OPEN ACCESS
`
`Research Article
`
`Thomas von Woedtke1
`Axel Kramer2
`
`1 INP Greifswald e.V. (Leibniz
`Institute for Plasma Science
`and Technology), Greifswald,
`Germany
`2 Institute for Hygiene and
`Environmental Medicine,
`University Greifswald,
`Germany
`
`The limits of sterility assurance
`
`Die Grenzen der Sterilisationssicherheit
`
`Abstract
`Sterility means the absence of all viable microorganisms including vir-
`uses. At present, a sterility assurance level (SAL) of 10–6 is generally
`accepted for pharmacopoeial sterilization procedures, i.e., a probability
`of not more than one viable microorganism in an amount of one million
`sterilised items of the final product. By extrapolating the reduction rates
`following extreme artificial initial contamination, a theoretical overall
`performance of the procedure of at least 12 lg increments (overkill
`conditions) is demanded to verify an SAL of 10–6. By comparison, other
`recommendations for thermal sterilization procedures demand only
`evidence that the difference between the initial contamination and the
`number of test organisms at the end of the process amount to more
`than six orders of magnitude. However, a practical proof of the required
`level of sterility assurance of 10–6 is not possible. Moreover, the attain-
`ability of this condition is fundamentally dubious, at least in non-thermal
`procedures. Thus, the question is discussed whether the undifferentiated
`adherence to the concept of sterility assurance on the basis of a single
`SAL of 10–6 corresponds with the safety requirements in terms of patient
`or user safety, costs and energy efficiency. Therefore, in terms of prac-
`tical considerations, a concept of tiered SALs is recommended, analog-
`ous to the comparable and well-established categorization into “High-
`level disinfection”, “Intermediate-level disinfection” and “Low-level dis-
`infection”. The determination of such tiered SALs is geared both to the
`intended application of the sterilized goods, as well as to the character-
`istics of the products and the corresponding treatment options.
`In the case of aseptic preparation, filling and production procedures, a
`mean contamination probability of 10–3 is assumed. In automated pro-
`cesses, lower contamination rates can be realized. In the case of the
`production of re-usable medical devices, a reduction of at least 2 lg in-
`crements can be achieved through prior cleaning in validated cleaning
`and disinfecting devices. By chemical disinfection, a further reduction
`of ≥5 lg increments is achieved. In the case of sterilized surgical instru-
`ments, an additional concern is that they lay opened in contaminated
`air for the duration of the operation, at least in conventionally ventilated
`operating theaters. Finally, the amount of pathogens necessary to cause
`an infection must be considered. By logical consideration of all aspects,
`it seems possible to partially reduce sterility assurance levels without
`any loss of safety. Proceeding from this, we would like to make the fol-
`lowing suggestions for tiered SAL values, adjusted according to the re-
`spective sterilization task:
`• SAL 10–6 for heat-resistant pharmaceutical preparations (parenter-
`als), suggested term: “Pharmaceutical sterilization”,
`• SAL 10–4 for heat-resistant medical devices, suggested term: “High-
`level sterilization”,
`• SAL 10–3 for heat-sensitive re-usable medical devices, under the
`precondition of a validated cleaning efficacy of >4 lg increments,
`suggested term: “Low-level sterilization”.
`Keywords: sterility, sterility assurance level (SAL), draft of tiered SAL
`values
`
`GMS Krankenhaushygiene Interdisziplinär 2008, Vol. 3(3), ISSN 1863-5245
`
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`I
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`Novartis Exhibit 2200.001
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`

`von Woedtke et al.: The limits of sterility assurance
`
`Zusammenfassung
`Sterilität bedeutet die Abwesenheit aller vermehrungsfähigen Mikroor-
`ganismen einschließlich Viren. Derzeit wird für Sterilisationsverfahren
`ein Sterilitätssicherheits-Wert (SAL-Wert) von 10–6 gefordert, d.h. in einer
`Menge von einer Million sterilisierten Gütern darf höchstens ein lebens-
`fähiger Mikroorganismus zu erwarten sein. Durch Extrapolation der
`Mikroorganismen-Reduktionsraten nach artifizieller extremer Ausgangs-
`kontamination (=106 Test-Mikrorganismen pro Prüfobjekt) wird zum
`Nachweis eines SAL-Werts von 10–6 eine theoretische Gesamtredukti-
`onsleistung des Verfahrens um mindestens 12 lg-Stufen abgeleitet
`(„Overkill“). Demgegenüber verlangen andere Empfehlungen lediglich
`den Nachweis, dass die Differenz zwischen Ausgangszahl und Zahl der
`Testorganismen nach Ende des Prozesses mehr als sechs Zehnerpoten-
`zen beträgt. Da der praktische Nachweis des geforderten Sterilisations-
`sicherheitsniveaus von 10–6 unmöglich ist und zumindest bei nichtther-
`mischen Verfahren die Erreichbarkeit dieses Zustands grundsätzlich
`zweifelhaft ist, wird die Fragestellung diskutiert, ob ein undifferenziertes
`Festhalten an dem gegenwärtigen praktizierten Konzept der Sterilisati-
`onssicherheit auf der Basis eines SAL-Wertes von 10–6 unter dem Aspekt
`der Patienten- bzw. Anwendersicherheit sowie der Kosten und des
`Energieverbrauchs den tatsächlichen Sicherheitsanforderungen ent-
`spricht.
`Unter praktischen Gesichtspunkten wäre daher ein Konzept von abge-
`stuften SAL-Werten analog der Differenzierung in „High-level“, „Interme-
`diate-level“ bzw. „Low-level disinfection“ sinnvoll, deren Festlegung sich
`sowohl an der vorgesehenen Anwendung des Sterilisierguts als auch
`an dessen Eigenschaften und den damit verbundenen Behandlungs-
`möglichkeiten orientiert.
`Bei aseptischen Zubereitungs-, Abfüllungs- und Herstellungsverfahren
`wird von einer mittleren Kontaminationswahrscheinlichkeit von 10–3
`ausgegangen, bei automatisierten Prozessen können auch geringere
`Kontaminationsraten realisiert werden. Im Fall der Aufbereitung wieder
`verwendbarer Medizinprodukte ist durch die vorausgehende Reinigung
`in Reinigungs-Desinfektions-Geräten eine Reduktion um mindestens
`2 lg-Stufen erreichbar. Durch die chemische Desinfektion wird nach der
`Reinigung eine weitere Reduktion um ≥5 lg-Stufen erreicht. Bei sterili-
`siertem chirurgischem Instrumentarium kommt hinzu, dass das Sterilgut
`im konventionell belüfteten Operationssaal für die Dauer der Operation
`geöffnet auf dem Instrumentiertisch lagert. Schließlich muss auch die
`Erregermenge berücksichtigt werden, die eine Infektion auszulösen
`vermag. Bei konsequenter Berücksichtigung aller Teilaspekte erscheint
`es möglich, die terminale Sterilisationsbehandlung ohne Sicherheitsver-
`lust deutlich zu reduzieren. Hierfür wird ein Vorschlag für an die jeweilige
`Sterilisationsaufgabe angepasste abgestufte SAL-Werte unterbreitet:
`• SAL 10–6 für thermostabile Arzneizubereitungen, vorgeschlagener
`Terminus: „Pharmazeutische Sterilisation“,
`• SAL 10–4 für thermostabile Medizinprodukte, vorgeschlagener Ter-
`minus: „High-level-Sterilisation“,
`• SAL 10–3 für thermolabile Medizinprodukte unter der Voraussetzung
`einer validierten Reinigungseffektivität >4 lg-Stufen, vorgeschlagener
`Terminus: „Low-level-Sterilisation“.
`Schlüsselwörter: Sterilität, Sterilitätssicherheits-Wert (SAL), Konzept
`abgestufter SAL-Werte
`
`GMS Krankenhaushygiene Interdisziplinär 2008, Vol. 3(3), ISSN 1863-5245
`
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`I
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`Novartis Exhibit 2200.002
`Regeneron v. Novartis, IPR2021-00816
`
`

`

`von Woedtke et al.: The limits of sterility assurance
`
`The concept of sterility assurance
`– a compromise between overkill,
`viability and safety requirements
`Sterility of a product or object means the complete ab-
`sence of viable microorganisms, including viruses, which
`could pose a risk during administration [67], [81], [61],
`[22]. A sterility assurance level (SAL) of 10–6 is currently
`required for sterilization procedures, i.e., a probability of
`not more than one viable microorganism in one million
`sterilized items of the final product [18].
`The inherent problem with these requirements is that
`evaluating the success of such sterilization by means of
`a final inspection is all but impossible, since contamina-
`tion rates on the order of an SAL of 10–6 cannot be recor-
`ded in experiments [59], [67], [46], [26].
`Thus, model situations have to be created, with the help
`of which conclusions can be drawn regarding the treat-
`ment conditions necessary to attain sterility meeting the
`SAL. Therefore, representative test organisms with a
`maximum resistance to the procedure to be examined
`are used for the purposes of auditing and qualifying
`sterilization procedures. Consequently, the inactivation
`of such highly resistant microorganisms encompasses
`all less resistant organisms, including most pathogens.
`Furthermore, the test organisms should be cultivable
`under simple and most easily reproducible conditions. In
`general, innocuous bacterial endospores fulfill these re-
`quirements [41], [50], [31], [4]. In order to verify the ef-
`ficacy of sterilization procedures, and to determine
`treatment parameters, extremely large volumes of these
`test organisms are used, typically ≥106 bacterial spores
`per test object, e.g., in the form of bio-indicators [18].
`When using the “Half-cycle method”, the action of half of
`the intended sterilization cycle – usually half of the
`treatment period – is examined. If, following this action,
`a certain number of bio-indicators contaminated with 106
`resistant bacterial spores are inactivated, it can be con-
`cluded that when applying the full cycle, an SAL of 10–6
`is guaranteed at a theoretical spore-inactivation rate of
`≥12 lg increments. This corresponds to “Overkill condi-
`tions” [37], [41], [30], [26].
`However, in a course of action of this kind, which is based
`on the complete inactivation of a limited number of test
`objects, it must be taken into account that there is a
`statistical connection between the mean number of
`contaminated test objects after treatment and the total
`number of identically treated test objects. Accordingly,
`the more test objects are included in the test, the greater
`the duration of treatment, concentration or dosage of
`antimicrobially active agents must be to completely inac-
`tivate a limited number of bio-indicators [66], [64], [87],
`[65].
`In contrast, other directions and recommendations – also
`for heat sterilization procedures – only demand evidence
`that the difference between the initial number of test
`microorganisms and the number of test organisms at the
`end of the process amount to more than six orders of
`
`magnitude, i.e., an inactivation rate ≥6 lg increments in
`order to consider the SAL of 10–6 as attained and indicate
`the product treated in this way as sterile [14], [3].
`What all methods used to prove the efficacy of steriliza-
`tion procedures have in common is that the conditions
`necessary to attain an extremely low probability of con-
`tamination of 10–6 are inferred from the treatment condi-
`tions required to reduce extremely high artificial test
`contaminations. This procedural method is based on the
`general assumption of exponential inactivation kinetics
`for microorganisms under the influence of antimicrobially
`effective parameters, from which a linear mortality curve
`results, given a semi-logarithmic diagram.
`There have been detailed trials and discussions, in par-
`ticular concerning heat inactivation kinetics. As early as
`1921, Bigelow put forward logarithmic inactivation kinet-
`ics for microorganisms under the influence of heat. The
`monograph by Konrich and Stutz [35], which was con-
`sidered to be a standard work for many years, as well as
`works by Machmerth [39], Pflug and Holcomb [48], Rus-
`sell [51], Gould [23] and Knöller [34] provide detailed
`analyses and discussions of the relevant literature, includ-
`ing experiments on this issue. The inactivation kinetics
`for microorganisms initially postulated for heat treat-
`ments, which corresponded to first-order reaction kinetics,
`was also applied in principle to the conditions of the ac-
`tion of ionizing irradiation as well as chemical agents with
`antimicrobial effect [28], [83], [63], [42], [53].
`Divergence from a strict semi-logarithmic course of the
`mortality curve is usually explained by inhomogeneity of
`the test organism populations used. The actual complexity
`of microbiological inactivation kinetics cannot be clearly
`and comprehensively described using simple mathemat-
`ical models, so that there is no uniform theory which takes
`into account all possible courses of mortality curves for
`the inactivation of microorganisms under the influence
`of noxa with antimicrobial effect [83]. Consequently, while
`in many cases the assumption of the linearity of inactiva-
`tion kinetics in the semi-logarithmic standard simplifies
`the actual circumstances, it offers the only practicable
`possibility for interpretation and utilization of data gained
`through experiments [39], [48], [37], [81], [34]. The cur-
`rent European Pharmacopoeia also puts forward the view
`that “the inactivation of microorganisms by physical or
`chemical means follows an exponential law...” [18].
`The fact that the objective of sterilization is to ensure a
`rate of microbial contamination of ≥10–6 surviving microor-
`ganisms per test object results in the necessity of extra-
`polating mortality curves from the area that can be recor-
`ded in experiments to determine SAL-compatible steriliz-
`ation parameters. As the description of the inactivation
`kinetics by means of a simple mathematical model, such
`as first-order kinetics, already shows an approximation
`within the area ascertainable in experiments, the associ-
`ated uncertainties must considerably increase in the case
`of extrapolation outside this area [83]. Nevertheless,
`procedural parameters intended to guarantee the attain-
`ment of sterility that meets the SAL are usually defined
`on the basis of extrapolation of this kind [41], [48], [4].
`
`GMS Krankenhaushygiene Interdisziplinär 2008, Vol. 3(3), ISSN 1863-5245
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`

`von Woedtke et al.: The limits of sterility assurance
`
`Thus, both the determination of procedural parameters
`for sterilization processes and the proof of a SAL ≤10–6
`as the quantitative end-point which has to be guaranteed
`by a sterilization process are not based on scientifically
`proven data, but are only rules of thumb and approximate
`values [34].
`Conclusion: Using the SAL concept as a basis for the
`evaluation of the performance of sterilization procedures
`constitutes a situation (which is probably unique) in which
`the certain attainment of a condition is required by law,
`but there is no way of proving the attainment of this
`condition in practical terms [22], [25], [24], [2]).
`
`Concept for proving sterilization
`assurance for heat-sensitive
`medical devices
`In our own tests to examine new concepts for the gentle
`sterilization of
`irritable goods, applying non-thermal
`physical and/or chemical treatment procedures and using
`Bacillus subtilis spores as test organisms, test conditions
`were selected that, after treatment, either viable test or-
`ganisms were still in evidence on the test objects or both
`sterile and unsterile test objects were present simultan-
`eously. By combining direct cell counting methods using
`classical microorganism recovery and counting techniques
`[65], [48], [1] with the “fraction negative method” [64],
`[65], [48], [62], mortality curves on the basis of experi-
`mental data were maintained in a range of about 8 orders
`of magnitude (≥106 to around 10–2 test organisms per
`test object). With reference to the known initial contam-
`ination of the test objects, exact reduction factors could
`be stated for the treatment parameters applied in each
`case, to derive from these the treatment conditions ne-
`cessary to attain sterility assurance in accordance with
`the SAL [73], [74], [75].
`Under this condition, the attainable degree of reduction
`of the test organisms can be exactly quantified on the
`one hand, and the actual inactivation kinetics can be
`depicted on the other, at least in the range that can be
`recorded using microbiological methods of proof. Addi-
`tionally, possible inhomogenities of the mortality curves
`can be taken into account for extrapolation into the SAL
`area.
`Conclusion: Exact quantitative statements as to the inac-
`tivation kinetics and, accordingly, the antimicrobial effic-
`acy can be made only if not all test objects are fully inac-
`tivated, starting from a known initial contamination follow-
`ing sub-effective treatment. Thus, for the quantitative
`characterization of the efficacy of antimicrobial proced-
`ures, the experimental conditions are to be chosen so
`that in the result of treatment, surviving test organisms
`are detectable for various individual treatments.
`Various experiments by the current authors demonstrated
`that inactivation kinetics within the experimentally access-
`ible range between 106 and 10–2 bacterial spores per test
`object never exhibited a linear course [33], [77], [76],
`
`[74], [75]. In part, there were very pronounced concave
`curves, consisting of an initial steep section which then
`levelled off. At least with the non-thermal antimicrobially
`active treatments examined, it was apparently possible
`to reduce the high starting incidence in the test objects
`to a low level using relatively short action duration, low
`substance concentrations or low irradiation. However,
`the efficacy against the residual contamination was con-
`siderably lower. Consequently, an extrapolation of the
`steep section of the respective inactivation curve into the
`SAL range of 10–6 would result in treatment durations,
`irradiation doses or substance concentrations which are
`actually much too brief to guarantee an adequate sterility
`assurance. On the other hand, an extrapolation of the
`second, flat part of the mortality curve would result in all
`cases in extreme sterilization conditions, which could not
`be applied practically.
`Analogous mortality curves can be found again and again
`primarily in older publications. Seidl et al., for example,
`reported on experiments on radio sterilization of medicinal
`products, in which it was possible to destroy 99.9% of a
`test organism population with an irradiation dose of
`0,1 Mrad (1 kGy); for the remaining 0.1%, however, a
`dosage at least five times stronger was necessary [57].
`Pronounced concave mortality curves for Bacillus subtilis
`spores dependent on varying gamma irradiation doses
`can also be found in Wallhäußer [80]. Pfeiffer also reports
`non-linear inactivation kinetics when applying ionizing ir-
`radiation, because of disproportionate survival of radi-
`ation-resistant microorganisms in the range of higher ir-
`radiation doses [45], [47]. Furthermore, various works
`on sporocidal efficacy of hydrogen peroxide show con-
`cave, flattening-out mortality curves [72], [11], [5]. Van
`Ooteghem describes non-linear survival curves for microor-
`ganisms under the influence of preservatives [71].
`Such inactivation curves, also called “tailing” curves, are
`explained predominantly (e.g., by Hermann [28] as well
`as Wickramanayake and Sproul [83]) by the existence of
`microorganism populations on the test object with incon-
`sistent resistance to the antimicrobially active treatments
`examined. Consequently, the less resistant fraction is
`killed first (steep curve section); the predominance of the
`surviving, more resistant fraction then results in a flatter
`course of the mortality curve. Spicher explains this phe-
`nomenon in detail, coming to the conclusion that the
`determination of the parameters of a sterilization proced-
`ure must be oriented to such extreme values represented
`by highly resistant test organism fractions, since that re-
`flects the actual circumstances [67].
`The range between experimentally detectable contamin-
`ation rates up to ca. 10–2 test organisms per test object
`and the SAL of 10–6, still encompasses 4 lg increments,
`which cannot be proven by experimental data. Due to the
`inhomogeneity of the inactivation kinetics already present
`in the experimentally accessible range, it is not possible
`to make any certain statement regarding the continuing
`course of such mortality curves, which may increasingly
`flatten out. Spicher already in 1993 expressly pointed
`out that, in unfavorable cases, the highly resistant test
`
`GMS Krankenhaushygiene Interdisziplinär 2008, Vol. 3(3), ISSN 1863-5245
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`

`von Woedtke et al.: The limits of sterility assurance
`
`organism fractions, and thus the flat part of the mortality
`curve, may no longer be determined using the usual mi-
`crobiological testing procedures. As a result, the custom-
`ary extrapolation of such curves into the SAL range in-
`volves considerable risks [67].
`Thus, it is clear that the general assumption of first-order
`exponential inactivation kinetics, which was used origin-
`ally to describe heat inactivation of bacteria, cannot be
`applied without restrictions to non-thermal processes.
`One approach for the interpretation of these experimental
`results is the hypothesis that there is a basic difference
`between thermal and non-thermal antimicrobial modes
`of action, from which follows the assumption that such
`non-linear mortality curves are not attributable solely to
`the inhomogeneity of the test populations. Possibly, they
`display a characteristic that can be generalized for all
`inactivation procedures based on non-thermal actions
`[77], [76]. From those treatment parameters that effect
`elimination of the conventionally high test organism
`numbers present on bioindicators for sterilization control,
`it is therefore not permissible – for sterilization proced-
`ures in which efficacy relies on irradiation and/or chem-
`ical effects – to directly derive treatment conditions which
`are intended to guarantee a reduction into the experiment-
`ally no longer detectable but for sterilization assurance
`essential range. Consequently, treatment conditions that
`ensure an SAL-compliant reduction up to a contamination
`rate of at least 10–6 cannot be clearly determined for such
`procedures. The conclusion to be drawn from this is that
`when non-thermal treatment procedures are applied, the
`condition of sterility that would conform with a contamin-
`ation probability of 10–6 cannot in fact be guaranteed.
`Thus, the SAL concept is not a procedure suitable for
`showing the efficacy of non-thermal sterilization pro-
`cesses.
`Consequently, only steam sterilization and sterilization
`using dry heat should be described as sterilization pro-
`cedures in the proper or traditional sense.
`Sterility according to a SAL of 10–6 should, logically, only
`still be required for medical devices and preparations
`that can be subjected to steam or hot air sterilization
`using the required standard and equivalent procedures.
`This is because it is possible that a homogeneous linear
`mortality curve, and thus the sufficiently certain determ-
`ination of the treatment conditions necessary to guaran-
`tee an SAL of 10–6, can be presumed only in thermal
`procedures.
`Other authors who carried out a detailed mathematical
`analysis of the intrinsic uncertainties of the exponential
`model of mortality of test organisms reach the same
`conclusions in principle (while also taking into account
`thermal inactivation kinetics). They explain that for dec-
`ades, this evidently inadequate theoretical basis has been
`adhered to without question, using the argument that,
`due to extreme safety premiums, the safety of sterilized
`products in practice is secured by assuming higher con-
`tamination rates with extremely resistant test organisms
`when examining sterilization procedures [8], [9].
`
`In order to ensure the highest possible level of safety in
`the application of non-thermal procedures, a proof of
`“Antimicrobial efficacy on the highest experimentally ac-
`cessible level” should be required.
`This proof should show that the inactivation kinetics de-
`pendent on the number of test organisms in the entire
`range ascertainable in experiments can be evidenced
`with performance data. As a rule, the performance char-
`acterization for non-thermal antimicrobial procedures
`should be carried out using test bodies contaminated
`with low levels of highly resistant test organisms. This is
`in order to reflect the fact that, apparently in contrast to
`the relatively simple option of reducing high numbers,
`the inactivation of low levels of residual contamination
`is disproportionately more difficult to achieve. Test mi-
`croorganisms with a high level of resistance to the pro-
`cedure to be examined should be used as test organisms,
`e.g., bacterial spores. It must be proven that a reduction
`of the number of test organisms by at least five lg incre-
`ments up to a contamination rate of 10–2, which can only
`just be proven in experiments, has been achieved. In
`general, only the strict concentration on data that can be
`recorded in experiments affords the possibility of being
`able to directly compare various procedures and proced-
`ural steps using inactivation kinetics and, consequently,
`to make available differing, but equivalent inactivation
`procedures for various products. The extrapolation of
`such inactivation kinetics recorded in experiments by
`merely one additional lg increment to a contamination
`level of 10–3 would guarantee a sufficient "Safety premi-
`um” for the determination of the necessary treatment
`parameters. Here, a “tiered” SAL of 10–3 could be intro-
`duced for non-thermal sterilization procedures. This value
`is also referred to repeatedly in the literature on this
`subject [22], [24], [86].
`In order to differentiate it from actual sterilization with
`an SAL of 10–6, which should be restricted to thermal
`procedures, such a gentle sterilization procedure targeted
`at a contamination probability of 10–3 could be called
`”Low-level sterilization”. The efficacy of ”Low-level steril-
`ization” concentrates primarily on the range of low levels
`of residual contamination with highly resistant microor-
`ganisms on goods to be sterilized, following effective
`preparation (aseptic processing and/or cleaning and
`subsequent disinfection), which is very important in
`practice.
`
`Do the current theoretical sterility
`assurance requirements reflect the
`actual safety requirements?
`It was ascertained that the practical proof of the required
`level of sterility assurance of 10–6
`is not possible.
`Moreover, the attainability of this condition is fundament-
`ally questionable, at least in non-thermal procedures.
`Furthermore, it is questionable whether the undifferenti-
`ated adherence to the currently practiced concept of
`
`GMS Krankenhaushygiene Interdisziplinär 2008, Vol. 3(3), ISSN 1863-5245
`
`5/10
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`I
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`Novartis Exhibit 2200.005
`Regeneron v. Novartis, IPR2021-00816
`
`

`

`von Woedtke et al.: The limits of sterility assurance
`
`sterility assurance on the basis of an SAL of 10–6 complies
`with the actual safety requirements in terms of patient
`or user safety [2]. The practical relevance of an SAL of
`10–6 is not conclusively proven, i.e., it is practically im-
`possible to find a difference in the rate of infection result-
`ing from the application of products that were sterilized
`with a (theoretical) SAL of 10–6 on the one hand, or on
`the other hand were treated using a procedure in which
`only a contamination likelihood of 10–3 is ensured [85].
`By extrapolating the reduction rates following extreme
`artificial initial contamination to an SAL of 10–6, a theor-
`etical overall performance of the procedure can be estab-
`lished by at least 12 lg increments (overkill conditions).
`Such an SAL, however, is neither evidence based nor can
`it be accurately ascertained in experiments, or attained
`practically.
`Furthermore, it must be taken into consideration that, by
`adhering to an SAL of 10–6 as a “gold standard” for steril-
`ization, unnecessarily high costs are incurred, the intro-
`duction of new sterilization procedures is made consider-
`ably more difficult [2], and no allowance is made for the
`requirement of sustainable development.
`Therefore, for practical reasons, a concept of tiered SALs
`would make sense, the establishment of which is oriented
`towards the intended application of the sterilized goods,
`the characteristics of such goods, and the corresponding
`treatment options. In disinfection procedures, an analog-
`ous categorization of “High-level disinfection”, “Interme-
`diate-level disinfection”, and “Low-level disinfection”, has
`been customary for some time already [69], [10], [52],
`[55], [60].
`Since the mid-1990s, there have been repeated sugges-
`tions for the definition of tiered sterility assurance values,
`such as a “Concept of tiered SALs”, the suggestion of the
`declaration of “Asepsis assurance values” [22], [86], or
`a declaration as “Aseptic: Safe for its designated use”,
`which is oriented more towards the intended application
`of the product [24], [2].
`The determination of the respective safety level should
`be oriented towards both the features and the quality of
`the products (e.g., new products being used for the first
`time and single-use products, as opposed to re-usable
`and re-processed goods etc.), as well as the intended
`application.
`In practice, in the course of modern quality-controlled
`production and preparation procedures for pharmaceut-
`icals and medical devices, only a very low level of contam-
`ination is to be expected. In the case of aseptic prepara-
`tion, filling and production procedures, a contamination
`probability of 10–3 is assumed; lower contamination rates
`can also be realized in automated processes [18], [45],
`[41], [85], [81], [17], [24], [13], [44]. For the use of
`gamma radiation for sterilization, possibilities of determ-
`ining dosage by ascertaining the number, type and radi-
`ation resistance of microbiological contaminants on the
`goods to be sterilized have been discussed and applied
`in practice for several years [45], [49], [27], [47].
`The possible inclusion of the real contamination risk for
`the products to be sterilized in the course of the quality
`
`control of production processes could become general
`practice to determine the necessary sterilization treat-
`ment conditions and the resulting level of safety. This
`would also correspond to the suggested concentration
`of the proof of efficacy of a sterilization procedure on the
`low level of residual contamination with more highly res-
`istant microorganisms.
`In addition to the degree of contamination prior to steril-
`ization, the period between sterilization and intended use
`as well as the risk of recontamination and/or proliferation
`of microorganisms during the storage period must also
`be taken into account. Thus, e.g., for aqueous parenterals,
`it is by all means reasonable to demand an SAL of 10–6.
`In the case of the production of re-usable medical devices,
`a reduction of at least 2 lg increments can be achieved
`through prior cleaning in validated cleaning and disinfect-
`ing devices (CDD). In CDDs for containers for human
`egesta, the reduction rate was 3 to >5 lg increments,
`depending on the procedure [82]. Due to the require-
`ments for chemical disinfection, a further reduction by
`≥5 lg increments is achieved through disinfection follow-
`ing cleansing in the reprocessing procedure [21]. This
`means that because of the preceding reprocessing pro-
`cedure for medical devices, at least 7 lg increments are
`usually added to the actual sterilization performance.
`Based on a consistent consideration of the reduction
`performance of all partial steps of reprocessing and
`sterilization procedures, it would be possible to markedly
`reduce the final sterilization treatment without a reduction
`in safety. The prerequisite for this is the validation of all
`partial process steps, which is required in any case.
`In the case of sterilized operating instruments, an addi-
`tional factor must be taken into account in the risk assess-
`ment. In conventionally ventilated operating theaters,
`sterile goods are removed from their sterile packaging in
`the theater and are stored opened on the instrument
`table for the duration of the operation. During this time,
`they are exposed to the risk of contamination from sedi-
`mentation of airborne or particle-borne pathogens, re-
`leased mainly by the operating team. Even in the case of
`low-turbulence displacement flow (LDF) and subject to
`the premise that the sterile goods are opened within the
`LDF and the table of instruments is also situated com-
`pletely within the LDF, according to DIN 1946-2 and -4,
`up to 10 CFU/m3 (CFU – colony forming units) may be
`contained in the airflow following the filter emissions of
`the ventilation equipment (around 10 cm apart) in an
`empty, unused, and previously cleaned and disinfected
`operating theater [15], [16]. The pathogens released by
`the operating team, which cannot be fully removed by
`the LDF, are added to this. Thus, the ov