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

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).

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.

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).

Discussion

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.