As part of their submission to the Food and Drug Administration (FDA) and submissions to regulatory bodies around the globe, manufacturers of reusable medical devices must validate that the reprocessing instructions provided to healthcare facilities will produce a device that is safe, capable of being cleaned, and ready to use on the next patient. Since the testing often must be done in an independent laboratory setting, and not with devices used for clinical procedures on patients, the medical device needs to be inoculated with test soil(s) that closely simulate soiling that would occur during clinical use. The medical device manufacturer needs to scientifically demonstrate that the formulation of simulated-use soil(s) is clinically relevant and closely approximates the challenge to reprocessing that actual soiling presents. Test soils are often characterized based upon various properties including biomarkers (e.g., protein, hemoglobin, total organic carbon [TOC]) and physical properties (such as viscosity and adhesion). These properties give the test soils their “product dimensions” and can be used to compare one soil to another. This article will discuss the means and methods for physically characterizing simulated-use test soils as well as clinically occurring soiling. This information will be useful to medical device manufacturers, independent testing labs, academicians, regulators, and standards-writing groups.

The composition of the test soil plays an important role in soil selection. Most ingredients used to prepare test soils for validations closely simulate components found in the bodily fluids medical devices are contaminated with during clinical use.

The recently published FDA guidance on this subject directs medical device manufacturers (MDMs) to use at least two biomarkers when validating the effectiveness of their cleaning instructions for reprocessing their medical device.1 Protein is often selected with another biomarker, such as TOC, hemoglobin, or some other measure. While the test methods are not completely standardized, they are, for the most part, well studied and documented in the literature. For example, there are several means to detect and measure protein; ortho-phthalaldehyde (OPA) and micro bicinchoninic acid (Micro BCA) are two common methods. They may provide somewhat different results, but their limitations and advantages are well documented in peer-reviewed studies.2 In a laboratory setting, the starting composition of the soil contamination can be defined, so the use of any detection/quantification method after cleaning can be compared to the starting point to demonstrate a level of cleaning effectiveness. As outlined by the FDA guidance, the positive control can be determined by inoculating a defined volume of test soil on the carrier where the amount of biomarker, such as protein, can be calculated based on the protein content of the test soil. Alternatively, the positive control protein level can be determined by harvesting the inoculated carriers prior to cleaning and comparing this protein level to what remains when inoculated carriers are harvested post cleaning. Regardless of which method is used, it is necessary to establish the harvesting efficiency for the recovery method used.

Other clinically relevant soil markers can be more difficult to detect and quantify. For example, a recent article in BI&T demonstrated that measuring for residual bone can be a challenge.3 So the field of biomarkers detection/quantification remains an opportunity for further study. Nonetheless, for many medical devices there are well-studied biomarkers that can be used to validate the effectiveness of device reprocessing instructions in a simulateduse laboratory setting.

Less well discussed and analyzed are methods for characterizing the test soil(s) in terms of physical parameters. For example: How viscous is it? How “sticky” is it? How water-soluble is it? This limitation extends not just to test soils, but to the actual clinically occurring soils. The viscosity of blood, for instance, at 37.5°C is well studied.4 But how about at room temperature, which is likely the temperature it will be on a medical device by the time reprocessing begins?

This article presents methods that can be used to describe the physical characteristics of soils, both clinical and simulated, that can be used to better understand the dynamics of cleaning a medical device and recreating those cleaning dynamics in a laboratory setting for reprocessing validation purposes.

The viscosity of a liquid will influence how it flows and, ultimately, contaminates a medical device.

Test soils can be characterized based upon various physical properties. These properties can then be used to compare one test soil to another. These properties can also be used to compare a test soil to clinically occurring soiling of a medical device. This can further help demonstrate the appropriate selection of a test soil for a given device. Many medical procedures use reusable medical devices that are exposed to a variety of components present in the body. To adequately represent clinical use of these devices, the test soil formulation should be based on scientific data to allow standardization and reproducibility. These test soil formulations should ideally mimic components such as secretions, blood, tissue, bone/cartilage, or any other patient-derived component that could contaminate the device during clinical use. There are some published data that can help determine the target composition of test soils.5 Less well studied are the physical characterizations of test soils. This article will discuss various methods for characterizing key physical properties of test soils used for simulated-use testing of cleaning efficacy.

Viscosity is a measure of a fluid's resistance to flow. It is one of the most important properties of a fluid and therefore is a vital consideration when selecting test soils for validation. The viscosity of a liquid will influence how it flows and, ultimately, contaminates a medical device. When reusable medical devices are exposed to different applications during use, viscosity plays a major role in terms of coverage and adherence. This will not only influence the functionality but also the ability to clean the device. Although viscosity is usually not a measurement that is identified during validation testing, it is something that should be considered because soils that are particularly viscous can be a greater challenge to the effective reprocessing of the device. It is strongly recommended that the viscosity of the test soil mimic the secretions and bodily fluids the device would encounter during use to simulate actual procedural conditions. The viscosity of the test soil can be increased or decreased with additives that meet the purpose of the test.

Selecting a Viscosity Test Method

There are a number of ways to measure viscosity, as reflected in the standard methods listed below. A viscosity value is temperature dependent. The standard units for viscosity measurement are mPa·s (millipascal second) and cP (centipoise)—for example, 1 mPa·s = 1 cP. One significant consideration, particularly when measuring clinically occurring soils, is the volume of soil required to conduct a given viscosity test method. Two different types of viscometers that are well suited to using relatively low sample volumes are plate and cone viscometers and vibrational viscometers.

Standard Methods for Measuring Viscosity

  • ASTM D445-15: Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (and Calculation of Dynamic Viscosity)

  • ASTM D4212-102014: Standard Test Method for Viscosity by Dip-Type Viscosity Cups

  • ASTM D7042-14: Standard Test Method for Dynamic Viscosity and Density of Liquids by Stabinger Viscometer (and the Calculation of Kinematic Viscosity)

  • ASTM D7867-13: Standard Test Methods for Measurement of the Rotational Viscosity of Paints, Inks and Related Liquid Materials as a Function of Temperature

  • ASTM D4287-002014: Standard Test Method for High-Shear Viscosity Using a Cone/Plate Viscometer

Viscosity Test Methods

  • Rotational viscosity testing: The principle of operation of a rotational viscometer is that a rotor is submerged in the liquid to be tested and the force applied to overcome the resistance against rotation is measured.

  • Vibrational viscosity testing: A vibrational viscometer sensor submerged in fluid resonates with a high frequency, and the energy is lost to the fluid because of its viscosity. This dissipated energy is measured and equated to the viscosity.

  • Cone and plate viscosity testing: With a cone and plate viscometer, a cone spins in the sample fluid on a stationary plate and a torque meter senses the resistance to rotation caused by the fluid between the cone and the plate.

  • Capillary viscosity testing: The process of a capillary viscometer is that the test fluid is made to flow through a narrow tube by gravity and the viscous behavior of the liquid is measured based on the flow time.

Viscosity Readings of Soils

The purpose of this article is not to compare various soils, clinical or simulated, but to describe useful methods for characterizing these soils. However, it is instructive to present some data to demonstrate the viscosity for different simulated-use test soils that are currently used in reprocessing validations (Table 1). The viscosity of each of these soils was tested after mixing/rehydrating (where applicable) as directed by the manufacturer's instructions for use (IFU).

Table 1.

Average rotational, vibrational, and cone and plate viscosity readings of some test soils

Average rotational, vibrational, and cone and plate viscosity readings of some test soils
Average rotational, vibrational, and cone and plate viscosity readings of some test soils

It is important to note that there can be a significant difference between viscosity test methods. This can be due to the shear characteristics and the components used to prepare the soil. For this reason, it is important to compare the results obtained from the same viscosity test method and not between different viscosity test methods.

Another important physical characteristic of a test soil is its surface adhesion properties. The more strongly a soil adheres to a surface, the more difficult it is likely to be to remove during cleaning. Some key factors that influence adherence to the surface of the medical device are the thickness and the “stickiness” of the soil itself, the time and conditions for drying of the soil on the surface, and the surface composition of the medical device being tested. For example, egg yolks used for test soils pose a challenge to cleaning because, when dried onto the surface, the yolk component of the egg hardens on the surface of the device, making it difficult to remove during reprocessing. The type of substrate (e.g., stainless steel, plastic, silicone) and the condition of the surface finish (e.g., smooth, textured, scratched) play a role in the adhesion properties of a soil. It has been noted and observed that the time given for a selected test soil to dry onto the test surface plays a role in the ability to clean a device (i.e., the longer the dry time the more difficult it is to remove the test soil).

Methods for Testing Adhesion

ASTM D33596 describes a method for evaluating the adhesion of a coating to a substrate as determined by the ability of adhesive tape to remove the soil. A modified method can be used to evaluate and compare the adhesive characteristics of simulated-use and clinically occurring test soils to a variety of substrates. In the modified method, the removal of soil is evaluated through gravimetric measurement (i.e., dry weight determination) (Figure 1).

Figure 1.

Overview of ASTM D3359 adhesion test method

Figure 1.

Overview of ASTM D3359 adhesion test method

Close modal

Table 2 represents the soil and substrate: A) the weight of the test coupon before soiling; B) weight after soil has been allowed to dry overnight (at room temperature, approximately 25°C and 60% relative humidity [RHP]); C) the weight after the lattice cut and brush; D) the weight after the tape test. The results presented are from averaging multiple (three) replicates.

Table 2.

Gravimetric data for ASTM D3359 adhesion testing of various test soils

Gravimetric data for ASTM D3359 adhesion testing of various test soils
Gravimetric data for ASTM D3359 adhesion testing of various test soils

Key Variable Test Parameters

The ASTM D3359 method provides an effective means of comparing one soil on a substrate to another. It is important to point out that there are some key parameters that are likely to affect the exact outcome of testing:

  • Temperature and humidity will affect the degree of drying over a given period of time. Time for drying will also affect the state of the test soil.

  • The pressure used to apply the tape to the surface. In a manual process, this is likely to differ between individuals and between laboratories.

  • As with the pressure used to apply the tape, the speed with which the tape is removed is likely to affect the amount of soil removed.

ASTM D33307 Test Method F describes a method for evaluating the adhesion of pressure-sensitive adhesive tape to a surface utilizing a force tester. Following this method for conducting the pull test provides a means to reduce variability between test runs. When compared to the manual method (ASTM D3359), the removal of soil from various substrates was similar. The use of a mechanical force tester (Figure 2), however, would better allow for comparison of results between individual testers and for comparing inter-laboratory test results. Further, the force tester also provides a measure for the force required to peel the tape. This is another measure of the adhesive quality of a given test soil to a substrate.

Figure 2.

Force-test machine for the ASTM D3330 adhesion testing

Figure 2.

Force-test machine for the ASTM D3330 adhesion testing

Close modal

As with the ASTM D3359 method, there may be variability due to environmental conditions and due to the pressure used to apply the tape. While not investigated in time for this article, a method to use the force tester to apply the tape could probably be developed, improving the repeatability of test results between laboratories.

See the results in Table 3 for various test soils on a stainless steel substrate. Results include the average and maximum load during the test, as well as the amount and percentage of soil removed.

Table 3.

Adhesion testing using the force-test machine

Adhesion testing using the force-test machine
Adhesion testing using the force-test machine
Table 4.

Removal of various test soils from various carriers after static exposure to water

Removal of various test soils from various carriers after static exposure to water
Removal of various test soils from various carriers after static exposure to water

Water alone and water with a cleaning agent (i.e., detergent) are the most common solvents used for cleaning medical devices in a healthcare facility (Figure 3). These, combined with mechanical action (spray, brushing, flushing, agitation, etc.) are the means by which medical devices are typically cleaned. Water is an excellent solvent, as many patient-derived secretions and bodily fluids, (e.g., blood, urine, mucosal secretions) are water soluble. However, for the thorough cleaning of a medical device, particularly when the clinical soils have dried or contain insoluble material (e.g., feces or lipidic materials such as fats and oils), additional resources such as mechanical action and detergent agents are required. Detergents usually contain enzymes and surfactants that help denature and clean the debris from the devices, which makes them a valuable addition to ensure adequate cleaning. As with adhesion testing, the substrate can also make a significant difference when comparing how well a soil is removed from a surface by water alone. Another factor is the dwell time between use of a device and when reprocessing begins.

Figure 3.

Overview of the static water removal test

Figure 3.

Overview of the static water removal test

Close modal

A test soil that readily dissolves in water without the assistance of these other additives is likely a poor challenge and should not be considered for cleaning validations. Characterizing how test soils dissolve in water in comparison to the target clinical soil may be a useful step.

ASTM D72258 Section 7 describes a method for testing a soiled coupon in a static cleaning solution. This same method could be used to test with water alone. When using water alone and the test soil includes proteins, it is recommended to use a water temperature below 45°C to avoid denaturing the proteins.9 It is best to use water at a temperature that is recommended in the IFU.

Soil removal may behave differently inside a lumen compared to on the external surface of a medical device. Inside a lumen, soil will likely take much longer to dry, allowing the test soil to reach areas within the lumen that may have small cracks and crevices. To characterize soil within a lumen, one method10 described below was used for comparison purposes.

The tubes were cut into 30.5 cm lengths. It was determined there was 2.5 mL of volume space in each 30.5-cm tube; therefore, 2 mL of test soil was injected into each tube. After the test soil was injected, excess soil was flushed out of the lumen using a syringe air flush. This consisted of 10 mL of air for each flush repeated a total of three times (i.e., a total of 30 mL of air flushed through the tubing). After the soiled tubes were dried overnight, the tubes were weighed. Then water alone was used to flush the lumen to determine how much of the dried soil could be removed. This was achieved using a syringe to flush 7.5 mL water (three times the tube volume) through the tube. Each tube was then flushed with 7.5 mL of air and dried overnight. The tubes were then weighed to determine the amount of test soil remaining after the water flush. These tests, in turn, gave the total soil removed and the percent of soil removed (Table 5).

Table 5.

Removal of various test soils from a lumen carrier after a water flush

Removal of various test soils from a lumen carrier after a water flush
Removal of various test soils from a lumen carrier after a water flush

Note: For most reliable results with this test method, the soil inside the lumen must be completely dry. If not completely dry the weight “after drying” likely will be overstated. Further, soil that is still liquid would likely more easily be removed when flushed with water than soil that is completely dry.

Medical device reprocessing has come under increased scrutiny in recent times. Performing a worst-case cleaning validation using a worst-case test soil is essential to ensure the cleaning validation appropriately addresses the real-world exposure the device will be subjected to in a healthcare setting.

As previously stated, the focus of this article is to suggest methods that can be used to characterize some of the physical characteristics of simulated-use test soils and clinically occurring soils as a means of scientifically selecting and demonstrating the appropriateness of a chosen simulated-use test soil. Disinfection/sterilization is the deactivation of potentially infectious agents. In contrast, cleaning is the removal of potentially harmful contaminants (e.g., organic material and microorganisms). When properly performed in series, these steps will render a medical device that is safe to use on the next patient.

The definition of a high-level disinfected or sterile medical device has been relatively well established in the industry. But what is clean? Or, as the AAMI/FDA summit report from 201111 so well put it, “How clean is clean?” Each medical device manufacturer has the responsibility to state the target end points of clinically relevant markers that reflect contamination and demonstrate in a simulated-use method (i.e., in a laboratory) that the instructions they provide to healthcare facilities will adequately eliminate the contaminant. It is hoped that the state of the art will be improved by better characterizing the physical and compositional characteristics of test soils so they accurately represent the soil challenge the medical device encounters during clinical procedures. This approach will ensure that adequate rigor is used by device manufacturers for establishing their cleaning IFUs and thereby increase confidence that a fully reprocessed medical device is safe to use on the next patient.

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About the Authors

Ralph J. Basile is vice president of marketing and regulatory affairs of Healthmark Industries Company, Inc. in Fraser, MI. Email: ralphjb@usa.net

Alpa Patel is senior scientist of Nelson Labs in Salt Lake City, UT. Email: apatel@nelsonlabs.com

Kaumudi Kulkarni is manager of research and development of Healthmark Industries Company, Inc. in Fraser, MI. Email: kkulkarni@hmark.com