitivity toward SHS. The study demonstrated that RRSCBIs could detect SHS at 121.75°C under laboratory conditions. Further studies are required to establish the response of RRSCBIs toward SHS when used in healthcare facility settings.
Funding
Sponsored by a research grant from 3M (St. Paul, MN).
Acknowledgments
To William Leiva (3M) and Axel Abels (3M) for invaluable assistance throughout the course of the work.
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