Hemocompatibility: Not All Devices Are Created Equal

An MD&DI January 1997 Feature Article

Sharon J. Northup

Blood-contacting devices such as needles, cannulae,blood containers, and dialyzers all have very different usage requirements. Ofcourse, the hemocompatibility concerns for each will differ as well. Forexample, a needle may reside in the bloodstream for only a short time; a cannulamay be implanted for much longer. The primary hemocompatibility problem for a needle would be hemolysis, the destruction of red blood cells as a result of chemical interaction with theneedle material. For a cannula, however, a more likely hemocompatibility problemwould be thrombogenicity, or clotting, which can be caused not only by chemicalinteraction, but also by the flow rate of the blood.

There are standards available for testing whether medical devices can be usedsafely with blood. But these standards are mostly horizontal, addressing broadgroups of products rather than specific devices, and many blood-contactingdevices are not adequately covered by them.

The few vertical, or device-specific, standards that have been written are notenough. They also often do not take into account variations in the way aparticular device is used. For example, if an anticoagulant is used with adevice in some procedures and not in others, this will change the testingrequired to establish hemocompatibility for all possible procedures.

To create specific standards that will ensure the hemocompatibility of all typesof medical devices will require concerted effort on the part of biomedicalspecialists not only to develop appropriate tests, but also to bring the testsinto widespread use.

CURRENT HORIZONTAL STANDARDS

The standards developed by TechnicalCommittee 194 of the InternationalOrganization for Standardization (ISO) are recognized as the minimumrequirements for biocompatibility testing of all medical devices today. Afterapproval at the international level, regulatory authorities in participatingnations have adopted the ISO standards as written or with minor modifications.For example, ISO 10993-1, "Guidance on Selection of Tests," wasadopted by FDA in 1995 withmodifications for intrauterine and some other types of devices.^1,2Both ISO and FDA standards recommend hemocompatibility testing for medicaldevices intended for direct or indirect blood exposure.

These horizontal guidelines group medical devices for testing according to routeof exposure. For example, percutaneous circulatory support systems,extracorporeal oxygenators, and apheresis equipment are all classified asexternally communicating devices, and are therefore given the same testingrecommendations. But because they are used for very different lengths of time,they present different risks for air emboli at the blood-air interface andprotein denaturation from foaming. Apheresis equipment, for example, is used forhours, whereas circulatory support systems must be designed for days of use.Also, anticoagulants are used for only part of the treatment with circulatoryassist devices, but are used throughout therapy with oxygenators or apheresisequipment. Obviously, anticoagulant use will dramatically affect measurement ofthrombogenicity.

The current standards describe some hemocompatibility tests in detail. Forexample, the assayfor hemolysis has been described in the research and clinical literature andhas been developed as a standard method for medical device testing by the American Society for Testingand Materials(ASTM) in a draft annex to ISO 10993-4, "Selection of Tests forInteractions with Blood" and in several vertical standards for specificmedical devices.

But depending on the application, various factors can affect the results ofhemolysis testing, and the standards do not account for these possible cases.For example, all the standards that describe hemolysis recommend measuringhemoglobin in a spectrophotometric assay using absorbency at 540 nm; nonementions that the absorption spectrum may shift in the presence of variouschemicals.³ Ethanol, propylene glycol, polyethylene glycol 400,dimethylsulfoxide, and dimethylacetamide will shift the absorption spectrum ofhemoglobin. The hemolysis standards are also silent about possible interferencefrom fixatives. In the presence of small amounts of formaldehyde orglutaraldehyde, the red blood cell membranes are cross-linked (tanned) and thusare less easily ruptured, making the test results misleading.

Future revisions of the standards for hemolysis and other types ofhemocompatibility testing should include sources of variation and methodologiesfor ascertaining their effect on the interpretation of the assays.

One of the most important factors affecting the results of hemocompatibilitytesting is how the device contacts the blood. For example, although needles andcannulae serve similar functions, their methods of exposure to blood are quitedistinct.

Needles are designed for fast penetration and short exposure time. Most oftenmetallic, they require sharp points and lubricious barrels for nearly painlessentry into the vessel. The points may be beveled concavely to increasesharpness. The barrels are lubricated and, in some variations, wall thickness isreduced to enhance tissue penetration.

In-dwelling cannulae, by contrast, are designed to reside in the body for muchlonger. Made of plastic materials, they have blunt tips that minimizeintravascular irritation. Cannulae generally have thick walls to prevent kinkingand occlusion through muscular contraction. Their barrels are seldom lubricatedbecause this would lessen skin adherence and increase the likelihood ofmicrobial infection.

The standards recommend only hemolysis testing for both needles and cannulae.But because thrombogenicity is a common failure mode for cannulae, these devicesshould undergo an implant test for thromboresistance as well. Measurements mightinclude blood flow rate, duration of flow, and cellular deposits on the surfaceor downstream from the cannulae.

Using a thrombogenicity test for cannulae would have the added benefit of takinginto account the reaction of muscles to the devices. When the vascular systembecomes irritated by a foreign object, the smooth muscles contract vigorously.This constriction could result in collapsing or kinking of a cannula, whichwould be evident by the loss of blood flow through it. The strength of thecannula must match or exceed that of the muscular constrictions at the vascularaccess site.

CURRENT VERTICAL STANDARDS

As noted above, there are some vertical standards for device hemocompatibility:blood container standards are good examples. Blood collection sets are coveredin ISO 1135-3, "Blood-Taking Set," and ISO 3826-4, "PlasticsCollapsible Containers for Human Blood and Blood Components." ^4,5Both documents list requirements for cell culture cytotoxicity, short-termintramuscular implantation, hemolysis in vitro, delayed contact sensitization,intracutaneous irritation, pyrogenicity, and sterility.

The customary measurements for a whole-blood container are total hemoglobin,hematocrit, and cell counts. The preferred hemolysis test is a static assayoccurring under the usage conditions of 21 days of storage at 4°–8°Cwith citrate phosphate dextrose solution or 42 days with citrate phosphatedextrose adenine solution. Common measurements on containers for red cellconcentrate are erythrocyte adenosine phosphate (ATP), lactate, and glucose asindices of an energy source and utilization. Red cells may also be evaluatedmicroscopically for morphological changes.

Depending on whether the patient whose blood is in the container has beentreated with an anticoagulant, these tests may not go far enough to ensuredevice safety. If an anticoagulant has been used, measurements of the stabilityof the anticoagulant over the product's shelf life—for example, the pH andconcentration of each additive—should also be made.

Containers for platelets would also require assays that are specific to theproducts they hold, such as pH, aggregation, morphology, glucose consumption,lactate accumulation, and cell counts.

COMPLEX DEVICES

Standards for complex or invasive devices are particularly in need ofdevelopment. For some of these devices, there is no consensus among existingstandards on what are appropriate tests. For example, there is disagreement onthe testing of hemodialyzers. The French and German standards require hemolysistesting on an eluate from the hemodialyzer,^6,7 whereasFDA guidelines require thefollowing tests:

The FDA requirements do not delineate the biological system and exposureprotocol that are necessary for interpretation of the measurements. Awhole-blood clotting time assay, for example, may not be meaningful becauseheparin anticoagulants are used during hemodialysis procedures.

Complement activation has been included in the guidelines to lessen thepotential for dialysis-induced chronic lung disease.⁹ Yet measuringonly complement activation as a potential source of inflammation ignores therole of platelet activation in the initiation of free radicals that couldcontribute to chronic lung disease. Also, some studies have shown that somemembrane materials used for hemodialyzers actually absorb complements, makingthe measurement irrelevant for those materials.

IMPLANTS

Medical devices that are implanted in the vascular system offer an even morechallenging task for creating specific testing standards. Hemocompatibility forimplantable devices is highly dependent on the material, shape, function, andlocation of the implant. Many of the currently marketed vascular graft materialsare hemolytic. Although it is not clear whether this is due to the specificmaterial or the air-blood interface, clinicians have used this property to sealthe grafts before implantation.¹⁰ The hemolytic property rapidlycreates a tribological surface between the materials of construction and thebiological environment by deposition of fibrin and other proteins and globulins,thereby enhancing the biocompatibility of the device and preventing seepage ofblood immediately after surgery. For grafts, then, the hemolysis assay haslittle predictive value for problems encountered in clinical use.

THE WORK AHEAD: VALIDATION

The main difficulty in creating specific standards is the amount of workrequired to validate tests and thus make them available for widespread use.Numerous assays for hemocompatibility have been and continue to be developed inresearch laboratories, but widespread adoption of the tests is often stalledbecause they are not rigorously validated. Validation establishes thecredibility of a candidate test through intra- and interlaboratory assessmentsand database development.

Intra- and interlaboratory assessments are used to determine the sensitivity,selectivity, and predictive value of an assay.¹¹ For a test to bevalid, it must be adequate in terms of these three factors. Sensitivityis the percentage of positive results, and selectivity is the percentageof negative results. Predictive value is the percentage of correct testresults, and is correlated with prevalence, the ratio of positive results to allsubstances tested. The predictive value of a test may be correlative ormechanistic.

Validating hemocompatibility assays will require reference materials, databases,and reference laboratories. For a particular test, a reference material is acharacterized material or substance that yields a reproducible result whentested. A standard reference material is a universally available material thatis characterized in regard to elemental composition, formulation, structure,phase and phase distribution, and impurity level in a prescribed physical formincluding, but not limited to, shape, surface character, and electrical charge.Biomedical scientists are a long way from developing a consensus on thecharacteristics of reference materials and establishing a repository of standardreference materials with defined biocompatibility and universal availability. Inthe early 1990s, the ISO task force on sample preparation and referencematerials sought to prepare a list of reference materials and standard referencematerials, but could find only a few.¹²

Other requirements for validation—consensus on what constitutes validation of anew material, a central repository for test performance data, an establishednetwork of reference laboratories capable of carrying out interlaboratoryassessments, and an understanding of the mechanisms of bloodbiocompatibility—will also require a committed effort.

CONCLUSION

A review of relevant medical device standards shows that the availablehemocompatibility assays are not always predictive for specific devices. In somecases, the assays may not be sensitive or selective enough. The community ofbiomedical specialists needs to recognize these limitations and work towardcreating a framework for validating assays that will establish standards forhemocompatibility that are applicable to all blood-contacting medical devices.

REFERENCES

1. "Biological Evaluation of Medical Devices, Part 1: Guidance on Selectionof Tests," ISO 10993-1, EN 30993-1, Geneva, International Organization forStandardization (ISO), 1992.

2. "Required Biocompatibility Training and Toxicology Profiles forEval-uation of Medical Devices," Blue Book Memorandum G95-1,Rockville, MD, FDA, Center for Devices and Radiological Health (CDRH), Office ofDevice Evaluation, 1995.

3. Reed KW, and Yalkowsky SH, "Lysis of Human Red Blood Cells in thePresence of Various Cosolvents," J Par Sci Technol, 39(2):64–68,1985.

4. "Transfusion Equipment for Medical Use, Part 3: Blood Taking Set,"ISO 1135-3, Geneva, ISO, 1986.

5. "Plastics Collapsible Containers for Human Blood and Blood Components,"ISO 3826-4, Geneva, ISO, 1988.

6. "Medical Surgical Equipment, Single Use Sterile Hemodialyzers and HemoFilters," French Standard NF S 90–302, Paris, Association Françaisede Normalisation (AFNOR), 1990.

7. "Extracorporeal Circuit Hemodialysis Dialyzers and Blood-Line SystemsMade of Plastics, Requirements and Testing," DIN 58 353, Part 3, Berlin,Deutsches Institut für Normung e.V. (DIN).

8. "Guidelines for Premarket Testing of New Conventional Hemodialyzers, HighPermeability Hemodialyzers and Hemofilters," Rockville, MD, FDA, CDRH,Bureau of Medical Devices, March 1992.

9. Moinard J, and Guenard H, "Membrane Diffusion of the Lungs in Patientswith Chronic Renal Failure," Eur Respir J, 6(2):225–230, 1993.

10. Vann RD, Ritter EF, Plunkett MD, et al., "Patency and Blood Flow in GasDenucleated Arterial Prostheses," J Biomed Mat Res, 27:493–498,1993.

11. Goldberg AM, Frazier JM, Brusick D, et al., "Framework for Validationand Implementation of In Vitro Toxicity Tests: Report of the Validation andTechnology Transfer Committee of the Johns Hopkins Center for Alternatives toAnimal Testing," J Am Coll Toxicol, 12:23–30, 1993.

12. "Biological Evaluation of Medical Devices, Part 12: Sample Preparationand Reference Materials," ISO 10993-12 (draft 4), Geneva, ISO, 1992.

Sharon Northup, PhD, is a managing associate with the WeinbergGroup, Inc. (Washington, DC).

Copyright © 1997 Medical Device & Diagnostic Industry