Updated review of wet pack complications in pulse vacuum pressure steam sterilisation processes in central sterile supply departments

Abstract

The quality of sterilisation and disinfection in a central sterile supply department is directly related to the quality of the hospital services and the patients’ safety. Wet packs occasionally occur following the process of pressure steam sterilisation; reducing this occurrence is an important issue. Therefore, the causes of wet pack following sterilisation were analysed to identify the influencing factors and suggest improvements to prevent its occurrence. Understanding the sterilisation process and possible causes of exposure helps with risk assessment and identifying necessary corrective measures.

Keywords

Wet packs central sterile supply departments pulse vacuum pressure steam sterilisation control measures

Introduction

Medical device-related infection is an increasingly prevalent problem, greatly increasing the costs and burdens of healthcare systems.¹ Medical equipment in wet packs must be repackaged and re-sterilised according to regulations, which increases the costs and causes significant waste of both materials and time; each sterilisation cycle, including loading and unloading, takes approximately 2 h. In addition to increased costs, this labour-intensive task affects operations, supply centres and clinics.² High-quality cleaning, disinfection and sterilisation of reusable medical devices is vital.³ The central sterile supply department (CSSD) is the key responsible unit.^4,5

Table 1.

Causes and solutions of wet packs.

Cause of wet pack		Solution
Device causes	Performance of vacuum pumps. Malfunction of the steam drain valve of the steriliser. Inner chamber baffle deformation. The drying system malfunctions and the drying time is too short. Water vapour separator malfunction. Unstandardised operation.	Before packaging, the devices and articles should be cleaned and dried in the drying cabinet or with low-fibre wadding cloth. The devices and dressings should be placed reasonably during packaging, and the lumen devices should be coiled to keep the lumen unobstructed. The package should not be too large and heavy, according to the requirements of the specification, the binding tightness is appropriate. Sterilised items should be loaded on sterilisation shelves or blue baskets; the loading quantity should be according to the specification. After sterilisation, the item in the steriliser should be unloaded after they are completely cooled. The storage area for sterilised item should be equipped with air conditioning device to keep the relative humidity below 70% and the temperature below 24 °C. If the combustion boiler provides pressure steam, sufficient time should be allowed to drain the condensed water and preheat the steriliser before sterilisation operation. It is indispensable to maintain the equipment well.
Packaging	High number of pieces of equipment in one pack. Instruments and pots and pans are not packaged individually. Poor package preparation. Residual moisture. Moisture absorption in the package.
Loading	Incorrect loading position of sterilisation items. The sterilised item is tightly attached to the inner wall or furnace door of the steriliser. The capacity of sterilisation packaging is too tight and dense. Pots and bowls are not placed diagonally and the opening is not facing one side. The items packaged in paper bags or paper plastic bags are not placed on their sides and have gaps.
Unloading	The unloading items are placed at the cold air outlet, and the cooling time and method are incorrect.
Steam quality	High steam moisture content. Poor insulation of the steam pipeline of the steriliser. Design issues for steam pipelines.

Table 2.

Primary causes and control measures for wet packs.

Causes	Control measures
Device issues
Inadequate pre-heating of the steam steriliser	Ensure the steam steriliser is fully pre-heated before use
Obstructed steam discharge pipe	Regularly inspect and clear the steam discharge pipe
Unclean inner wall of the sterilisation cabinet	Clean the inner wall and filter net regularly
Steam quality issues (e.g. wet or insufficient steam)	Monitor steam quality and maintain a stable supply
Sudden drop in steam pressure	Ensure stable steam pressure during sterilisation
Lack of operator training	Provide professional training for operators
Packaging
Incorrect packaging methods	Follow standardised packaging guidelines
Use of inappropriate absorbent materials	Use absorbent materials as per standardised guidelines
Loading
Non-standard loading methods	Follow standardised loading methods
Mixing different types of loads	Avoid mixing different types of sterilisation loads
Unloading
Items not completely cooled	Allow items to cool completely before unloading
Steam quality
High humidity	Use dehumidifiers and maintain optimal room conditions
Insufficient steam dryness	Use saturated steam with a dryness value of not less than 95%
Incomplete drainage of condensate from the steam supply pipeline	Install a trap valve assembly on the main steam line to collect condensed water
Unstable supply of water and electricity	Ensure a stable supply of water and electricity during the operation of the sterilisation equipment

Pulse vacuum pressure steam sterilisation is a common and effective technique.6 This method has the advantages of low cost, intense steam penetration, a short sterilisation cycle and no chemical residues or by-products. Furthermore, it is considered one of the safest, most reliable, economical and environmentally friendly sterilisation methods.⁶ Steam has strong penetration abilities, effectively eliminating microorganisms. In addition, vacuum pressure steam sterilisation causes minimal damage to sterilised items and keeps them dry, while producing thorough sterilisation.⁷

Residual moisture inside or outside a sterilised package is termed ‘wet pack’⁸ and is a risk factor for microbial contamination from the external environment to the packaged item. The selection of packaging materials, loading method, steam quality and the item being sterilised impact whether wet packs occur, and choosing effective procedures can effectively control its incidence.^9,10 The overall occurrence of wet packs is low at approximately 1%.^11,12 Studies on the problem of wet packs have been reported,^13,14 but a comprehensive overview is lacking.

Method

Pulse vacuum pressure steam sterilisation is commonly used in hospital CSSDs for the wet disinfection of heat-resistant surgical instruments. When utilising a fluctuating pressure steam system, the first step¹⁵ is to use a vacuum pump to place the sterilisation container under negative pressure and thoroughly eliminate cold air. Next, saturated steam is released into the sterilisation chamber containing the load to be sterilised. The hot steam quickly disinfects the items loaded inside the chamber; however, the heat load must be maintained to reach the appropriate sterilisation temperature and pressure. The process is a heat exchange: saturated steam comes into contact with the loaded items’ surfaces, increasing the surface temperature, and then condensing into water. The appropriate sterilisation time wherein the steam is in contact with the items to be sterilised is based on the denaturation of bacterial microbial proteins.

During the cycle, condensed water is discharged from the inner chamber of the steriliser prior to the sterilisation stage before the remaining water evaporates and the sterilisation procedure is completed with the drying stage.¹⁶ In this last stage, the vacuum pump utilises the physical phenomenon that water boils at lower temperatures as the pressure decreases. As the vacuum strength increases, more condensed water is converted into water vapour and extracted by the vacuum pump, and the sterilised load is thoroughly dried.^16,17 However, wet pack occurs when the condensed water in the sterilised packages does not evaporate completely (Table 1).¹⁸

Findings

Packaging with non-woven fabrics, paper or plastic has certain advantages, including moisture resistance and excellent air permeability, ensuring good steam penetration.¹⁹ However, they have significant disadvantages regarding maintenance costs and sterile storage compared with traditional materials. Despite this, an increasing number of hospitals use these packaging materials to replace traditional textile packaging materials.²⁰

Based on the size of the steriliser, the weight of a load before sterilisation is approximately 50 kg. If the total load weight increases by >1% or the weight of a single metallic instrument increases by >0.2% after sterilisation, it is identified as a wet pack.²¹ An external wet pack is noticeable when visible water stains or droplets are present on the package's exterior, or there is noticeable wetness coupled with an increase in the package's overall mass.²²

The Association for the Advancement of Medical Instrumentation's ST79 standard states that if there is moisture on the surface or the inside of two or more sterilised packs, the whole batch should be regarded as a wet batch and, therefore, as contaminated.²³ The incidence of wet packs varies from 0.093%²⁴ in India to 1.46% in China but can rise to 14%–19.7%^25–27 for non-standard loading of paper–plastic packaged batches.^28,29 Therefore, preventing and controlling the incidence of wet packs remains an urgent problem in many sterile supply centres.

Problems and control measures

The common problematic areas within pulse vacuum pressure steam sterilisation are shown in Figure 1.

Figure 1.

Schematic diagram of faulty areas in pulse vacuum pressure steam steriliser.

The following problem areas were identified and suggestions for control measures formulated (Table 2):

Pre-heating

Pre-heating the steam steriliser is essential; failing this, water may remain in its interlayer, the steam discharge pipe may become bent or blocked, or its trap valve may be obstructed. These events can obstruct the discharged steam and may affect the discharge of condensed water. In addition, if the inner wall of the sterilisation cabinet is not clean, particles in the filter net will not be removed timeously, which can block the vent and filter net, preventing the removal of condensed water and reducing the vacuum rate. Poor drying can result, therefore increasing the occurrence of wet packs.³⁰

Temperature

In practice, steam and sterilisation temperatures may be different. When the steam contains >3% condensed water (a dryness level of <97%), its temperature will gradually decrease as it passes through the condensed water film, thus lowering the temperature and resulting in ineffective sterilisation. The presence of air can also impact the steam's sterilisation temperature by forming a ‘cooler bubble’, preventing the cycle from achieving an appropriate temperature and leading to sterilisation failure.

Packaging

Incorrect packaging and material selection have a vital impact on the effect of pressure steam sterilisation. Packaging too tightly prevents steam from escaping after sterilisation, while the sparseness of perforation size in trays can cause water to accumulate at their corners. Water-absorbent materials in packages, such as cotton, disposable medically absorbent paper and other materials, may soak up water. Furthermore, if an absorbent towel extends beyond the edge of a basket, it will absorb excessive water, particularly if it is repeatedly folded. Subsequently, the heat released when metal instruments are cooled may be insufficient for drying them completely, causing wet packs.

Cleaned instruments, utensils and dressings must be dried thoroughly before sterilisation. In addition, stacking containers without using absorbent cloth or paper for separation and not placing plates, bowls and basins upside down is not conducive to steam penetration. For surgical instruments stacked directly without baskets, overlapping and packing loads that are too large, heavy or tight increases the load density, producing resistance to steam penetration and affecting late-stage drying.

Furthermore, sterilisation processes in which surgical instruments are unpackaged and placed in a rectangular box may cause water droplets to aggregate in the container and become trapped. As such, the water does not evaporate in the drying stage, resulting in wet packs.³¹ In addition, complex surgical instruments, especially those with shafts and lumens, can easily retain water.³²

A device's functioning should be carefully checked beforehand to ensure it is intact. Surgical instruments should be placed in baskets or perforated trays for supportive packaging and should be evenly distributed. All utensils should be opened in one direction and separated with blotting paper. Lumen instruments should be wound to keep the lumen unobstructed, and two layers of packaging material should be used for double packing, ensuring a sterilised and smooth bag surface. The size and weight of the sterilised package should be noted to ensure that it meets the requirements.

Attention should also be given to selecting the packaging materials, as they provide a sterile barrier to maintain sterility before use.³³ The selected materials should adequately cover the articles and be tightly wrapped to prevent cracks, rolls or pores. Finally, the packaging should allow steam and air in but prevent bacteria from entering.³⁴ This paper does not consider whether or not using fungicides is an important factor.

Loading

Non-standard loading is a crucial factor that can lead to wet packs.³⁵ If the loading method is incorrect, instruments can accumulate under gravity, and the condensed water formed during the sterilisation process can concentrate, leading to incomplete drying; this occurrence is especially pronounced if different load types are mixed in the same batch, for example, allowing condensed water to drop onto fabric dressing loads. Open containers must be placed with their opening facing downwards to prevent water collection. If too many loads are added, the smooth entry and discharge of steam is prevented, thus reducing the sterilising effect and the drying process.

All surfaces of items should be directly exposed to steam, and free steam flow should be ensured around each item.³⁶ Dress-type loads or metal instrument bags should be placed vertically without overlapping. A gap of >2.5 cm should be left between each load to facilitate air exclusion, steam penetration and drying. During loading, the devices’ packaging should not touch the steriliser wall.³⁷ For paper–plastic bags, sterilisation loading racks can effectively control the wet pack problem. During sterilisation, utensils and instruments of the same material should be placed in the same batch. In the case of mixed items, the loading sequence should follow the loading principle: from top to bottom and from light to heavy (e.g. textile, paper and plastic on the top and metal instruments on the bottom).

Steam quality

Approximately 60% of equipment-related causes of wet packs can be attributed to steam problems.⁶ For example, steam superheat is an important factor; the European standard EN 285 requires that the superheat temperature does not exceed 25°C.¹⁹

High humidity, insufficient steam saturation, irregular boiler room operation, adding water to the boiler when injecting gas, high gas-water content, steam with high water content and incomplete condensate drainage from the steam supply pipeline before sterilisation may produce unsuitable steam. Pressure steam sterilisation requires saturated steam with a dryness value of >95% to sterilise medical instruments.³⁸ Insufficient steam dryness (i.e. excessive water content) can lead to the premature humidification of the load, which will affect the temperature increase and prolong the time to reach the sterilisation temperature. If the steam supplied is too wet or insufficient, pressure instability and an increase in the amount of non-condensable gas can occur, causing wet packs. In addition, the surface wetting of packaging materials affects the steam penetration and drying processes, thus reducing the sterilisation effect.³⁹ In addition, a sudden drop in steam pressure of <210 kPa during steam sterilisation can cause wet packs.

Professional training on the theory and practice of the sterilisation equipment is mandatory. Attention should be paid to the layout of pipelines when installing a new pressure steam steriliser. In addition, equipment maintenance should be performed strictly according to the manufacturer's operating instructions, including vacuum pump maintenance. Traps should be cleaned regularly, and pressure steam sterilisation parameters should be inspected annually.⁴⁰ During the sterilisation process, operators must closely monitor critical parameters, including the time of sterilisation and the equipment's temperature and pressure.⁴¹

For sterilisation equipment connected to an external steam source, professional personnel should regularly inspect the evaporator to identify and rectify problems early. A steam separator must be installed at the evaporator outlet of the boiler to filter the condensed water, and a trap valve assembly should be installed on the main steam line to collect this water. The equipment's liquid-level detection device should be cleaned regularly to ensure the accuracy and sensitivity of the sensor. During the operation of the sterilisation equipment, a stable supply of water and electricity should be guaranteed to ensure the quality of the steam.

Unloading

After unloading, sterilised items not completely cooled must be placed in a cold, ventilated area. Contact between high-temperature sterilisers and low-temperature air can cause wet packs. Therefore, after the sterilisation cycle is complete, items should be unloaded and stored correctly.⁴² A package should remain inside the steriliser cabinet for at least 30 min for cooling to below 60°C. Next, the operator should open the steriliser unloading door, remove the sterilised items from the interior part of the chamber, place them in a position without cold air ventilation and let them cool naturally to room temperature. The operator must avoid touching or handling the package during this time to ensure sufficient drying. Depending on the package and environmental temperature requirements, sterilised items should be cooled within 2 h. After cooling, a sterilised bag with water droplets inside or outside should be considered a wet pack and not stored or issued. Meanwhile, the cause of the wet pack should be identified. Dry sterile bags should be kept in the storage rack with a layer of sterile cotton cloth.

Footnotes

Acknowledgements

Not applicable.

Authors’ contributions

HQY and TJ conceived of the study, and ZJ and TH participated in its design and data analysis and statistics and HY and HGX helped to draft the manuscript. All authors read and approved the final manuscript.

Availability of data and materials

All data generated or analysed during this study are included in this published article.

Consent for publication

Not applicable.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Ethics approval and consent to participate

An ethics statement is not applicable because this study is based exclusively on published literature.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: Fund project: 2021 Scientific research project of Xiangtan Medical Association (order number:2021-xtyx-17). Funding agencies did not play a role in study design, data collection, analysis and interpretation, and manuscript writing.

ORCID iDs

Yan Huang

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