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New technology for sterilization and disinfection
New Technology for Sterilization BERTIL
NYSTR~M,
M.D.,
stockho/m,
Sweden
Sterilization with low temperature steam and formaldehyde is a well-known process in many European countries, but little known in the United States. It sterilizes reliably and reproducibly at temperatures 2 65°C. With a well-designed cycle, it leaves residues of formaldehyde on sterilized items below 5 pg/cm’, measured on a standard filter paper. Formaldehyde levels in air near the autoclave are well below official exposure limits, if at all measurable. Occurrence of late growers in bioindicators, and penetration of the sterilizing media into long narrow lumina, should be validated for new processes. Automated cleaning and disinfection in closed washer-disinfectors and flushing disinfectors are likewise processes relatively little known in the United States. Disinfection is achieved by a final rinse with hot water or steam. Washer-disinfectors are used for surgical instruments, nondisposable anesthesia and other equipment, flushing disinfectors for nondisposable bedpans, washbowls, urinals, and similar equipment. They clean well, washer-disinfectors excellently so, and disinfect reliably. With the use of such equipment in wards, surgical departments, and other areas, reliance on chemical germicides can be dramatically reduced and disposables can be replaced by disinfectable nondisposables.
From the Division of Hospital Infection Control, Department of Clinrcal Microbiology, Huddinge University Hospital, Stockholm, Sweden. Requests for reprints should be addressed to Bertil Nystrom, M.D., Department of Clinical Microbiology, F 71, Huddinge Hospital, S-141 86 Huddinge, Sweden.
38-2648
September
16, 1991
and Disinfection
The American
Journal
of Medicine
T
he purpose of this article is to describe, mainly for American readers, a sterilization and a disinfection process, both of which have been developed in northern Europe and have been used there and elsewhere for a long period of time with favorable experiences, but which are little known and used in America.
STERILIZATION WITH LOW TEMPERATURE STEAM AND GASEOUS FORMALDEHYDE Sterilization with low-temperature steam and gaseous formaldehyde (LTSF) in its modern shape was first described by Alder et al 111 in the United Kingdom. It was initially used mainly for cystoscopes and similar urologic equipment. It was of interest as an alternative method to ethylene oxide sterilization for heat-labile equipment in hospitals when at that time reliable and safe ethylene oxide autoclaves for hospital use were hardly available on the European market. Since then LTSF equipment has been much developed and reliable and safe equipment is now marketed by several European manufacturers. LTSF sterilizes reliably at temperatures 2 65°C. Sterilization is achieved by gaseous formaldehyde in the presence of saturated steam. The combination of these two sterilization media evenly distributed in the autoclave chamber is essential for a succesful process. A complete process takes approximately 2 hours at 65°C shorter at higher temperatures, including aeration to remove formaldehyde residuals from the goods. Condensation of steam to water into which formaldehyde vapor can dissolve must be avoided, as must polymerization of formaldehyde onto the inner chamber surfaces. Important contributions to the technology in these respects have been given by Weymes et al [2] and by Handlos [3]. Penetration of the sterilization media into long narrow lumina is important, for the process is often used for sterilization of catheters and other tube-formed devices. Several authors [4,51 have suggested test pieces to monitor this penetration. Some such test piece must be used in validating a new process, but in our experience this is not necessary in routine monitoring. The choice of suitable bioindicators is, of course, important. In Europe various Bacillus stearothermophilus preparations are generally used. In SweVolume
91 (suppl
3B)
CONFERENCE
den, sterilization efficacy is measured by bioindicators of two kinds-one with B. stearothermophilus spores and one with Bacillus subtilis spores with an efficient moisture protection. The B. stearothermophilus preparation is more resistant to formaldehyde and detects defects in formaldehyde concentration; the B. subtilis preparation has a more efficient moisture protection and detects deficient hydration. Experiences by Christensen and Kristensen [6] have demonstrated that delayed growth may occur with inadequate processes. In validating new processes, therefore, prolonged incubation periods up to 6 weeks for bioindicators are important. Spicher and Peters [7] have demonstrated that bacterial spores exposed to sublethal concentrations of formaldehyde can be revived by heat treatment. This, too, should be taken into account when validating a new process. In routine monitoring, however, prolonged incubation or heat treatment of bioindicators should not be necessary. It is also important to ensure that formaldehyde residues in the indicator carrier does not give rise to false-negative culture results [81. Formaldehyde is well known to be a highly toxic substance. In Sweden, at present, the occupational exposure threshold limit value is 0.6 mg/m3 as a time-weighted average with a short-term exposure limit of 1.2 mg/m3. Available equipment easily meets these levels. According to unpublished experiences from my hospital, levels of approximately 0.25 mg/m3 have been measured within the chamber for approximately a minute after opening the door at the end of a cycle. Around the autoclave, levels are lower than 0.1 mg/m3. Are formaldehyde residues in sterilized equipment a hazard? If so, what should the upper limit be? These questions have no obvious answers. The Swedish Association for Sterilization and Hospital Infection Control has suggested a limit of 5 Fg/cm’ measured on a standard filter paper. Handlos [91 has described an appropriate analysis method and has demonstrated residue levels on different materials [lo]. The suggested limit has been set as low as can be obtained with good presently available equipment at temperatures z 65°C and with a reasonably short cycle. It has been argued that the suggested level is, in fact, unnecessarily low. In the absence of reliable studies demonstrating this, we prefer to stay with the lowest possible level. In many hospitals in Scandinavia and the United Kingdom, LTSF autoclaves have been used for many years for the sterilization of heat labile equipment like nonflexible endoscopes, electric equipment, various objects made of heat-labile plastics, among others. We do rely on them and find them preferable to ethylene oxide sterilization September
16, 1991
ON NOSOCOMIAL
INFECTIONS
/ NYSTRiiM
in hospitals, partly because of a much shorter cycle, aeration included, and less potential occupational and environmental hazards. LTSF covers virtually all hospital needs for sterilization of heat-labile equipment, with some few exceptions if they choose to resterilize some types of expensive very heat-labile instruments intended for single use. This highly controversial problem will, however, not be discussed here. FLUSHING AND WASHER-DISINFECTORS Flushing disinfectors and washer-disinfectors are automated and closed equipment to clean and disinfect a number of objects, from bedpans, urinals, washbowls, suction bottles, among others, to complicated surgical instruments, anesthesia tubes and masks, and other similar objects. The first-mentioned type of equipment can be cleaned and disinfected in flushing-disinfectors. They have a short cycle of a few minutes. They clean by flushing with warm water, possibly with the addition of a detergent, and then disinfect by flushing the items with hot water at approximately 9O”C, or with steam. They can also be used as a slop basin. In this way, for example, a used bedpan can be emptied, cleaned, and disinfected in the same machine. They are most useful in wards and can be found in most hospital wards in the Scandinavian countries. They are also available and much used in many other European countries, such as Germany and Austria. In Sweden their cleaning capacity is validated by running pieces of relevant equipment contaminated with dried blood in a fully loaded machine. At least 95% of the objects must be clean by ocular inspection after the completed cycle. Their disinfecting capacity is validated with the use of closed plastic capillaries filled with a broth dilution of a Salmonella phage ill]. The capillaries are situated where the temperature conditions are the worst during the cycle according to earlier thermocouple measurements. The inactivation factor on objects must be 25 x lo7 and on the inner sides of the machine chamber 2 5 x 105. The machines should be annually revalidated with temperature measurements. The use of this equipment eliminates a lot of manual cleaning, and equipment is reliably disinfected after each use [12]. In this way manual cleaning is eliminated, fewer disposables are needed, and less chemical germicides, thus reducing the environmental pressure from the hospital waste and sewage. Surgical instruments and anesthesia and other equipment are more difficult to clean. They are run in washer-disinfectors with a longer cycle of some
The American
Journal
of Medicine
Volume
91 (suppl
36)
3B-26%
CONFERENCE
ON NOSOCOMIAL
INFECTIONS
/ NYSTRijM
20-30 minutes and with the use of a detergent. These machines, too, disinfect by hot water at approximately 90°C. In Sweden, their cleaning and disinfection efficiency is validated in a similar way to the one described for flushing-disinfectors. They clean extremely well, so well that doubts have arisen of whether the disinfection phase is necessary or microorganisms are removed well enough by the physical cleaning [131. Washer-disinfectors are mainly used in surgical departments, in intensive care units, and in wards with an uncommonly high instrument usage. They have similar advantages as flushing-disinfectors: cleaning by hand is eliminated, fewer disposables, and fewer chemical germicides are needed.
REFERENCES 1. Alder VG, Brown AM, Gillespie WA. Disinfection of heat-sensitive material by low-temperature steam and formaldehyde. J Clin Pathol 1966; 19: 83-9. 2. Weymes C, White J, Harris C. Studies in the use of low concentrations of formaldehyde with steam at sub-atmospheric pressure as a means of sterilizing non-porous heat sensitive loads. Greater Glasgow Health Board Sterilization Research Centre 1975, note 4. 3. Handlos V. Formaldehyde sterilization. II. Formaldehyde-steam steriliza-
3B-266s
September
16,
1991
The American
Journal
of Medicine
tion, the process and its influence on the formaldehyde
residuals.
Arch Pharm
Chem Sci Ed 7 1979: l-11. 4. Line SJ, Pickerill JK. Testing a steam-formaldehyde sterilizer for gas penetration efficiency. J Clin Pathol 1973; 26: 716-20. 5. Spicher G, Borchers U. Abhangigkeit der mikrobiologischen PrOfungsergebnisse eines Formaldehyd-Gassterilisationsveriahrens von der Form der zu steriliserender Objekte. Hyg Med 1983; 8: 7-9. 6. Christensen EA, Kristensen H. Gaseous sterilisation. in: Russell AD, Hugo WB, Ayliffe GAJ (eds). Principles and practice of disinfection, preservation and sterilisation. Oxford: Blackwell Scientific Publishers 1982; 548-68. 7. Spicher G, Peters J. Resistenz mikrobieller Keime gegeniiber Formaldehyd. I, Vergleichende quantitative Untersuchungen an einigen ausgewdhlten Arten vegetativer Bakterien, bakterieller Sporen, Pilze, Bakteriophagen und Viren. Zentralbl Bakt Hyg, [Bl 1976; 163,486-508. 8. Line SJ, Hoxey EV, Hurrell DJ, Soper CJ, Davies DGJ. Selection of a formaldehyde inactivator to assist the recovery of survivors from biological monitors for LTSF. In: Proceedings of the 1st Eucomed Workshop on Biological Monitoringof Sterilization, London: Eucomed, 1986: 217-8. 9. Handlos V. Formaldehyde sterilization. I. Determination of formaldehyde residuals in autoclave-sterilized materials. Arch Pharm Chem Sci Ed 5 1977: 163-9. 10. Handlos V. Formaldehyde sterilization. III. The behaviour of the loaded autoclave and the permeability of plastic materials to formaldehyde. Arch Pharm Chemi Sci Ed 7 1979: 12-8. 11. Nilehn B. A method for the quantitative microbiological check of heat decontaminators. Stand J Infect Dis 1972; 4: 245-53. 12. Nystram B. Disinfection in bed-pan washers. J Hosp Infect 1983; 4: 191-8. 13. NystrBm B. Disinfection of surgical instruments. J Hosp Infect 1981; 2: 263-8.
Volume
91 (suppl3B)
NYSTR~M,
M.D.,
stockho/m,
Sweden
Sterilization with low temperature steam and formaldehyde is a well-known process in many European countries, but little known in the United States. It sterilizes reliably and reproducibly at temperatures 2 65°C. With a well-designed cycle, it leaves residues of formaldehyde on sterilized items below 5 pg/cm’, measured on a standard filter paper. Formaldehyde levels in air near the autoclave are well below official exposure limits, if at all measurable. Occurrence of late growers in bioindicators, and penetration of the sterilizing media into long narrow lumina, should be validated for new processes. Automated cleaning and disinfection in closed washer-disinfectors and flushing disinfectors are likewise processes relatively little known in the United States. Disinfection is achieved by a final rinse with hot water or steam. Washer-disinfectors are used for surgical instruments, nondisposable anesthesia and other equipment, flushing disinfectors for nondisposable bedpans, washbowls, urinals, and similar equipment. They clean well, washer-disinfectors excellently so, and disinfect reliably. With the use of such equipment in wards, surgical departments, and other areas, reliance on chemical germicides can be dramatically reduced and disposables can be replaced by disinfectable nondisposables.
From the Division of Hospital Infection Control, Department of Clinrcal Microbiology, Huddinge University Hospital, Stockholm, Sweden. Requests for reprints should be addressed to Bertil Nystrom, M.D., Department of Clinical Microbiology, F 71, Huddinge Hospital, S-141 86 Huddinge, Sweden.
38-2648
September
16, 1991
and Disinfection
The American
Journal
of Medicine
T
he purpose of this article is to describe, mainly for American readers, a sterilization and a disinfection process, both of which have been developed in northern Europe and have been used there and elsewhere for a long period of time with favorable experiences, but which are little known and used in America.
STERILIZATION WITH LOW TEMPERATURE STEAM AND GASEOUS FORMALDEHYDE Sterilization with low-temperature steam and gaseous formaldehyde (LTSF) in its modern shape was first described by Alder et al 111 in the United Kingdom. It was initially used mainly for cystoscopes and similar urologic equipment. It was of interest as an alternative method to ethylene oxide sterilization for heat-labile equipment in hospitals when at that time reliable and safe ethylene oxide autoclaves for hospital use were hardly available on the European market. Since then LTSF equipment has been much developed and reliable and safe equipment is now marketed by several European manufacturers. LTSF sterilizes reliably at temperatures 2 65°C. Sterilization is achieved by gaseous formaldehyde in the presence of saturated steam. The combination of these two sterilization media evenly distributed in the autoclave chamber is essential for a succesful process. A complete process takes approximately 2 hours at 65°C shorter at higher temperatures, including aeration to remove formaldehyde residuals from the goods. Condensation of steam to water into which formaldehyde vapor can dissolve must be avoided, as must polymerization of formaldehyde onto the inner chamber surfaces. Important contributions to the technology in these respects have been given by Weymes et al [2] and by Handlos [3]. Penetration of the sterilization media into long narrow lumina is important, for the process is often used for sterilization of catheters and other tube-formed devices. Several authors [4,51 have suggested test pieces to monitor this penetration. Some such test piece must be used in validating a new process, but in our experience this is not necessary in routine monitoring. The choice of suitable bioindicators is, of course, important. In Europe various Bacillus stearothermophilus preparations are generally used. In SweVolume
91 (suppl
3B)
CONFERENCE
den, sterilization efficacy is measured by bioindicators of two kinds-one with B. stearothermophilus spores and one with Bacillus subtilis spores with an efficient moisture protection. The B. stearothermophilus preparation is more resistant to formaldehyde and detects defects in formaldehyde concentration; the B. subtilis preparation has a more efficient moisture protection and detects deficient hydration. Experiences by Christensen and Kristensen [6] have demonstrated that delayed growth may occur with inadequate processes. In validating new processes, therefore, prolonged incubation periods up to 6 weeks for bioindicators are important. Spicher and Peters [7] have demonstrated that bacterial spores exposed to sublethal concentrations of formaldehyde can be revived by heat treatment. This, too, should be taken into account when validating a new process. In routine monitoring, however, prolonged incubation or heat treatment of bioindicators should not be necessary. It is also important to ensure that formaldehyde residues in the indicator carrier does not give rise to false-negative culture results [81. Formaldehyde is well known to be a highly toxic substance. In Sweden, at present, the occupational exposure threshold limit value is 0.6 mg/m3 as a time-weighted average with a short-term exposure limit of 1.2 mg/m3. Available equipment easily meets these levels. According to unpublished experiences from my hospital, levels of approximately 0.25 mg/m3 have been measured within the chamber for approximately a minute after opening the door at the end of a cycle. Around the autoclave, levels are lower than 0.1 mg/m3. Are formaldehyde residues in sterilized equipment a hazard? If so, what should the upper limit be? These questions have no obvious answers. The Swedish Association for Sterilization and Hospital Infection Control has suggested a limit of 5 Fg/cm’ measured on a standard filter paper. Handlos [91 has described an appropriate analysis method and has demonstrated residue levels on different materials [lo]. The suggested limit has been set as low as can be obtained with good presently available equipment at temperatures z 65°C and with a reasonably short cycle. It has been argued that the suggested level is, in fact, unnecessarily low. In the absence of reliable studies demonstrating this, we prefer to stay with the lowest possible level. In many hospitals in Scandinavia and the United Kingdom, LTSF autoclaves have been used for many years for the sterilization of heat labile equipment like nonflexible endoscopes, electric equipment, various objects made of heat-labile plastics, among others. We do rely on them and find them preferable to ethylene oxide sterilization September
16, 1991
ON NOSOCOMIAL
INFECTIONS
/ NYSTRiiM
in hospitals, partly because of a much shorter cycle, aeration included, and less potential occupational and environmental hazards. LTSF covers virtually all hospital needs for sterilization of heat-labile equipment, with some few exceptions if they choose to resterilize some types of expensive very heat-labile instruments intended for single use. This highly controversial problem will, however, not be discussed here. FLUSHING AND WASHER-DISINFECTORS Flushing disinfectors and washer-disinfectors are automated and closed equipment to clean and disinfect a number of objects, from bedpans, urinals, washbowls, suction bottles, among others, to complicated surgical instruments, anesthesia tubes and masks, and other similar objects. The first-mentioned type of equipment can be cleaned and disinfected in flushing-disinfectors. They have a short cycle of a few minutes. They clean by flushing with warm water, possibly with the addition of a detergent, and then disinfect by flushing the items with hot water at approximately 9O”C, or with steam. They can also be used as a slop basin. In this way, for example, a used bedpan can be emptied, cleaned, and disinfected in the same machine. They are most useful in wards and can be found in most hospital wards in the Scandinavian countries. They are also available and much used in many other European countries, such as Germany and Austria. In Sweden their cleaning capacity is validated by running pieces of relevant equipment contaminated with dried blood in a fully loaded machine. At least 95% of the objects must be clean by ocular inspection after the completed cycle. Their disinfecting capacity is validated with the use of closed plastic capillaries filled with a broth dilution of a Salmonella phage ill]. The capillaries are situated where the temperature conditions are the worst during the cycle according to earlier thermocouple measurements. The inactivation factor on objects must be 25 x lo7 and on the inner sides of the machine chamber 2 5 x 105. The machines should be annually revalidated with temperature measurements. The use of this equipment eliminates a lot of manual cleaning, and equipment is reliably disinfected after each use [12]. In this way manual cleaning is eliminated, fewer disposables are needed, and less chemical germicides, thus reducing the environmental pressure from the hospital waste and sewage. Surgical instruments and anesthesia and other equipment are more difficult to clean. They are run in washer-disinfectors with a longer cycle of some
The American
Journal
of Medicine
Volume
91 (suppl
36)
3B-26%
CONFERENCE
ON NOSOCOMIAL
INFECTIONS
/ NYSTRijM
20-30 minutes and with the use of a detergent. These machines, too, disinfect by hot water at approximately 90°C. In Sweden, their cleaning and disinfection efficiency is validated in a similar way to the one described for flushing-disinfectors. They clean extremely well, so well that doubts have arisen of whether the disinfection phase is necessary or microorganisms are removed well enough by the physical cleaning [131. Washer-disinfectors are mainly used in surgical departments, in intensive care units, and in wards with an uncommonly high instrument usage. They have similar advantages as flushing-disinfectors: cleaning by hand is eliminated, fewer disposables, and fewer chemical germicides are needed.
REFERENCES 1. Alder VG, Brown AM, Gillespie WA. Disinfection of heat-sensitive material by low-temperature steam and formaldehyde. J Clin Pathol 1966; 19: 83-9. 2. Weymes C, White J, Harris C. Studies in the use of low concentrations of formaldehyde with steam at sub-atmospheric pressure as a means of sterilizing non-porous heat sensitive loads. Greater Glasgow Health Board Sterilization Research Centre 1975, note 4. 3. Handlos V. Formaldehyde sterilization. II. Formaldehyde-steam steriliza-
3B-266s
September
16,
1991
The American
Journal
of Medicine
tion, the process and its influence on the formaldehyde
residuals.
Arch Pharm
Chem Sci Ed 7 1979: l-11. 4. Line SJ, Pickerill JK. Testing a steam-formaldehyde sterilizer for gas penetration efficiency. J Clin Pathol 1973; 26: 716-20. 5. Spicher G, Borchers U. Abhangigkeit der mikrobiologischen PrOfungsergebnisse eines Formaldehyd-Gassterilisationsveriahrens von der Form der zu steriliserender Objekte. Hyg Med 1983; 8: 7-9. 6. Christensen EA, Kristensen H. Gaseous sterilisation. in: Russell AD, Hugo WB, Ayliffe GAJ (eds). Principles and practice of disinfection, preservation and sterilisation. Oxford: Blackwell Scientific Publishers 1982; 548-68. 7. Spicher G, Peters J. Resistenz mikrobieller Keime gegeniiber Formaldehyd. I, Vergleichende quantitative Untersuchungen an einigen ausgewdhlten Arten vegetativer Bakterien, bakterieller Sporen, Pilze, Bakteriophagen und Viren. Zentralbl Bakt Hyg, [Bl 1976; 163,486-508. 8. Line SJ, Hoxey EV, Hurrell DJ, Soper CJ, Davies DGJ. Selection of a formaldehyde inactivator to assist the recovery of survivors from biological monitors for LTSF. In: Proceedings of the 1st Eucomed Workshop on Biological Monitoringof Sterilization, London: Eucomed, 1986: 217-8. 9. Handlos V. Formaldehyde sterilization. I. Determination of formaldehyde residuals in autoclave-sterilized materials. Arch Pharm Chem Sci Ed 5 1977: 163-9. 10. Handlos V. Formaldehyde sterilization. III. The behaviour of the loaded autoclave and the permeability of plastic materials to formaldehyde. Arch Pharm Chemi Sci Ed 7 1979: 12-8. 11. Nilehn B. A method for the quantitative microbiological check of heat decontaminators. Stand J Infect Dis 1972; 4: 245-53. 12. Nystram B. Disinfection in bed-pan washers. J Hosp Infect 1983; 4: 191-8. 13. NystrBm B. Disinfection of surgical instruments. J Hosp Infect 1981; 2: 263-8.
Volume
91 (suppl3B)