Effects of Time, Temperature, and Humidity on Soil Drying on Medical Devices

Abstract

In the healthcare environment, delays can occur that prevent reusable devices from being processed within the specified time outlined in manufacturers‘ instructions for use. It has been suggested in the literature and industry standards that residual soil components, such as proteins, may undergo a chemical change when they are exposed to heat or experience prolonged drying times under ambient conditions.

However, little experimental data are available in the literature to document this change or how is may be addressed for cleaning efficacy. This study presents the effects of time and environmental conditions on contaminated instrumentation from the point of use until the cleaning process begins. It demonstrates that soil drying after a period of eight hours changes the solubility of the soil complex, with a significant change occurring after 72 hours. Temperature also contributes to chemical changes in protein. Although no significant difference occurred between 4°C and 22°C, temperatures greater than 22°C demonstrated a decrease in soil solubility in water. An increase in humidity prevented the soil from completely drying and prevented the chemical changes affecting solubility from occurring.

Most reusable medical devices are intended to be cleaned immediately after use or stored in a way that does not allow for the remaining clinical soil (e.g., blood, mucus, tissue) to dry on surfaces. These point-of-use treatment instructions are conveyed in medical device instructions for use (IFU) and are intended to be performed by healthcare personnel (e.g., perioperative staff).

However, following these instructions may not always be possible, resulting in soil drying on a device. Guidance documents (e.g., the Association of periOperative Registered Nurses' Guidelines for Perioperative Practice,¹ ANSI/AAMI ST79:2017²) suggest that changes to soil may occur if it's allowed to dry, but little evidence exists within the literature for how soil drying may affect the cleaning process. In the current study, a series of experiments were conducted to elucidate how time, temperature, and humidity may affect the solubility of soil, if allowed to dry.

Review of Literature

The microbiological quality of a product is defined as all activities that provide confidence that the product is microbiologically safe according to its intended use.³ This is particularly important in critical situations, such as perioperative practices. It is important to understand that product quality goes beyond whether a product contains microorganisms and the associated risk of infection.

An immune response can occur from microbiological contamination and other toxic compounds on the surface or eluting from the device. As an example, the potential toxicity of protein concentrations were measured using cytotoxicity tests, and it was found that when the concentration of known toxic proteins was increased to greater than 8 μg/cm², cell death occurred.⁴ Although this was a potentially exaggerated response, as the L29 mouse cells used in the study had no immune system, the evidence demonstrates that residual protein can be cytotoxic. Other studies also have demonstrated that chemical residue, such residual cleaning agent, can be cytotoxic.^5,6

Overall, the microbiological and chemical contamination on a product, which includes residual chemicals and particulates, may elicit an immune response in a patient. Manufacturers are responsible for ensuring that medical devices are manufactured with the intended microbiological quality and delivered to the healthcare facility with the appropriate instructions, thereby allowing for safe and effective use throughout a device's lifetime.

During the previous 25 years, country-specific and global standardization committees have worked to standardize the validation of manufacturers' device processing IFUs. Standardizing IFUs in terms of their development and validation allows for the successful communication of a device's microbiological quality from the manufacturer to the healthcare facility. The standardization of validation requirements in this area requires a collaborative effort and has resulted in performance requirements documented in country-specific, regional, and international consensus standards.^7–10 The validation of applicable cleaning process instructions typically involves anticipating a worst-case scenario in the healthcare facility. Thus, testing usually omits point-of-use treatment steps and allows test soil to be visibly dry before processing steps, which are designed to remove soil to predetermined levels, are followed. Therefore, processing IFUs contain a margin of safety.

The responsibility for the microbiological quality and safe reuse of the device transfers to the healthcare facility with the IFU⁹—a responsibility involving processing the device after each patient use throughout the device's lifetime. However, the margin of safety established in the IFU validation can be eroded under the constraints of a busy healthcare environment, if the responsibility at each point in device processing is not well appreciated.

The time and conditions in which devices are stored prior to decontamination can play a critical role in the effectiveness of the cleaning process. Kaibara and Fukada¹¹ demonstrated that blood soils exposed to high temperatures (>45°C) cause fibrin to denature, sometimes irreversibly. Protein likely is adsorbed to the device material if allowed to dry, and water alone is not enough to remove it during the cleaning process.^12,13 However, proper storage of soiled devices in a moist environment can reduce protein adsorption.^13,14 This is the foundational science behind device manufacturers' instructions for point-of-use treatment prior to decontamination. Still, the literature does not address how long it takes for soil to denature and absorb to the device material or whether environmental conditions affect this process.

Healthcare Facility Practices

A typical sterile processing decontamination room in a healthcare facility is responsible for processing a wide variety of items that have been used throughout the facility. This includes items used in surgery, which often produces the largest volume of used devices throughout the day. Devices also are used in areas such as outpatient clinics, the emergency department, labor and delivery, radiology, endoscopy, and nursing units. It is not unusual for devices from these areas to be used at any time from the beginning to end of a shift. The devices then are delivered to the decontamination room at a scheduled time, typically at the end of the shift, resulting in soiled devices sitting for hours before being transported to the decontamination room.^15,16

Processing soiled devices can pose many challenges, but the most frequently reported problem area is cleaning. Device cleaning begins at the point of use to moisten and remove gross soil and assist in the prevention of soil from drying on devices.¹ Devices then are transported to the sterile processing decontamination room.

These deliveries are erratic and point-of-use treatment may not have been performed. Due to shifting priorities, devices can sit for many hours before undergoing thorough cleaning.^16,17 However, some medical devices have a mandated time for cleaning, as stated in manufacturers' IFUs. For example, flexible endoscopes are required to be processed within one hour of use. Delivery of devices from the operating room (OR) to sterile processing may be based on surgical case completion times, and device volumes are sporadic. In addition, some devices may be on a “turns list,” meaning that they will be needed for another case later in the day.

Loaned device sets may be received in sterile processing at any time throughout the day, and the types and amounts vary. Loaned device sets often consist of multiple trays and many types of instruments and/or implants. These items can be complex, sophisticated devices that are new to the facility and require processing before use.² The processing IFU for loaned sets may need to be reviewed in detail to clean complex devices, thereby requiring additional time to determine the proper steps. After the loaned instrumentation is used, it must again undergo decontamination before being sent to another facility.

The equipment used in a decontamination room can cause a bottleneck if adequate equipment is not available. Washer-disinfectors, which mechanically wash devices with water and detergent, are standard automatic decontamination equipment used in healthcare facilities to process most devices. Ultrasonic cleaning equipment is used to remove soil from joints, crevices, lumens, and other areas that are difficult to clean via other methods. Certain instruments undergo both types of cleaning. An inadequate amount of automatic processing equipment in a decontamination room can result in delayed device processing, with instrumentation waiting in a queue for an available washer. Equipment breakdowns can also occur, causing additional delays.

Inadequate staffing also affects the speed at which devices are processed. Some medical devices, such as flexible endoscopes, must be cleaned within one hour of use and require numerous steps, which can result in delays processing other devices. Newer devices tend to be more complex, with IFUs often requiring longer processing times, and facilities may not have increased staffing to meet these needs. Inadequate staffing due to call offs and employee replacement/turnover also can affect device processing times.

External Transportation

A growing trend is to centralize processing at one facility, in order to concentrate sterile processing expertise, free up valuable space in the hospital, and better utilize capital processing equipment. This results in soiled medical devices being transported over roadways. Currently, no guidance is available that addresses external transportation of medical devices, though the Association for the Advancement of Medical Instrumentation (AAMI) is developing the technical information report TIR109, External transport of medical devices processed by health care facilities.

Centralized processing involves new processing concerns because the soiled instrumentation is exposed to temperature and humidity. In addition, unlike transporting items in a hospital with smooth floors, the instrumentation may be subjected to the shock and vibration during travel. In addition, changes in temperature and humidity can occur as devices are moved from the facility, to the transport vehicle, and to the centralized processing facility. External transportation also results in additional time before the instrumentation can undergo cleaning. Instead of transportation taking minutes to hours before cleaning, it may take hours to days before devices are received in the decontamination room.

Conceptual Framework

To address the research aim, four experiments were conducted to evaluate the variables of time, temperature, and humidity on soil drying. The framework of each is described below.

1. Visibly dry: This experiment explored how devices typically are dried during a manufacturer's cleaning validation using visible soil or tactile moisture as the drying measurement.

2. The effects of time on soil solubility: The time soil dries on a device was evaluated to investigate protein adsorption rate on the device materials and measured by solubility (i.e., the ability to be dissolved, especially in water).

3. The effect of temperature on soil solubility: High temperatures is expected to have an effect on the soil chemistry and adherence to device material. In this experiment, a range of temperatures was evaluated to investigate the relationship between temperature and soil solubility.

4. The effect of humidity on soil solubility: During transport, temperature and humidity can affect soil drying. Humidity was explored as an independent variable to assess soil solubility.

Design and Methods

For each experiment, a stainless steel test coupon with the dimensions of 25 × 50 × 6 mm and surface area of 34 cm² was used to evaluate material adsorption, while a hemostatic clamp (VWR no. 10807-438; VWR International, Radnor, PA) with a surface area of 159 cm² was tested to evaluate the impact of device features (i.e., surrogate device). Prior to testing, the coupons and surrogate device samples were cleaned by soaking in a cleaning agent (CIP 100; STERIS, Mentor, OH) for 60 minutes, followed by brushing to remove all residual soil. Coupons were rinsed under running critical water and allowed to dry completely before a presoiling weight was taken.

The test soil (modified coagulated blood soil), which contained constituents equivalent to worst-case clinical exposure, was prepared by blending 100 mL fresh egg yolk (Eggland's Best, Malvern, PA), 100 mL heparinized sheep blood (Rockland, Royersford, PA) and 2 g dehydrated hog mucin (Sigma-Aldrich, St. Louis, MO). To reverse the anticoagulant, 0.05 mg protamine sulphate (Thermo Scientific, Waltham, MA) was added to each 5 mL of soil, mixed, and used in the test system within 10 minutes.

The average protein concentration of modified coagulated blood was determined to be approximately 560,214 μg/mL, using the BCA protein residual test, and the maximum protein found on highest soiled devices was determined to be 117,758 μg/cm².¹⁸ The appropriate soil volume calculated for the coupon was 82,042 μg, delivered gravimetrically as 0.22 g. Due to the complexity of the surrogate device, the soil was applied evenly over one side but not delivered gravimetrically. For both the coupon and surrogate device samples, weights were taken postsoiling to ensure all samples received an equivalent soil amount. The soil was applied using a foam-tipped paint brush with the necessary coats to reach the desired soil weight.

A solubility test was performed using a gravimetric analysis for three of the four experiments. Solubility included placing soiled samples into a preheated, 45°C/113°F extraction bag (Whirl-Pak, Madison, WI) filled with the appropriate volume of extraction fluid (150 mL critical water, as defined in AAMI TIR34:2014¹⁹) for the coupons and 500 mL critical water for the surrogate devices, allowing for complete submersion. Extraction bags with samples were warmed in a water bath at 45°C/113°F for 60 minutes. Samples then were removed from the extraction bags slowly to avoid shearing soil from the sample surface. The sample was placed on a polypropylene drying sheet and allowed to dry completely before postextraction weight was recorded.

Visibly Dry

Coupons and surrogate devices (n = 25) were soiled and allowed to dry at ambient laboratory conditions (~22°C/71.6°F, 50% relative humidity [RH]). At 0.5, 1, 2, 3, 4, 6, 19, 24, and 36 hours, samples were weighed using an analytical balance (Mettler Toledo, Columbus, OH) capable of measuring to 0.0001 g.

How Time Affects Soil Solubility

For each time point tested (1, 2, 4, 6, 19, 24, 48, and 72 h), samples of 12 coupons and 12 surrogate devices were soiled and allowed to dry in an environmental chamber (Thermo Scientific) set to 22°C/71.6°F and 50% RH. After drying for the allotted time, samples were tested for solubility.

How Temperature Affects Soil Solubility

For each temperature point (4°C/39.2°F, 11°C/51.8°F, 22°C/71.6°F, 35°C/95°F, 45°C/113°F, and 55°C/131°F), 25 samples were soiled and dried for 24 hours at the specified temperature and 50% RH in an environmental chamber (Thermo Scientific). After drying for the allotted time, samples were tested for solubility.

How Humidity Affects Soil Solubility

For each humidity point (30%, 50%, 80%, and 100% RH), 25 samples were soiled and dried for 24 hours at 45°C/113°F with the specified RH in an environmental chamber (Thermo Scientific). After drying for the allotted time, samples were tested for solubility.

Results

Visibly Dry

For the “dry-to-the-touch” experiment, the coupon data set analysis showed that the most drying occurred between soiling time and 0.5 hours, with a 65.0% change in soil weight. After the 0.5-hour mark, the percent change at 1 hour dropped to 3%, then continued to approach 0%. Analysis of variance (ANOVA) demonstrated that at the 1- to 2-hour percent change mark, no statistical difference was seen for the change in soil weight (P = 0.134). This allowed for the conclusion that after 1 hour, the soil on the coupon samples could be considered dry.

The surrogate device data set analysis showed that the most drying occurred over the course of 0.5 hours, with a 55.4% change in soil weight. After the 0.5-hour mark, the percent change to 1 hour dropped to 4.6% and continued to approach 0%. These levels were similar to those observed in the coupon studies, but further weight loss was observed at longer hold times. ANOVA demonstrated that at the 4- to 6-hour percent change mark, no statistical difference was seen for the change in soil weight (P = 0.131). This allowed for the conclusion that after 4 hours, the soil on the surrogate device samples could be considered dry.

How Time Affects Soil Solubility

The solubility rate was inversely related to percent soil retention. A higher percentage of soil retained on a sample after the solubility test indicated lower solubility rates. When evaluating the effect of time on the soil chemistry, visually soiled coupons dried for 1 hour had almost no visible soil, while the 72-hour coupons retained almost all of the applied soil (Figure 1).

Figure 1.

Left to right: Soiled coupons, ranging from drying times of 1 to 72 hours.

With both the surrogate devices and coupons, the percentage of soil retention increased with time, but it was not significantly different until the 8-hour time point compared with the 1-hour time point using ANOVA (coupon P = 0.041 and surrogate P = 0.066). The soil retention continued to increase over time but more slowly with the surrogate device. The most soil retention was observed at the 72-hour dry time point (Figure 2).

Figure 2.

Effect of time on soil solubility. Left: Coupon soil retention versus dry time. Right: Surrogate soil retention versus dry time.

How Temperature Affects Soil Solubility

Following the principles of the Arrhenius equation, in which the rate constant of a reaction is dependent on the absolute temperature of the reaction, an additional temperature experiment was conducted. This experiment investigated whether, when soil is exposed to a higher temperature for a shorter time, the solubility rate will differ from the controlled experiment using the dry time of 24 hours. At the challenge time points of 2, 4, and 6 hours, coupons exposed at 45°C/113°F and 50% RH had an average soil remaining of 12.68% ± 8%, 46.99% ± 25%, and 34.92% ± 13%, respectively, compared with the 24-hour time point tested at 22°C/71.6°F with 50% RH at 12.10% ± 7%. These results showed that with increased temperatures, the time required for the solubility rate to shift decreased.

For the impact of temperature on drying, a positive correlation was observed between increased temperature and soil retention for temperatures greater than 22°C. Using ANOVA, temperatures of 4°C, 11°C, and 22°C were statistically similar in terms of soil retention (coupon P = 0.215 and surrogate P = 0.214). Temperatures greater than 22°C had a P value of 0, demonstrating no statistical similarity (22°C compared with higher temperatures). This demonstrated that as temperatures increased, soil retention increased and soil solubility decreased (Figure 3).

Figure 3.

Effect of temperature on soil solubility. Left: Coupon soil retention versus dry temperature. Right: Surrogate soil retention versus dry temperature.

How Humidity Affects Soil Solubility

In the humidity experiment, soiled devices exposed to 100% RH did not visibly dry and, therefore, did not present the same challenge as other humidity conditions. The points of 30%, 50%, and 80% RH were statistically different (P = 0 compared with 100% RH), with 50% RH showing the most soil retention for both the coupon and surrogate device (Figure 4).

Figure 4.

Effect of humidity on soil solubility. Left: Coupon soil retention versus humidity. Right: Surrogate soil retention versus humidity.

Discussion

The drying of residual patient soil should be a concern for all those responsible for the microbiological quality of reusable medical devices. The results of this study confirmed that time (>8 h) and the environmental conditions of temperature (>22°C) and RH (>80%) affected soil solubility in water. Lower water solubility rates may be indicative of an increased cleaning challenge because water is the primary solvent used during the cleaning process.

Although manufacturers validate with a margin of safety by allowing soil to become visibly dry, as demonstrated in the first experiment, “dry to the touch” can be achieved within the first one to two hours of soil drying. However, chemical changes continue to occur, as demonstrated in the “how time affects soil solubility” experiment. The time for water to fully evaporate from soil depends on the thickness of the soil, and medical devices with complex features can harbor soil in a concentration that changes the evaporation rate, as seen by the longer hold time with the surrogate device.

The period between 2 and 8 hours was shown to be the most critical drying period for soil adhesion. For both the coupon and surrogate device, soil retention during this time period did not change the solubility rate significantly. After 8 hours (for coupons and surrogate devices), soil retention increased significantly as time increased. After the 8-hour mark, a steady decrease in solubility (i.e., expressed in increase in soil retention) suggested that a steady decrease in water from the soil occurred in the presence of a consistent variable (e.g., being on a flat surface, uniform soil thickness).

When complications associated with medical device features are included in the test system, the loss of solubility over time is less consistent. This may be influenced by how complex device features accumulate soil (leading to inconsistent soil thickness) and prevent air flow penetration to facilitate water evaporation. Regardless of the effect of the feature, after water has fully evaporated from the soil (i.e., after 72 hours of drying), the solubility of the soil decreases significantly, especially for complex features such as the box hinge and serrated features depicted in Figure 5.

Figure 5.

Soil retention in surrogate device following solubility test at the 72-hour dry time point (22°C/50% relative humidity).

Kaibara and Fukada¹¹ investigated the rigidity of fibrin gel with increasing clotting temperatures and showed that the dissociation of cross-linking resulted in a decrease in solubility. The current study builds on this concept, as it showed that an increase in temperature decreased the soil solubility at temperatures greater than 22°C/71.6°F. Colder temperatures did not affect the solubility rate of the soil.

In general, RH has a lower impact on the drying of soil. The difference between 30% and 80% RH is negligible, but as RH approached 100%, the evaporation of water from the soil decreased, causing the solubility rate to increase. Conditioning soiled devices in a humid environment does not allow the soil to bind to the device material surface and therefore is beneficial to maintaining high solubility rates.

Implications for Perioperative Nursing

Residual soil remaining on a device after processing can be a costly issue for healthcare facilities. OR time is expensive, and the devices used during surgeries are expected to be processed for reuse with a speed that allows for optimization of the resources to keep the OR in service as much as possible. Cleaning steps that include a combination of manual and automated methods can be time consuming, but residual soil on a device can affect patient safety.

Time and environmental conditions can influence the ability to clean medical devices in accordance with manufacturers' IFUs if soil has the opportunity to dry. It is becoming increasingly common for soiled medical devices to be transported, sometimes over long distances, to centralized locations for device processing. Therefore, risks of soil drying during this transport should be minimized as much as possible. The first and most important part of the device processing cycle is point-of-use treatment.

Point-of-use treatment is a critical aspect of device manufacturers' IFUs. Validating cleaning instructions at the point where soil solubility is the lowest would make efficient processing of medical devices in healthcare facilities difficult. The point-of-use treatment steps in device IFUs mitigate the risk of soil drying while waiting for the decontamination process to begin. Most IFUs include steps for immediate rinsing of visible soil and packaging in a manner that will prevent soil from drying during transport.

Devices should be kept moist, transported expeditiously, and decontaminated within 6 to 8 hours after being soiled, unless otherwise instructed by in IFUs (e.g., for endoscopes). Although point-of-use treatment is effective at removing visible soil, devices may have features that harbor unseen soil; therefore, packaging devices to prevent soil drying and ensuring swift transport for decontamination should be standard practices. Transport should be in a conditioned space that is approximately 22°C/71.6°F and 50% RH. Colder temperatures can be used, but devices that are soiled and dry should not be exposed to high temperatures (i.e., >35°C/95°F) if possible.

High humidity is a favorable condition for soiled medical devices. Conditions with high temperatures near the dew point will not allow soil to dry and are acceptable to maintaining target solubility rates. Humidity within the packaging can be maintained through methods such as covering devices with wet towels, using a humectant foam spray, or including a humidity pack in the transport container. However, at the same time, it is important to note that such humidity levels can also allow for bacterial growth and potential biofilm development. Therefore, methods that can retard microbial growth are preferred.

Transport time must also be added to the queue time for processing in the sterile processing department. Delivering large batches of devices at the 6-hour time point does not mean that processing will be completed within the desired 8-hour window. Time must be considered for all steps prior to decontamination, and management of device flow should be communicated effectively across all departments responsible for maintaining the microbiological quality of the reusable medical device(s).

Conclusion

The responsibility to maintain the microbiological quality of a reusable medical device rests with everyone who handles the device during its life cycle. Infection prevention starts with point-of-use treatment through mitigating the risk of soil drying. Time and environmental conditions can have an impact on the effectiveness of device cleaning and, therefore, must be considered for patient safety. It is also important to understand that point-of-use treatment instructions provided in the manufacturers' IFUs should always be followed for effective cleaning. The features of a device can affect the function of soil drying and the accessibility of water and cleaning agents to solubilize the soil.

Although the current study does not promote a time delay for cleaning, it identifies an increased risk to the effectiveness of the cleaning process when soil is dry or exposed to unfavorable environmental conditions.