Validation of disinfection procedures in aseptic manufacturing


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A technical discussion of the available data and viewpoints

At present, the topic of disinfection, also outside of the production of critical products such as pharmaceuticals, medical devices or cosmetics, has moved more into the focus of society. However, the use of disinfectants alone does not automatically lead to the desired result, a sufficient reduction of microorganisms and thus to a minimisation of risk for people and products. This goal can only be achieved if the application is correct and suitable for the intended purpose. This is currently shown by the large number of misapplications by untrained laypersons in the context of corona control. But even among experts there are always different views on correct applications, depending on the areas of use, the regionally valid testing methods and applicable guidelines.
Due to such differing views, the following discussion arose between three practitioners in the field of disinfection on an important aspect of the disinfection procedure: the exposure time.

Thorsten Hinken:
When searching for "exposure time" on the internet, I first came across an article from 2017 (, in which you, Axel, state that the EN standards do not consider it necessary to keep hands moist, although drying is unlikely due to the test procedure under "normal" conditions. However, keeping hands moist is definitely required for hand disinfection (30 sec. for hygienic hand disinfection). Similarly, the understanding of an effect in the constant presence of liquid disinfectant is much more understandable and logical. In EN13697, although the amount of disinfectant for testing is given as 100?l and the area is given as a round coupon of 2cm diameter, the liquid is not distributed evenly but is given as drops onto the previously dried microorganisms. Thus, drying is unlikely within 5 -15 minutes even under high air exchange rates, especially if the surface tension of the liquid is high. For this reason, among others, many users and auditors demand a completely humid contact time.

Axel Schroeder:
I agree with you that the topic is still relevant, and disinfection issues are of course receiving even more attention in the current phase of COVID-19 and Annex 1 revision. As far as dampening is concerned, I don't think it is correct to lump all applications together and generally require the surface to be dampened. Let's first look at the hand disinfection you mentioned, where keeping the hands moist is required for 30 seconds. 30 seconds is usually not a problem with a sufficient amount of alcohol, i.e. ? 3ml for 2 average hands. Neither the water nor the alcohol content nor the other active ingredients (QAV or others), depending on the product, evaporate during this time. In order to achieve the desired effect on the relatively difficult and not smooth surface "skin", this makes sense. But hand disinfection, as an antimicrobial treatment of a biological, i.e. living and animated surface, can only be compared with the disinfection of "technical" surfaces to a very limited extent.

However, let us now also consider classical surface disinfection. If the disinfectant is applied properly to the surface - I believe the manufacturer's recommendations are 20-60 ml/sqm (Rheinbaben/ Wolff, 2002) - a continuous moist film is formed that encloses any microorganisms that may be present. In this phase, the active ingredient is transferred to or into the microorganism and subsequently takes effect, e.g. at the cell membrane or in cell metabolism. Under normal environmental conditions, the wetting time is sufficient for this "transition phase", i.e. when drying takes place, the active ingredient is already where you want it to be. This is where it will continue to act after the surface has dried. In my opinion, one-time correct wetting should be the standard solution and other procedures the exception. That at least protects the environment and the wallet and minimises the handling of hazardous substances.

Thorsten Hinken:
Then let's talk about surface disinfection. Manufacturers say an amount between 20ml/m² and 80ml/m² should be applied. Below 20ml/m² a sufficient effect is unlikely and above 80ml it cannot be increased. (Disinfectant quantity and effectiveness in surface disinfection, K. Bansemir, Swiss Med 7 (1985) No. 3 b, 36-39). Is it now up to the user to determine what amount of liquid is sufficient to achieve the desired effect? Should the user carry out tests for drying and then determine whether drying after 5 min still has an effect on yeasts, although the test according to EN13697 specifies 15 min here? Similarly, the extrapolated amount of disinfectant in EN 13697 (2cm coupon 100?l = 100?l/3.14cm² = 318ml/1m²) is 318ml/m². The only chance to allow early drying with laboratory tests is the validation according to EN16615. Here it is possible to achieve a realistic simulation with your own cloth (mop) and your own surfaces.

If this validation does not exist, the surface must remain moist during the exposure time.

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Axel Schroeder:
In my opinion, we first have to clarify the terms for exposure times, wetting times and contact times. For me, the exposure time would be the time that an applied active substance or a mixture of active substances (i.e. it has reached the microorganism and is "effective") needs to reduce a given initial bacterial count by a desired factor.

The wetting time would be the time that a surface remains wet and during which application to/into the microorganism occurs.

The contact time would be the time of actual contact of the active ingredient with the microorganism - in the case of non-volatile active ingredients such as QAV, this would be the wetting time + the further exposure time until the microorganism is killed. Once applied to/into the cell, the active ingredient remains in contact and active. In the case of reactive active substance solutions, e.g. H2O2 or peracetic acid splitting agents (e.g. in powder form for preparation), on the other hand, the contact time would only be equal to the wetting time, since the chemical process of active substance formation ends with drying.

How long the contact time is generally depends on various factors, e.g. the initial bacterial count, the temperature and, if applicable, the surface structure and the type of microorganism. (Wallhäuser 1988; I. Schwenke, RKI 2017; ). Accordingly, the assessment of the initial situation - which bacterial counts are to be expected (data from monitoring/qualification), which surfaces are used, is there also contamination and are there temperature deviations - is of decisive importance. Therefore, the sense of obligatory exposure times as you mentioned (EN13697 - fungi 15 minutes) could be questioned. The authors of EN13697 have also recognised this and have removed both the mandatory 5 minutes for bacteria and the 15 minutes for fungi in the latest version of October 2019. The exposure time is now defined between 1-60 minutes by the manufacturer.

This brings us back to the situational approach - I evaluate my framework conditions process, equipment, environment, historical data and requirements for limits or specifications, follow the principle of quality risk management - as recommended in relevant guidance documents such as Annex 1 or ICH Q9 - and then decide whether there is an increased risk based on the drying time in certain cases. I simulate this in a test and/or with monitoring and derive from the data obtained whether the classical approach - apply disinfection, let it dry - is sufficient or whether special circumstances (e.g. disinfection in the form of a reaction solution) require rewetting, increasing the quantity applied/sqm or increasing the concentration.

Thorsten Hinken:
I can only agree with your comments now. But with this we have only shifted the problem from the manufacturer of the disinfectant to the user.

I hope that in the future further studies will take place that provide one or the other proof that the effect of a non-volatile, or even that of a volatile disinfectant, once it has reached the cell, is sufficient to kill the micro-organism. The point that now remains is to determine the time that is sufficient to transport the disinfectant into the cells - i.e. your "actual contact time". Is this a few seconds or minutes?

Axel Schroeder:
Maybe a different perspective will help us. Anastasija, you work on the validation of disinfection procedures every day. How do you see it?

Anastasija Schlicht:
At this point I would like to contribute a few more comments or additions. The topic of exposure time (contact time, moisture retention time or whatever you want to call or define the time) has been a much-discussed topic for some time now. In fact, no matter how you approach it, you cannot make a definite statement on whether surfaces need to be kept moist for long periods of time or not, due to a lack of data.

The easiest way to avoid this problem would of course be to use products for the disinfection of surfaces that have the shortest possible contact time (especially if the surfaces are walked on or used again quickly after disinfection). So far so good! If it weren't for the problem that the shorter the exposure time, the higher the concentrations of active ingredients that have to be used. In the worst case, this means that unnecessarily high concentrations of active substances are used at the expense of the staff, the environment and, last but not least, the material to be disinfected, even though it would be possible to do it differently.

Axel, you have already explained that the respective standards (e.g. EN 13697) do not require surfaces to be kept moist or to be rewetted, and with the high quantity of disinfectant applied, at least in laboratory tests, drying during the exposure time is almost non-existent. In the case of disinfectants that evaporate without leaving residues (e.g. alcohol-based disinfectants), care should certainly be taken to ensure that the surfaces are kept moist during the entire exposure time, as otherwise it will probably not be possible to achieve a sufficient effect (which is usually not a problem, however, because alcohols also manage with short exposure times). With other disinfectants, it should (hopefully) be sufficient if the surface is completely wetted, at least initially. The actual killing of the micro-organisms still takes place when the surface is already dry again; at least that is the theory so far, which I also currently hold.

Ultimately, however, I wonder whether all the discussions about exposure times and keeping surfaces moist are not purely academic. From a scientific point of view, it is of course very interesting to know when and how long a disinfectant needs to kill microorganisms or whether keeping surfaces moist is decisive for this (this is exactly the reason why we at Labor LS have been dealing with this topic for a long time now and hope to be able to publish something solid in the medium term). However, what exactly is the discussion supposed to achieve in practice? On the one hand, the efficacy of disinfectants is demonstrated in laboratory tests to prove the basic suitability of the products. But no matter if a disinfectant manufacturer writes on his product that the product has a "contact time" of 30 minutes and that the surface must be kept moist during this entire time (or not), all these laboratory tests will never be able to reflect the practical conditions at the user's premises (with regard to surfaces, environmental isolates, ambient temperature, air exchange rates, etc.).

And even if the user can show extensive disinfectant validation, this will not be able to replace daily monitoring. This is the only way to prove beyond doubt that the daily application of disinfectants - with all its imponderables - is actually suitable for keeping microbiological contamination of surfaces in check. And if I can show reasonable data in hygiene monitoring with my established cleaning and disinfection procedure with surfaces that have not been kept moist so far, that should be the only relevant factor that matters.

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Nevertheless, I hope to be able to shed a little more light on the subject soon. Who knows, maybe I will be proven wrong on the basis of our test data. And in the meantime, the following suggestion might be a practicable alternative, as already suggested by you, Axel: I look at how long my surfaces stay wet under real conditions in my company and check the effectiveness of the disinfectants in laboratory tests after exactly this time. If the effectiveness is then achieved in all cases, I no longer need to prepare an argument for the next inspection for keeping surfaces moist.

Axel Schroeder:
I think we have reached a point here where it becomes clear that no generalised recommendation can be made at present that is suitable for every user or for every production process. In the final analysis, the pharmaceutical manufacturer as the user bears the responsibility and will have to determine through a well-founded risk analysis, based on the spatial conditions, type and application of the disinfectants and equipment and its process sequences and with the help of its validation, whether longer-lasting dampening is necessary in certain cases beyond the classic procedure. Let us hope for new data soon that will help us in the future.

About the Authors
Dipl. Biol. Thorsten Hinken has been working for Ecolab in the life sciences sector since 2006 as an account manager for the pharmaceutical industry, manufacturing pharmacies and cosmetics manufacturers.
Anastasija Schlicht is the head of the department responsible for disinfectant testing at Labor L+S AG. In addition, she is accredited as an expert for the preparation of disinfectant reports at the Association for Applied Hygiene (VAH).
Dipl. Biol. Axel H. Schroeder has been working at CONCEPT HEIDELBERG since 2008 and is head of the microbiology department.

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