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Particulates that may be present on intravenous medi-cal devices such as guidewires, catheters, and stent delivery systems pose potential health risks to pa-

tients. When a device is deployed or exposed to the patient, particles may be released and become lodged in the patient’s vascular capillary system. Whether this is harmful to the patient depends on the size, shape, and composition of the particles, as well as where and to what degree they cause an occlusion. The response of the patient’s immune system to the particles is also a factor.1

The source of these particulates may be the manufac-turing process and environment, product packaging, or the device itself. The human body is the dirtiest source in a cleanroom (flecks of deodorant have been found). But clothing, machining, lubricants, and building materi-als can also be a source of particulates. The integrity of drug coatings found on some stent delivery systems can contribute to the total particulate load. Manufacturers of these types of devices are being asked by FDA to provide data regarding particulates to help evaluate the potential effect the use of their device may have on the patient.

The challenges facing manufacturers include deciding how to test devices to obtain accurate particulate counts and how to set reasonable acceptance limits for particulates. Standards addressing these issues are not available for most devices. Acceptance limits for particulates have not been es-tablished because of the wide range of device types and the unknown effects of particulate size, quantity, and potential toxicity to the patient due to the chemical composition of the particles. Most manufacturers set their own acceptance lim-

its or use the limits specified in USP 788, “Particulate Matter in Injections.” However, USP 788 is specific for injections and parenteral infusions and may not always be appropriate for medical devices.

This article evaluates the test methods used to analyze medical devices for particulates, the documentation FDA may request from medical device manufacturers, and the guidelines being developed to help manufacturers establish acceptance criteria for their devices.

Analyzing Particulate Matter on Medical DevicesManufacturers can employ test method strategies to minimize health risks of intravenous medical devices.

Susan Reynolds and Ryan Lunceford

Susan Reynolds is a technical writer at Nelson Laboratories (Salt Lake City). Ryan Lunceford is the director for particu-late matter studies at the company.

The microscopic method is used for samples that cannot be tested with light obscuration. It enables particulates to be identified by color and shape.

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Test MethodsUSP 788 describes two methods for analyzing particulates

in injections and parenteral infusions: light obscuration and microscopic (see Table I, p. 60).2 These two methods have been adopted by many manufacturers to analyze particulates on medical devices. For both methods, particles are extracted from the device with particle-free water (i.e., water that has been passed through a 0.22-μm filter) or other appropriate solvent, and the extraction fluid is analyzed.

Light obscuration, which uses laser diffraction technology in a liquid particle counter, is the easiest and most common method. It is an automated method that determines the size of the particles present in the extraction fluid and the number of each size. A laser diode in the particle counter is directed at the liquid sample, and as particles present in the sample flow through the sensor, they diffract or interrupt the laser diode. This produces a pulse for each particle, and the amplitude of the pulse is proportional to the size of the particle.

The major advantage of this method is that, because it is automated, the results are repro-ducible and nonbiased. Sample preparation and analysis is fast and easy. The liquid particle counter is capable of assessing a wide range of particle sizes from approximately 1.25 to 150 μm.3 The exact range varies by instru-ment. Another advantage is that the entire sample is not usually consumed in testing, so further analysis can be performed on the extraction fluid.

However, there are limita-tions to light obscuration. The liquid particle counter is unable to identify particulates by color, shape, or composition. It can only quantify the particulates and categorize them by size. Any colloid or surfactant pres-ent on the medical device may create bubbles when drawn into the sensor of the instru-ment and be counted as particles. The particle counter may also have difficulty sizing fibrous particulates because of their uneven shape. When such issues arise, the micro-scopic method is preferred.

The microscopic method is for samples that cannot be analyzed by light obscuration due to turbidity, viscosity, or the presence of colloids or surfactants. The extraction fluid is filtered through a ≤1.0-μm membrane filter. Surfactants and colloids pass through the membrane during filtration and do not affect the results. The filter is allowed to dry, and it is then examined microscopically by the analyst. Particles are counted and sized by means of a circular diameter graticule in the eyepiece of the microscope under 100× magnification. The particles can also be identified by color, shape, or other parameters. This is helpful in evaluating possible sources of contamination for remedial or preventive measures.

This method also has limitations. Analyzing samples by microscope is more time-consuming than light obscuration. Processing many samples at one time does not provide the typical economies of scale that can be realized with the light obscuration method. Counting particulates on filter slides can be tedious, detail-oriented work. Particles that are clear or the same color as the filter may be missed during analysis. Counts can differ significantly between analysts; therefore, the results are not as reproducible as the automated method. In addition, the entire sample is consumed during the filtra-tion process so further testing is not possible.

When evaluating a device in the laboratory, it is important to ensure that particulates are not introduced into the test system. USP 788 recommends that all testing be performed in an ISO Class 5 hood. All glassware must be washed with a warm detergent solution and rinsed thoroughly with particle-free water before being used in the test system.

An area designed for particulate testing with positive air pressure and HEPA filtration adds an extra level of assurance

for minimizing particulate con-tamination from the laboratory environment. Analysts should be trained in aseptic technique. These are important factors to ensure accuracy and reproduc-ibility and should be considered when evaluating laboratories.

What FDA May RequestFDA looks at what the total

particulate load will be to the patient during clinical use of the medical device. The agency is in-terested in the number and size of particles being shed by the de-vice and where the particles are coming from. It may also require the size and number of particu-lates from other components used in the test system, such as the guide catheter or guidewire. If the device has components of various sizes, FDA may want all

sizes tested for particulates. Some manufacturers have tried to divide their testing process into separate phases to deter-mine what part of the system generates particulates. If the medical device contains components from other manufactur-ers, it is a good idea to perform baseline particulate counts to determine their effect on the total particulate load of the test system.

A medical device may have a drug or other coating that can slough off during deployment, which can create particu-lates. This is especially common among drug-eluting stent systems. If there is a delay between sample preparation and analysis, such particulates may degrade and dissolve, pos-sibly resulting in lower particulate counts than if the sample was analyzed immediately. Therefore, it is recommended that samples of this nature be tested shortly after extraction to minimize the possibility of particulate dissolution.

Particulates with an unusual shapes or colors can be iden-tified using the microscopic method. But counting such items is tedious work.

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USP 788 requires that a test environment control be included when analyzing samples by either method. This control consists of all items that are part of the laboratory test system, such as the liquid particle counter, glassware, and extraction fluid. The items are rinsed with particle-free water, and the particulate counts of five 5-ml aliquots are determined. The control must meet acceptance criteria established in USP 788 before analysis of test samples can proceed.

In addition to the environment control, device manu-facturers may want to include a control that is prepared in the same manner as the samples. This control should include all items used in the clinical setting that are sepa-rate from the medical device being tested. Examples in-clude guidewires, catheters, hemostasis valves, cannulae, and needles. This control serves as a baseline count of the particulates being shed by components that are separate from the actual device.

A common way to evaluate particulates in terms of pa-tient exposure is through simulated-use studies. The device travels through a tortuous path consisting of either a glass or plastic vein that mimics how the device will travel through the patient’s vascular system. FDA finds the tortuous path design illustrated in ASTM 2394 to be clinically relevant and recommends that it be used. However, this model may not be applicable for all device systems, so manufacturers can design a similar system and include the rationale for its use when submitting their product for approval. It is advised that the tortuous path incorporate the entire length of the device to which the patient will be exposed. For devices that contain an inflatable balloon, the nominal pressure is routinely used in simulated-use studies. However, for particulate testing, the rated burst pressure may be preferred.

When designing a tortuous path, include a variety of angles that represent clinical use. However, avoid creating a simu-lated-use system that stresses the device more than it would be when used clinically. Doing so could falsely elevate counts.

It is recommended that the simulated deployment system be validated prior to use, because FDA may request a copy of the validation report. To validate the system, predetermined amounts of National Institute of Standards and Technology (NIST) traceable standard particles are introduced into the

tortuous path. These particles should be the same size used in testing (usually 10 and 25 μm at minimum). Then the percent recovery is determined. FDA has established acceptance cri-teria of a minimum of 90% recovery for 10–25-μm particles and a minimum of 80% recovery for ≥25-μm particles.

The particulate size ranges counted according to the USP 788 method are ≥10 and ≥25 μm. Data for larger particu-lates such as 75 μm, >100 μm, or particulates at the upper limits of the liquid particle counter used in testing are often requested. The presence of large particles on vascular medi-cal devices is a concern because they may have a greater influence on patient health.

It may also be useful to identify the composition of the particulates to determine whether they are homogenous or heterogenous. Knowing the composition distribution of the particulates can provide information about their source, as well as their potential risk to the patient. This is especially important when the counts are high.

Heterogenous particles are usually environmental in origin, and their size and numbers are of more importance than their composition. Homogenous particles usually indicate a single source. For example, 90% of the total particulate count for a medical device may be due to packaging materials with only 10% coming from the device itself, or there may be a tenfold increase in particulates when a balloon catheter is inflated. Homogenous distribution may point to a specific phase of the manufacturing process as the source of particulate con-tamination. High numbers of homogenous particles should also be evaluated for their potential toxicity to the patient.

GuidelinesAlthough some devices have standards for particulate

count limits, such as gravity-fed infusion sets (ISO 8536-4), cardiac pacemakers (EN 45502), and autologous transfu-sion devices (ANSI/AAMI AT6-2005), the majority of device categories do not have guides. Most vascular device OEMs use USP 788 or set their own acceptance limits. USP 788 should be used only as a general guide. Its use is accept-able as long as the small volume criteria specified in the method are met. If the particle counts of the device exceed these limits, an OEM must justify to FDA why its counts are acceptable.

An AAMI technical information report (TIR) document on particulates is currently being drafted. The goal is to provide manufacturers with guidelines for testing and establishing acceptable particulate levels for vascular devices.4 It also focuses on best practices to help manufacturers minimize particulates on their devices through design, development, and manufacturing. The TIR addresses possible sources of particulates from raw materials, the manufacturing process, or the environment and provides guidelines for control-ling contamination. The document is expected to provide guidelines for manufacturers to set their own acceptance limits based on such factors as category of the device (i.e., the length of patient exposure to the device) and its clinical application, as well as appropriate particle sizes and their po-tential toxicity. The authors anticipate that specifications will be in counts per device or, for multiple components, counts per system. Specifications may be tied to the category of device.

The light obscuration method, shown here, is automated and may be the easiest particle-counting method.

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Test methods for sizing and counting particulates are expected to be addressed in the TIR, as are guidelines for sizing particulates by the microscopic method. A list of test methods for analyzing the composition of the particulate contamination and the type of information that can be de-rived from each method will also be provided.

ConclusionMany of the recommendations described in this ar-

ticle were gathered through the authors’ experience in performing particulate tests and may not apply to all devices. Nevertheless, until a concrete standard is set for particulates on vascular medical devices, it is neces-sary to consider all FDA recommendations regarding particulate matter testing when submitting a product for approval. Consulting with FDA prior to developing a final draft of a testing protocol can help clarify what data are required for specific products. A contract labo-ratory that performs particulate testing can also be a

valuable resource for what to consider when developing a testing protocol.

AcknowledgmentsThe authors thank Tonya Morris, who is on the AAMI

Medical Device Particulates Committee responsible for drafting the TIR document.

References1. LJ Pesko, “Physiological Consequences of Injected Particulates,” in

Liquid- and Surface-Borne Particle Measurement Handbook, ed. JZ Knapp, TA Barber, and A Lieberman (New York: Marcel Dekker, 1996): 661–685.

2. USP 788, “Particulate Matter in Injections” (Rockville, MD: United States Pharmacopeial Convention, 2009).

3. Nelson Laboratories internal data, STP0011 Sizing/Counting Par-ticulate Matter.

4. AAMI TIR Draft, “Particulates Associated with Vascular Medical Devices,” (Arlington, VA: AAMI). ■

CIRCUMSTANCE LIGHT OBSCURATION METHOD MICROSCOPIC METHOD

Large volume of samples Many samples can be processed in a relatively short amount of time.

Involves time-consuming manual enumeration of particulates per sample. Not ideal for large-volume work.

Colloids, surfactants, or high density of proteins in sample.

High likelihood of such materials being counted as particles. Could drastically increase false positive results.

Substances normally pass through a filter membrane used for microscopic analysis.

Cost for contract laboratory Comparatively low. Less time required by a technician to run samples.

Comparatively high. More time needed for technician to enumerate each membrane.

Cost for in-house testingHigh initial investment to purchase

equipment. Low cost per sample for large volume of samples.

Low initial cost due to simple equipment used for microscopic testing. High labor costs to perform testing on many samples.

Measuring large particles Depending on the model of sensor, may or

may not be able to accurately size and count very large particles.

Can measure or detect large particles up to 1000 μm.

Measuring small particlesCan count and size particles of 2 μm or

smaller. Such sizes are required by some specifications.

Difficult to accurately measure and size particles <10 μm on a light microscope at 100×.

Accuracy Nonbiased and reproducible. Human error can be a factor because of reliance on manual counting.

Consumption of sample

Only a portion of the sample, 20 ml, is typically required for USP 788. Other testing can be performed on the remaining sample.

Typically consumes the entire sample in filtration. A portion of the sample can be filtered to perform calculations that determine the total particulate count.

Dilutions If sample is less than 20-25 ml dilutions are required for sufficient flow rate. Dilutions are atypical unless sample is viscous.

Flow rate limitations Flow rate requirement can be met with dilutions if necessary.

No flow rate requirement. Samples that are too viscous may cause filter clogging.

Caustic, corrosive, or other strong chemicals used in testing

Most particle counters are designed to facilitate a variety of stronger chemicals. (Read the manual prior to testing).

Hard to find USP-specified filter (black, grey, and gridded) that is resistant to chemicals. Standard cellulose does not react well to strong chemicals.

Identification of particulate properties other than size and count

Can only size and count particulates. Does not provide color, shape, or composition.

Provides shape, color, and possible composition (e.g., metal or fiber) of individual particles.

Comparison of the two particulate methods. Light obscuration is the method preferred by USP and should be the default method unless a sample necessitates the microscopic method.

Reprinted with permission from MEDICAL DEVICE & DIAGNOSTIC INDUSTRY, May 2009. On the web at www.devicelink.com.© A Canon Communications LLC Publication. All rights reserved. Foster Printing Service: 866-879-9144, www.marketingreprints.com.

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