Command of the chain of sterilisation

According to the 9^th edition of the Pharmacopoeia française : « The efficacy of the method of sterilisation depends on the initial count of contaminating micro-organisms. All stages of the production, or of the preparation of the material to be sterilised, are carried out in such a way as to reduce the contamination as far as possible. The rules of personal hygiene and those of the workplace are respected in order to decrease the risks of contamination. Handling is reduced as far as possible. »

The chain of sterilisation is the set of the procedures to be carried out from the end of the use of an instrument until it is ready for re-use in a sterile condition (^{table 1}). There are three main phases : pre-disinfection/cleaning, sterilisation and storage (together with maintenance of the sterilisation register).

Pre-disinfection

This is the basic stage which facilitates cleaning as well as assures protection for the staff (^{Prost et al., 1998} ; ^{Goulet, 2001}). It is performed immediately after the patient's treatment has been completed. A standard product for decontamination is poured into a watertight container that is provided with a perforated basket and a lid.

For small instruments (e.g. burs, cups, ultrasonic inserts), it is necessary to use small, perforated receptacles specifically for the purpose.

Preparation of the decontamination solution

The product used for decontamination is diluted in accordance with Afnor standards. If two standards are specified, use the higher concentration.

Manufacturers provide information on the dilution required. The solution can be prepared a short time in advance and can be stored in a bottle for several days without significant deterioration. Used solutions remain effective for one day.

The solution must be changed as soon as it becomes contaminated.

Post-treatment decontamination (fig. 1, 2 and 3)

It is essential to wear a mask and household gloves or overgloves. The soiled instruments are transferred to the container without touching them (by using appropriate tongs). Hinged instruments (e.g. scissors, forceps) should be disassembled if possible, otherwise left open. Instruments must be totally immersed and the lid of the container closed.

After a minimum soaking time of 15 minutes, the whole container is taken to the sterilisation room (wet room) where the instruments are to be cleaned.

Cleaning

The objective of cleaning is to lower the level of initial contamination as far as possible. This stage is essential to guarantee the efficacy of the final sterilisation process (^{Brisset and Lecollier, 1997}).

The cleaning of the instruments is undertaken in the wet area of the sterilisation room.

To be effective it must fulfill four conditions :

- a product should be used that has a chemical action to dissolve stains (this usually means an alkaline surfactant) ;

- there should be a mechanical action consisting scrubbing to remove debris ;

- heat should be applied to assist the cleaning ;

- sufficient time should be allowed for the cleaning.

Manual cleaning

It should not be assumed that cleaning must always be undertaken manually. This method raises the risk of infection from, for example, needle stick injuries and cuts. Also, the time taken is significant and not reproducible, based only on human reliability. It is necessary to reserve this method for objects not capable of being washed by machine or by ultrasonic baths, to some hollow instruments (e.g. aspiration cannulas), to spatulas soiled with cement and possibly to hinged instruments (scissors, forceps).

Ultrasonic cleaning

Cleaning by ultrasonic methods is effective and is used extensively in dental surgeries, but drying is not ensured. The principle of this type of cleaning is founded on three combined actions :

- the action of waves emitted by a transducer, with alternation of low pressure (with production of millions of microbubbles, i.e. cavitation) and of high pressure (implosion of the bubbles on the surfaces of the instruments causing the detachment of debris) ;

- thermal action, by raising the temperature in the bath to between 30 and 45 °C ;

- chemical action, by the use of a detergent and disinfectant solution. The simplest thing is to choose a product that is effective at the same concentration for ultrasonic baths as it is for pre-disinfection.

The method is as follows :

- the preparation of the bath is the same as for pre-disinfection baths ;

- the instruments (in one or two layers) are completely immersed (^{fig. 4}) ;

- the ultrasonication is maintained for 4 to 15 minutes. The time depends on the number and the type of instruments as well as the power of the apparatus ;

- the instruments are then rinsed for 5 minutes and fully dried.

The solution in the bath is renewed daily or as soon as it becomes dirty.

Machine cleaning

The use of washing machines has the advantage of performing all stages of the pre-disinfection, including drying of the instruments. Cleaning takes place in these machines in several stages :

- a pre-cleaning stage (cold rinsing in a detergent solution) ;

- a hot washing stage, with detergent solution ;

- a rinsing stage (alternate rinsing in hot and cold water), essential for the elimination of traces of disinfectant ;

- a drying phase.

Packaging

The packaging guarantees that sterility of the equipment is maintained over time and ensures control of the aseptic chain. It must :

- permit the action of the sterilising agent without being degraded by the permeability of the packaging material ;

- ensure that the sterility of its contents is maintained ;

- preserve the properties of the medical equipment it contains ;

- permit the removal and use of the sterilised objects in aseptic conditions.

The packaging and its constituent materials are subject to the EN 868 standard.

By preference, in surgery, instruments will be packed in rigid containers. Arranging instruments in kits means that surgery can be undertaken ergonomically and ensures that the aseptic chain remains intact.

Nevertheless, it is necessary to distinguish simple heat-sealed instrument packs and rigid containers with lids that incorporate filters and airtight seals.

Heat-sealed packs

This type of packaging is the most widespread. It permits instruments to be wrapped individually or grouped together in kits.

The packs are usually sachets made from smooth paper and plastic. They must be compatible with the method of sterilisation used. Sachets with extendable gussets are generally made from strong paper.

Sealing of the sachets or envelopes

The sachets are heat-sealed. The join, being an area of weakness that must be controlled, is 8 mm wide (^{fig. 5} and ⁶).

Two types of heat-sealer are currently available : guillotine sealers are practical but often perform less well than continuous sealers. The packs are sealed at temperatures between 160 and 180 °C.

Some sachets are glued. One should especially look at the width of glue contact because, if it is insufficient, the sachet can become unstuck.

Packaging must carry the date of sterilisation, the batch number and the date of expiry.

Expiry time

The period for which sterility is maintained within the sachets is generally accepted to be two months. However it depends on a number of factors such as :

- the method of packing (simple or double sachet). The latter can double or triple the time for which it can be maintained ;

- the material. For paper sachets, sterility is maintained for one month, for the metallic containers, three months and for the crepe paper, one day ;

- packs must be protected from light, dust and moisture. Storage in an enclosed space (e.g. cupboard, drawer) increases the time of conservation considerably.

Sterilisation

General factors

Probability of sterility

The NF EN 556 standard defines the assured level of sterility. « The product is considered to be sterile when it is free from of viable micro-organisms. » The sterility of treated items is defined in terms of the probability of there being a non-sterile item in the batch. So that a medical device having undergone full sterilisation can be labelled « sterile », the theoretical probability that a viable micro-organism is present on the device must be lower than or equal to 10^-6. It is essential that the material placed in the steriliser carries as few micro-organisms as possible.

Sterilisation and the different protocols to achieve it are defined by the following standards (^{Léglise, 1997}) :

- NF EN 554 (October 1994). The sterilisation of medical equipment. Routine validation and inspection for steam sterilisation ;

- ISO 11140-1 (July 1995). The sterilisation of health products. Chemical indicators. Part 1. General instructions ;

- NF EN 285 (February 1997). Sterilisation. Steam sterilisers. Large sterilisers of more than 54 litres ;

- NF EN 867-3 (June 1997). Non-biological systems intended for use in sterilisers. Part 3. Specifications for class B indicators intended to be used in the Bowie-Dick test ;

- EN 13060 (April 2004). Sterilisation. Steam sterilisers. Small sterilisers of less than 60 litres ;

- EN 556 (1995). The sterile state - obtained by validated processes.

Sterilising and disinfecting equipment

Several processes for sterilisation are available : dry heat, steam and gaseous chemicals (^{Ruty, 1998}). In practice, saturated water vapour (autoclaving) remains the process against which others are measured.

The ministerial circular of 20 October 1997 stipulates : « In the present state of knowledge, saturated water vapour under pressure must be the method to be employed when the object to be sterilised can withstand it […], the sterilisation must conform to the harmonised European standards currently in force […]. » In addition, circular DGS of 14 March 2001 (^{Ministry of Employment and Solidarity, 2001}) specifies that dry heat and gaseous chemicals are amongst those « processes very likely to fix residual infected material », meaning non-conventional transmissible agents (prions), and are therefore positively forbidden. The destruction of these agents will be effective (according to the same circular) in an autoclave at a temperature equal to, or greater than, 134 °C, maintained for a period of at least 18 minutes.

Given this fact, steam sterilisation with saturated water vapour must be the only method to be retained, especially as this process can be validated according to EN 554 standards.

The maintenance of a sterile state depends on the condition of the packs when the load is removed from steriliser. The packs must be dry ; if they are wet, micro-organisms can pass through pores of the paper and the contents may be contaminated. Wet, warm sachets provide an excellent support for microbial proliferation.

Therefore, a steriliser must, without question, ensure the desiccation of the sterilised items. A steam « steriliser » that doesn't have a drying cycle and does not have a vacuum pump, is referred to as a « disinfector ».

Conditions necessary for good steam sterilisation (Callanquin et al., 2001 ; Sinegre, 1997)

Purging of air

Warm moist air and cold air do not mix. Therefore, all the cold air must be evacuated from the chamber. In order to do so, the steriliser must be equipped with a vacuum pump that brings the pressure in the chamber to 0,2 millibar, total evacuation being unattainable.

The efficiency of the process depends on the method of evacuation : a single evacuation or (to be more effective), fractional evacuation involving several evacuations accompanied by new admissions of steam. These evacuations are necessary for items with complex structures, such as hollow objects or porous textiles and are essential for the sterilisation of rotary instruments where the complex internal parts will trap air (e.g. rotors, air and water tubes).

The vacuum must be maintained in an airtight sterilisation chamber. A Bowie-Dick test and the test program of the steriliser are used to monitor the effective evacuation of air. The Bowie-Dick test should be done every morning before using the steriliser.

Quality of the water

Water must be deionised.

Quality of the steam

To be most efficient, the steam must be saturated, that is to say in equilibrium with liquid water at the temperature in question. It is necessary to confirm the complete dryness of the sachets at the end of sterilisation.

Homogeneity of the load

It is necessary to avoid mixing objects of very different types in the same packs and in the same loads (e.g. compressible objects and instruments, tubes, turbines and other instruments).

Placing of the load in the sterilisation chamber

Objects must be spaced. Perforated cassettes that prevent instruments from touching each other, are especially appropriate. The sachets must be sufficiently separated from each other to permit a good circulation of steam between them.

The chamber must not be overloaded. The maximum load of a steriliser is less than 2/3 of the volume of the chamber.

Most efficient programming

The temperature used should be the highest possible, according to the load ; 134 °C assures the greatest security.

The « flash » cycle, can be used for unwrapped loads at a temperature of 105 °C. This is a rapid decontamination cycle and does not achieve complete sterility.

Sterilisation of prions

Ministerial circulars 138 and 139 of the 14 March 2001, relate to the precautions to be observed in order to reduce the risk of transmission of prions and decree the new precautions and new protocols for hygiene. The destruction of prions will be undertaken in steam autoclaves at temperatures above or equal to 134 °C, maintained for a minimum of 18 minutes (^{fig. 7} and ⁸).

Monitoring sterilisation

The monitoring process can be undertaken only after the end of the cycle and to ensure that the steriliser is functioning correctly, additional checks are taken into account (^{Builles and Goulet, 1997}). There are two arms to the process. The first concerns the functioning of the steriliser, the second the efficiency of the process itself.

Functioning of the steriliser

This involves routine tests to ensure the effective working order of the steriliser.

The Bowie-Dick test (class B indicator)

This test demonstrates successful extraction of the cold air, allowing the rapid and uniform penetration by saturated steam. At the end of the test, the chemical indicator must present a uniform colour change in its totality. This test should be done every morning before any sterilisation is carried out (as this is very costly, weekly or fortnightly tests may be satisfactory ; scientific studies on the necessary frequency should be undertaken in order to determine precisely the importance of its daily use).

Test of the vacuum seal

The equipment must have a cycle that tests how air-tight the chamber is against vacuum. The result is recorded on a printout.

Recording the parameters of sterilisation

A report on the stages of the sterilisation cycle is recorded as a graph or on a recording chart. It provides precise information on the various parameters of sterilization during the cycle ; temperature and pressure as a function of time. The diagram is analysed at the end of cycle in order to identify any anomaly in the sterilisation process. Entering the data on a computer facilitates stock control.

The efficacy of the process

Test of efficacy of sterilisation : physico-chemical indicators

Physico-chemical indicators are called upon to supplement the parametric data and to evaluate the results obtained. They are placed in the middle of the load, they measure two constants, temperature and pressure, each as a function of time. When these indicators integrate the three parameters, they are called « integrators ».

Classification of indicators

Indicators are classified as : class A, passage indicators ; class B, specific tests such as the Bowie-Dick test ; class C, variable unique (limited to dry heat sterilisers) ; class D, variable multiple ; class E, integration indicators.

Class A indicators, or passage indicators (^{fig. 9})

These are generally on the outside surface of the sachets and on adhesive bands. They are sensitive to heat and turn positive after 10 minutes at 121 °C or after 2 minutes at 134 °C, even if all parameters have not been reached. They merely indicate that the pack has passed through the steriliser. They do not show the efficacy of the sterilisation process.

Class D indicators (^{fig. 10})

These are named « multiple variable » indicators because they monitor two or more parameters. They are composed of materials such as wax, ink or reactive pigments that migrate or change colour. Chemical integrators have several zones that change progressively.

These indicators must be placed inside the packs.

The use of physico-chemical indicators

This type of indicator provides information on the conformity of the cycle and complements the parametric data provided on the printout, and the checks that confirm the proper evacuation and the airtightness of the chamber. In order to enable traceability of operations, it is advisable to use labelling appropriate to the indicator used.

Is it necessary to have an indicator by sachet or by kit (^{Missika and Drouhet, 2001}) ?

A class A indicator assures the patient that instruments have been exposed to the sterilisation process. The significance of class D indicators is to provide information on conformity with the cycle. It is important to know the strategic points where the efficiency of the sterilisation process is the most difficult to obtain. The manufacturer must provide information on these areas on request. Considering the laborious requirements for traceability, checking the sterilisation of the trays consists of placing an indicator in only one sachet. That sachet is, in turn, positioned in this strategic zone. This will effectively certify the quality of sterilisation of the other sachets. The practitioner will thus have assurance of similar sterilisation of the rest of load in that cycle. The indicator records the date of sterilisation and the batch number. On the other hand, it is advisable to place an indicator in every surgical kit or with every surgical instrument (^{fig. 11}).

This data, which has no medico-legal implication, will be kept in the sterilisation register (^{Missika and Drouhet, 2001}).

Register of sterilisation

After every batch has been sterilised, a list of instruments must be compiled showing the contents of the batch. A well-kept register of sterilisation will show :

- the batch number ;

- the physico-chemical « prion » test indicator (134 °C for 18 minutes), dated and numbered ;

- the date of sterilisation ;

- the list of instruments or kits concerned, or of the containers ;

- the summary of the printed parameters of sterilisation, for the related equipment ;

- in addition, there should be the dated and numbered result of the Bowie-Dick test for every day.

Recommendation

Especially in the case of implant, periodontal and oral surgery, the reference numbers of the surgery kit used (sterilisation number, date and load) are the basic data that should be entered into the patient's notes. For the same reason, the batch numbers of implants or other material used should also be attached.

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BIBLIOGRAPHIE

• Afnor. NF EN 556. Stérilisation des dispositifs médicaux - Exigence pour les dispositifs médicaux étiquetés « stériles » (indice de classement). La Défense : Afnor, 1995.
• Afnor. NF EN 554. Stérilisation des dispositifs médicaux - Validation et contrôle de routine. Pour la stérilisation à vapeur d'eau (indice de classement S98-107). La Défense : Afnor, 1995.
• Afnor. NF EN 285. Stérilisation - Stérilisation à la vapeur d'eau - Grands stérilisateurs. La Défense : Afnor, 1997.
• Afnor. NF EN 29001 (ISO 9001). Système qualité - Modèle pour l'assurance de la qualité en conception/développement, production, installation et soutien après vente. La Défense : Afnor.
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