Analysis of Wet Pack Incidence in Steam Sterilization: A Study in a Chinese Medical Center

Abstract

Background

Central sterile supply departments (CSSDs) play a vital role in hospital infection control. We investigate the factors associated with wet pack occurrence after steam sterilization.

Material/Methods

We designed a log sheet to record information concerning sterilized packs. The data included the type of sterilized pack; outside weather (sunny, overcast, or rainy); the item in the sterilized pack; packaging material; whether the item had been packaged in compliance with guidelines; whether the pack had been laid flat, upright, or leaning at an acute angle; which sterilizer was used for sterilization of the pack; whether the pack had been placed on the top or bottom shelf inside the sterilizer chamber; whether the pack had been loaded in compliance with guidelines; the drying time following sterilization; and cooling time after sterilization. The sterilized packs in our study were selected from all of the packs that were sterilized in the CSSD of the authors’ institution during June to December 2021.

Results

Factors associated with wet pack occurrence after steam sterilization include: outside weather on the day of sterilization; the item in the sterilized pack; packaging material; whether the item had been packaged in compliance with guidelines; whether the pack had been placed on the top or bottom shelf; and cooling time after sterilization. Statistically significant differences (P<0.05) in wet pack incidence were identified for all of these factors.

Conclusions

Various factors are associated with wet pack occurrence after steam sterilization. Recommendations for reducing the risk of wet packs include regular maintenance of the steam pipeline, regular replacement of thermal insulation materials for the steam pipeline, and extension of the drying time.

Introduction

Central sterile supply departments (CSSDs) play a vital role in hospital infection control [1]. Steam sterilization is the preferred method for heat- and moisture-tolerant surgical instruments, with the prevention of the occurrence of wet packs being a key criterion of CSSD quality control. A study conducted in 125 hospitals showed that wet packs occurred in 78% of the surveyed hospitals and the frequency of occurrence of wet packs varied significantly [2]. As noted by Seavey [3], wet packs constitute one of the most complex and time-consuming problems in sterilization. Studies by Sun et al [4] and Zhou [5] have shown that the rate of wet pack occurrence varies significantly in China, ranging from 1.67–19.7%. Basu [6] reported that the incidence rate of wet packs in eastern India is around 1%. Seavey [3] found that the bacterial culture positive rate in the central part of wet packs can be as high as 50%. Due to the wet environment inside and outside the pack, a channel connecting the inside and outside environment enables microorganisms to enter the pack due to the siphon principle, rendering the pack susceptible to secondary contamination, leading to hospital infections. The occurrence of a wet pack should be regarded as a sterilization failure [7] that can delay the regular supply of sterile instruments, affect surgical procedures, and undermine the quality of diagnosis and treatment. Moreover, repeated sterilization of the same instruments may affect the properties of instrument materials [8,9] and constitutes a waste of hospital labor, materials, and financial resources. In recent years, researchers worldwide have been paying attention to wet pack occurrence following pressure steam sterilization. Affected by various factors, wet pack occurrence has not been effectively solved. It is still a thorny issue, causing concern for disinfection and sterilization in hospitals. Wet packs can be caused by several factors, such as steam quality (60%), equipment performance (30%) and loading defects (10%) [10]. The results of Panta et al [11] have shown that approximately 60% of the wet packs caused by equipment factors can be attributed to steam problems. The results of He et al [12] have shown that when steam supplied by the boiler system is too wet or insufficient, the pressure can become unstable and the non-condensable air can result in the occurrence of wet packs. During sterilization, factors such as pressure sensor failure, water drainage filter net clogging, or an incomplete door gasket in the chamber can prevent steam evacuation from the sterilization chamber and lead to steam dampness. All of this can result in wet pack occurrence, accounting for approximately 30% of wet pack incidence [13]. The study of Huang et al [14] shows that if instruments packaged using ordinary cotton wraps are placed at random or stacked up during loading for sterilization, a large amount of condensed water will be generated in the upper packs and drop on the lower packs, resulting in wet pack occurrence. In this study, we investigated the factors associated with wet pack occurrence after steam sterilization and prescribe preventive measures for reducing the risk of wet packs.

Material and Methods

Study Setting

A total of 4099 sterilized packs were selected by quality controllers from all of the packs that had been sterilized in the CSSD of the authors’ institution during June to December 2021. The packs were sampled using random cluster sampling. Only packs that had been sterilized by steam sterilization were selected. Contaminated packs and packs with damaged packaging were excluded from this study.

Survey Tools

After consulting the literature [10–14] and CSSD guidelines [15–18], we listed the possible factors associated with wet pack occurrence using root cause analysis and brainstorming. Then, 5 CSSD experts were invited via email to consult on the factors associated with wet pack occurrence. The experts were working in CSSDs at different hospitals in different provinces, possessed more than 10 years of CSSD work experience, and were members of their respective provincial-level sterile supply committees. Of them, 2 were chief nurses and 3 were co-chief nurses. Factors associated with wet pack occurrence were revised through a consensus of experts. The following factors were identified: type of pack (eg, laparoscopic instrument pack, uterine instrument pack, surgical gown pack) as shown on the label outside the pack; weather conditions on the day that sterilization took place (sunny, overcast, or rainy); the item in the pack; packaging material; whether the item had been packaged in compliance with guidelines; whether the pack had been laid flat, leaning at an acute angle (defined as laid at between a 0° and 90° angle from the shelf), or upright (at a 90° angle from the shelf); the person responsible for sterilization; which sterilizer (of 2 sterilizers at the study hospital) was used for sterilization of packs; whether the pack had been placed on the top or bottom shelf inside the sterilizer chamber; whether the pack had been loaded in compliance with guidelines; the drying time following sterilization; and cooling time after sterilization.

We designed a log sheet for wet pack occurrence following pressure steam sterilization. We recorded information for the following variables: the type of the sterilized pack, as listed on the pack label; weather conditions; the item in the pack (instrument or dressing); packaging material; whether the item had been packaged in compliance with guidelines; whether the pack had been laid flat, leaning at an acute angle (defined as laid at between a 0° and 90° angle from the shelf), or upright (at a 90° angle from the shelf); the person responsible for sterilization; which sterilizer was used; whether the pack had been placed on the top or bottom shelf inside the sterilizer chamber; whether the pack had been loaded in compliance with guidelines; the drying time following sterilization; and cooling time after sterilization. The log sheet was filled out by quick response code scan. The distribution personnel strictly inspected the sterilized packages to identify wet packs and accurately recorded the information concerning wet packs.

Criteria for Determining a Wet Pack

According to the CSSD directive (Part 2: Standard Operating Procedure of Cleaning, Disinfection, and Sterilization) released by the National Health Commission of the People’s Republic of China [15], a wet pack is characterized by visible moisture or water droplets found in or on the pack after sterilization and cooling. According to the Technical Requirements for Large Steam Sterilizers – Automatic Type released by the Standardization Administration of the People’s Republic of China [16], a pack is considered wet if any of the following conditions are present: its wrapper feels wet; visible moisture or evident water staining is found on the wrapper or on the chemical indicator tape; water droplets, water mist, or water puddles are present inside the pack; or the dressing was moist after sterilization. The Association for the Advancement of Medical Instrumentation [17] and Centers for Disease Control and Prevention [18] of the United States define a wet pack as one in which moisture, water droplets, or water puddles are left in or on the pack following a complete sterilization cycle.

Based on the above-mentioned definitions, a pack was considered wet in our study if any of the following conditions were met: its wrappers felt wet; visible moisture or evident water staining was found in or on the wrapper or on the chemical indicator tape; presence of water droplets, water mist, or water puddles in the pack; or moist dressing after sterilization.

Data Collection

The log sheet for wet pack occurrence following pressure steam sterilization was used for collecting the data concerning wet packs. Data entry was double checked by 2 researchers. Training on relevant knowledge, work procedures and evaluation criteria was provided for CSSD staff members who participated in this study to achieve consistency and accuracy in the study and to strictly control any deviation from study protocol.

Statistical Analysis

IBM SPSS version 25.0 was used for data analysis. Categorical data are described as N. Pearson’s chi-square test was used to identify differences. The odds ratio (OR) and 95% confidence interval (CI) for each variable were analyzed by binary logistic regression to assess which factors were correlated with wet pack occurrence. A Pearson’s chi-square test was used to determine the factors associated with wet pack occurrence. All variables that were significantly correlated with the outcome (identified by P<0.05) were simultaneously entered into binary logistic regression models to determine their correlation with wet pack occurrence. A statistically significant difference was identified by α=0.05 and a 2-tailed P value <0.05.

Results

Single-Factor Analysis of Factors Associated with Wet Pack Occurrence

Of the 4099 sterilized packs, 128 (3.12%) were considered wet packs and 3971 (96.88%) were considered non-wet packs. Statistically significant differences (P<0.05) in wet pack presence were associated with the following variables: weather, the item in the sterilized pack, packaging material, whether the item had been packaged in compliance with guidelines, whether the pack had been placed on the top or bottom shelf inside the sterilizer chamber, and cooling time. No statistically significant differences were found for the other variables (P>0.05) (Table 1).

Table 1.

Single-factor analysis of factors associated with wet pack occurrence after steam sterilization.

Variable	Description	Wet packs	Non-wet packs	Chi-square (χ2)	P value
Weather	Sunny	41	1371	24.842	<0.001
	Overcast	24	1405
	Rainy	63	1195
Item in the sterilized pack	Dressing	22	2372	92.395	<0.001
	Instrument	106	1599
Packaging material	Cotton fabrics	13	1442	48.230	<0.001
	Non-woven fabrics	75	1306
	Paper-plastic combinations	40	1223
Whether the pack had been laid flat, leaning at an acute angle, or upright	Flat	46	1842	8.922	0.012
	Acute angle	51	1119
	Upright	31	1010
Person responsible for sterilization	Nurse	25	662	0.727	0.394
	Non-nurse healthcare worker	103	3309
Sterilizer used for sterilization of packs	No. 1	34	1116	0.146	0.702
	No. 2	94	2855
Placement of the pack on the top or bottom shelf inside the sterilizer chamber	Top shelf	34	2211	42.436	< 0.001
	Bottom shelf	94	1760
Whether the pack had been loaded in compliance with guidelines?	Yes	37	1442	2.950	0.086
	No	91	2529
Whether the item had been packaged in compliance with guidelines?	Yes	82	3540	75.877	<0.001
	No	46	431
Cooling time after sterilization	≥30 min	46	2723	60.252	<0.001
	<30 min	82	1248

The above statistically significant influencing factors (weather, the item in the sterilized pack, packaging material, whether the item had been packaged in compliance with guidelines, whether the pack had been placed on the top or bottom shelf, and cooling time) were regarded as the independent variables in a single-factor analysis, with wet pack occurrence regarded as the dependent variable. Assignments of the influencing factors are shown in Table 2.

Table 2.

Possible factors associated with wet pack occurrence and their assignments.

Factor	Code	Assignment description
Weather	X1	1=Sunny, 2=Overcast, 3=Rainy
Item in the sterilized pack	X2	1=Dressing, 2=Instrument
Packaging material	X3	1=Cotton fabrics, 2=Non-woven fabrics, 3=Paper-plastic combinations
Whether the pack had been laid flat, leaning at an acute angle, or upright	X4	1=Flat, 2=Acute angle, 3=Upright
Placement of the pack on the top or bottom shelf inside the sterilizer chamber	X5	1=Top shelf, 2=Bottom shelf
Whether the item had been packaged in compliance with guidelines?	X6	1=Yes, 2=No
Cooling time after sterilization	X7	1 ≥30 min, 2 <30 min

Logistic regression analysis results show that the probability of wet pack occurrence on overcast and rainy days was 1.878 and 2.432 times higher, respectively, than on sunny days; the probability of wet pack occurrence in the instrument packs was 3.297 times higher than in the dressing packs; the probability of wet pack occurrence in the packs packaged using non-woven fabrics was 3.808 times higher than those in cotton wraps; the probability of wet pack occurrence in the packs not packaged in compliance with guidelines was 2.830 times higher than the packs packaged in compliance with guidelines; and the probability of wet pack occurrence when the cooling time of the sterilized pack was less than 30 minutes was 2.209 times higher than when the cooling time was 30 minutes or more (Table 3).

Table 3.

Multivariate logistic regression analysis of factors associated with wet pack occurrence.

Variable	Description	Beta value	Standard error	Wald χ2	P value	Odds ratio	95% confidence interval
Weather	Sunny			15.769	<0.001
	Overcast	0.630	0.213	8.724	0.003	1.878	1.236–2.853
	Rainy	0.889	0.253	12.366	<0.001	2.432	1.482–3.992
Item in the sterilized pack	Dressing	1.193	0.260	21.074	<0.001	3.297	1.981–5.486
	Instrument
Packaging material	Cotton fabrics			20.224	<0.001
	Non-woven fabrics	1.337	0.310	18.601	<0.001	3.808	2.074–6.991
	Paper-plastic combinations	−0.032	0.213	0.023	0.880	0.968	0.638–1.470
Whether the pack had been laid flat, leaning at an acute angle, or upright	Flat			0.413	0.813
	Acute angle	0.040	0.245	0.027	0.870	1.041	0.644–1.682
	Upright	−0.100	0.249	0.161	0.689	0.905	0.556–1.474
Placement of the pack on the top or bottom shelf inside the sterilizer chamber	Top shelf	0.595	0.217	7.500	0.006	1.813	1.184–2.775
	Bottom shelf
Whether the item had been packaged in compliance with guidelines?	Yes	1.040	0.200	27.028	< 0.001	2.830	1.912–4.188
	No
Cooling time after sterilization	≥30 min	0.792	0.204	15.145	<0.001	2.209	1.482–3.292
	<30 min

Discussion

The occurrence of a wet pack can undermine the effectiveness of the microbial invasion prevention barrier, leading to the contamination of surgical instruments in the pack, thus posing a threat to the lives of patients and medical staff [19]. There are a number of factors, such as sterilization equipment, steam composition, and instrument processing procedures (including drying, packaging, sterilization, storage, and delivery), which may be associated with wet pack occurrence after steam sterilization. The results of Duan et al [20] and He et al [21] show that the main causes of wet pack occurrence include non-compliance in packaging or loading and unloading of packs; insufficient cooling time; function decline or poor sealing performance in the vacuum pump; and non-compliance in terms of water, electricity, and steam provision.

Our study found that the following factors were associated with wet pack occurrence and that these factors are preventable and controllable:

1. Weather. The hospital in this study relies on a central steam supply system. The autoclave room was only 150 m away from the CSSD. The steam pipeline is buried underground and a layer of high-temperature resistance film coats the pipeline’s external surface. The film had been used for 14 years and it is possible that the film had lost its protective effect due to long-term use. A substantial temperature difference between the interior and exterior of the pipeline on an overcast day, or the occurrence of standing water on the ground where the steam pipeline is buried underground, could cause the steam to cool, resulting in the occurrence of unsaturated steam. The drying time of the sterilizer was not extended accordingly, resulting in wet pack occurrence.

2. Item in the pack. Most of the packs in our study contained surgical instruments. Condensate water was produced when steam came into contact with the instruments. However, the metal instruments did not absorb this condensate water, so condensed water was retained on the metal surface of the instruments. By contrast, the dressing packs contained textiles with the function of moisture absorption (such as pads, covers, and surgical gauze), and thus the incidence rate of wet packs in dressing packs was lower than that in instrument packs in our study. This is consistent with the results of Fan et al [22], who reported that wet packs are more likely to be found in instrument packs.

3. Packing material. Cotton fabrics, non-woven fabrics, and paper-plastic combinations were the packaging materials in our study. The packs packaged with non-woven fabrics accounted for the largest number of wet packs in our study, possibly because non-woven fabrics, which are denser than cotton fabrics, influence the permeation and evacuation of steam. The results of our study are similar to those of Kang [23] who pointed out that steam is more likely to convert to water droplets on non-woven fabrics compared with cotton fabrics due to the difference in these materials in terms of surface tension.

4. Whether the pack had been placed on the top or bottom shelf. Our study shows that the number of wet packs found on the bottom shelves was larger than that on the top shelves. This is possibly because the air inlets of the sterilizers used in our hospital were located on the lower left side of the sterilizers. Such a long-distance central steam supply might produce unsaturated steam, which could enter the sterilizers and render the items moist. Our results are similar to those of Yuan et al [24], who pointed out that the closer the item is to the air inlet of the sterilizer, the more moisture the item has.

5. Whether the item had been packaged in compliance with guidelines. Packaging incompliances found in our study include the following: improper or incomplete drying of the instruments and too many instruments placed in the pack. In addition, Xu [25] confirmed that wet packs can be caused by the following factors: the instrument not being fully disassembled to the smallest parts for cleaning and drying; overly tight packaging; too many instruments; and no absorbent liner.

6. Cooling time. The CSSD directive (Part 2: Standard Operating Procedure of Cleaning, Disinfection, and Sterilization) released by the National Health Commission of the People’s Republic of China [15] stipulates that sterilized items can be delivered after 30 minutes of cooling. However, CSSD staff in the hospital considered in this study made decisions concerning item delivery based on the pack’s surface temperature felt by hand, which was perhaps a too-subjective method for determining the temperature. Whenever there arose a large surgical volume met by an insufficient supply of ready-to-use surgical instruments, urgent processing of surgical instruments was needed. In these circumstances, CSSD staff delivered the items in urgent need immediately after sterilization. A large temperature difference between the external surface of the pack and external cold air produced condensate water, resulting in wet pack occurrence [26].

Recommended Prevention Strategies

1. It is necessary to regularly maintain the steam pipeline, replace thermal insulation material of the pipeline whenever appropriate, and change the drying time of the sterilizer from 15 min to 20 min. Hospital engineers should conduct weekly inspections and maintenance of the sterilizer. If a hospital intends to build a new CSSD, purchasing autoclave steam generators is recommended so that the new CSSD can have its own steam supply. A stainless steel steam pipeline with a length of less than 20 m should be adopted to prevent steam from contaminating and rusting the carbon steel pipeline, thus avoiding wet pack occurrence caused by excess condensate water. In addition, the quality of water for steam should also satisfy the necessary requirements.

2. To reduce the risk of wet pack occurrence it is advisable to use perforated instrument sterilization baskets as the trays for loading of the instruments and to use an absorbent liner as a tray liner for absorbing the remaining condensate water.

3. The results of Kang [23] show that putting a cotton wrapper between 2 non-woven wrappers for packaging can effectively reduce the incidence of wet packs, compared with simply using 2 layers of non-woven fabrics. Following a discussion with operating room staff, we recommend adding a cotton wrapper into the pack containing instruments to reduce wet pack incidence and help retain the patient’s normal body temperature.

4. We suggest complete elimination of condensate water prior to sterilization on a daily basis. We also suggest extending the pre-heating time of the sterilizer and discharging condensate water in the steam pipeline ahead of using the sterilizer to ensure that the steam entering the sterilizer chamber is saturated. Sufficient circulation temperature may be ensured by making steam enter the interlayer and closing the autoclave door for preheating. When the steam pressure and temperature in the sterilizer rise to 0.5 Mpa and 40°C, respectively, the empty autoclave should be monitored.

5. Standardizing the loading procedures for the packs waiting for sterilization is recommended. The packs waiting for sterilization need to be loaded for safe and effective sterilization according to loading procedures stated in the guidelines for disinfection and sterilization. The packs to be sterilized should be placed on specific sterilization racks or into specific baskets and there should be a gap between the packs. It is appropriate to place instruments, appliances, and items of the same material in the same batch for sterilization [27]. Pang and Zhang [28] reported that the bottom of the pack waiting for sterilization should be as parallel as possible with the sterilization rack, and that a distance should be left between the pack and the walls and door of the sterilizer. During sterilization, condensate water on the packs on the top shelf in the sterilizer may drop onto the packs on the bottom shelf, resulting in wet pack occurrence on the bottom shelf. The large packs should be placed on the top shelf, while the small ones should be placed on the bottom shelf. Particularly when the instrument and dressing packs are sterilized simultaneously, the dressing packs should be placed on the top shelf and the instrument packs should be placed on the bottom shelf to prevent dressing packs from absorbing a large amount of condensate water [29].

6. Standardizing the packaging procedures is recommended. It is necessary to dismantle surgical instruments to their smallest parts following collection. An instrument with a long lumen or complex structure can be packaged after being thoroughly dried with a high-pressure air gun. The instruments must be counted and thoroughly dried prior to assembly. Long and slender lumen instruments and instruments with grooves must be carefully checked to ensure thorough drying. Grooves must be placed downward to prevent condensate water from accumulating in the groove. Both the weight and volume of each pack should satisfy the relevant guidelines. Oversized or overweight packs need to be sterilized after unpacking [30]. For utensils stacked in the same instrument pack, we recommend using a medical absorbent sheet to separate them so as to avoid condensation caused by tiny internal spaces within the pack, and also making sure that the open mouths of the utensils are orientated in the same direction (Order of placement: large basin is laid flat; stainless steel kidney-shaped tray is placed mouth up; stainless steel medication cup is placed mouth up; stainless steel surgical instrument is laid flat). Proper tightness of pack sealing is conducive to steam penetration.

7. We propose purchasing more surgical instruments to meet the demand of increasing surgical volume, as well as to reduce the incidence of urgent processing of surgical instruments, and ensure the delivery of sterilized items following cooling for 30 min. An infrared thermometer is recommended for accurately measuring the surface temperature of the pack, thus reducing the risk of contamination caused by repeated hand-touching of the sterilized pack. A warning sign indicating that the loading racks should not be proximate to the vent for cooling should be positioned near the vent. A cooling time of >30 min should be set in the traceability system, such that if the real cooling time is not the pre-set time, the traceability system will trigger an alarm indicating a non-conformity in item delivery [31].

Limitations

Our study has some limitations. First, we did not investigate the influence of the sterilization cycle protocol parameters on wet pack occurrence. Second, this was a cross-sectional study that analyzed the data concerning wet pack occurrence in a specific time period. Further, there should be more in-depth analysis of the role of sample size in future studies. Multi-center research is required for a more rigorous investigation of the influence of different sterilizers and different parameters on wet pack occurrence.

Conclusions

Our research findings show that the following factors are associated with wet pack occurrence after steam sterilization: outside weather, the item in the sterilized pack, packaging material, whether the item had been packaged in compliance with guidelines, whether the pack had been placed on the top or bottom shelf, and cooling time after sterilization.

Recommendations for reducing the risk of wet packs include: regular maintenance of the steam pipeline, regular replacement of thermal insulation materials for the steam pipeline, extension of the drying time, appropriate packaging of items in compliance with guidelines, purchasing specialized surgical instruments, adding a cooling time alarm system module to the traceability system, and inclusion of a handheld infrared thermometer for accurate measurement of the surface temperature of the packs.