An overview of platelet function testing

Platelet aggregation, as practiced by Drs. Born, O’Brien, and others in the 1960s, was a complicated but not a complex process. It opened an era of discovery in platelet and vascular biology, heritable diseases and their management, and drug development and therapies. In turn, these discoveries spawned new capabilities, technologies, and avenues to explore, along with new challenges.

Platelet function testing is transitioning from a confirmatory, diagnostic tool for heritable diseases to a functional one for platelet-centric therapies. Here is an overview of some of the issues in platelet function testing that are most relevant today.

Perceptions and misperceptions

Mention platelet aggregation or platelet function testing to many laboratorians or clinicians, and the image that may come to mind is of a beleaguered tech, working feverishly, alone in a small lab at the end of the hall. She alone handles specimen collection, processing, and sample preparation, and it is a long and tedious task. Testing, offered only on select days and times, is performed only by her, and it seems to involve a kind of wizardry. Results and their interpretation often do not include clinically useful answers or guidance.

The realities of today’s platelet function testing belie that stereotypical image. Today, platelet function itself is highly complex, and there is still much to learn. But the tools and technologies that are available to today’s laboratory have demystified the analytical process and organized test results. Contemporary aggregometers are computer-managed analyzers with operating features similar to other laboratory instruments. System software provides organized results and data analysis capabilities. Reagents and test kits are commercially available. Evacuated specimen collection tubes are now used to obtain specimens. Specimen processing can be accomplished in under ten minutes. Guidelines for pre-analytical, analytical, and post-analytical processes are available from a number of organizations. Clinical guidelines have been published.

Applications: Dx to Rx

The traditional focus of platelet function tests has been the detection and confirmation of the various heritable platelet diseases or dysfunction. Specialty laboratories are reasonably competent in doing this despite inherent differences in sample preparation, technology used, and test methods. Accessibility and inter-laboratory variability driven by the lack of common practices continue to be challenges for these laboratories.

Heritable platelet disorders affect about 1% of the population. This segment of the platelet testing market is flat and mature. Technology changes for mature markets tend to be evolutionary. The development, efficacy, and management of antiplatelet therapies have fulfilled Gus Born’s prophetic statement:“The inhibition of aggregation was the most important discovery made with optical aggregation because it established the therapeutic possibility of preventing arterial thrombosis by antiplatelet drugs and initiated the era of their beneficial use.”¹

The recent, rapid adoption of antiplatelet therapy, particularly dual antiplatelet therapy, and the ubiquitous interest in platelet reactivity in thrombotic disease and therapy have changed the focus of platelet function laboratories from diagnostics to therapeutics.

Science and technology

Light Transmission Aggregometry (LTA) is the gold standard for platelet function testing for diagnostic and therapeutic analyses.^2,3 LTA is the only platelet function test endorsed by platelet experts.⁴ LTA’s principle of operation is the measurement of optical changes in 37ºC, stirred Platelet Rich Plasma over time when an agonist is added, compared to a Platelet Poor Plasma blank.¹ Differences in commercially available LTA systems include various parameters, such as wavelength (405 – 1060 nm), stirring speeds, and
measurement methodologies.

Impedance Aggregometry (IA) measures changes in electrical resistance caused by platelets in a diluted blood sample adhering to charged electrodes in response to an agonist. IA cannot measure disaggregation. A number of factors can affect the accuracy of impedance measurement.⁵

There are also a number of near patient tests for platelet function available. These tests focus on the clinical need for the selection, efficacy, and monitoring of antiplatelet therapies. The appeal and use of such tests is logical. Evidence based on patient outcomes, however, does not support clinical use of these tests.^6-8

Laboratory developed tests (LDTs)

Clinical and medical specialty laboratories have a long and fruitful history of developing tests for special groups of patients and conditions—often because there were no commercial tests or other laboratories that could provide the needed information. The explosive growth of collegial and inter-laboratory communications in the last half of the 20^th century led to wider knowledge about these “home brew” or LDTs and their use outside the laboratories that developed them. The platelet function laboratory has been an active participant in the development and use of LDTs.

The components of LDTs have often been fine chemicals and other laboratory materials not regulated by the United States Food and Drug Administration (FDA). Available instrumentation was sometimes modified to perform the home brew test, or the test was not one that was within the scope or intended use of the instrument. More recently, software used to perform, control, or collect and analyze data from these tests has been itself developed in the laboratory.

Recent regulatory focus on LDTs has generated two new challenges for the laboratory. First, the use of an LDT could result in the laboratory being considered the manufacturer of an unapproved in vitro diagnostic or medical device. Second, the FDA has determined that, if an approved product is not used exactly as specified in its labeling, the modified test is an LDT and subject to FDA regulations.⁹

Unmet medical needs

There are two major unmet medical needs that could impact laboratory capacity and test volume as well as patient outcomes. The first is compliance testing, and the second is monitoring of high-risk drugs.

Former U.S. Surgeon General C. Everett Koop’s assessment of patient compliance was blunt: “Drugs don’t work in patients who don’t take them.¹⁰ Compliance rates for patients in developed countries who are on long-term therapies is between 40% and 50%.¹¹  The pharmaceutical industry estimates patient non-compliance results in lost revenues of $1.88 billion per year. Anti-platelet aggregation agents accounted for 2.5% of 2011 pharma revenue, or about $7.8 billion. Lost pharma revenue for anti-platelet drugs was calculated to be $3.86 billion. The additional burden on the healthcare system as a direct result of patient non-compliance is estimated to be $290 billion.¹²  There are no reports of the magnitude of the lost revenue for laboratories that did not perform compliance or monitoring tests on these patients.

The 2009 Cardiovascular Unmet Medical Needs Survey results show that 99% of cardiologists expect that some of their patients will experience subsequent cardiac events while on antiplatelet therapy, and 88% believe that these events are the result of the patient’s resistance or low response to therapy.¹³

Therapeutic drug monitoring is very underutilized, even when there is consensus that a particular drug should be monitored. Tjia et al report that the range for appropriate monitoring is 0% to 88%. Patients started on new therapies had less than 20% of the recommended testing. Patients with chronic conditions fared better but still had less than 40% of the recommended tests.¹⁴ There is an unmet demand of 60% to 80% of patients on medication who require laboratory monitoring, but do not have the testing performed. As laboratorians, we must ask ourselves whether the issue is accessibility, affordability, or the lack of a clinically useful test.

Laboratory monitoring of high-risk drugs

Pharmaceutical companies have two goals that are diametrically opposed. The ideal therapeutic is one that requires no monitoring. Variability in patient responses to therapy, particularly with high-risk drugs or those with a narrow therapeutic window, requires practical, clinically useful, and affordable monitoring.The development and use of companion diagnosticshas the potential to reinforce the need and appropriateness of laboratory monitoring of new drugs. Swinderman predicts that the use of companion diagnostics will be the standard of care in a decade.¹⁵

Standardization

Standardization has long been identified as, and remains, a significant challenge for laboratories performing platelet function tests. Although there are groups developing and updating standards and guidelines,there are wide variations in technology, pre-analytical practices, and analytical methods as well as the appropriate use of these tests. As a result, clinical correlations, patient outcomes, and reports can be confounding as well as conflicting.

Reimbursement

Reimbursement for platelet aggregation tests was reduced on the 2013 Centers for Medicare and Medicaid Services (CMS) Laboratory Fee Schedule. Some near patient tests are also classified as platelet aggregation tests. While the reductions of the National Fee Limits are about 3%, reimbursement rates at the state levels are as much as 63.9% less than the national limit. For the von Willebrand factor activity test, the National Limit Reduction was also about 3%, but state levels are as low as 27.1% of the National Limit.^16,17 Some manufacturers publish test or instrument specific reimbursement guides. The effect of the Affordable Care Act and other federal budget negotiations are not clear for the fiscal year 2014, which starts October 1, 2013.

To boldly go…?

Barry Collar has described the past 50 years as the Golden Age of Platelet Research and predicts that we are approaching the beginning of a new period in the exploration and understanding of the platelet’s many functions.¹⁸ The Golden Age has provided a foundation for the future. The full promise and potential of platelet function testing has yet to be attained. It is clearly recognized as a “fantastic marker of risk” according to Professor Giles Montalescot.¹⁹

There are multiple frontiers for platelet function and research. These include standardization, drug discovery, molecular mechanics, receptors, ligands, genetics and genomics, “resistance,” hypersensitivity, personalized therapies, and companion diagnostics. And, perhaps the most challenging of all: the function of platelets beyond the traditional roles in thrombosis and hemostasis.

1. Born GV. Aggregation of blood platelets by adenosine diphosphate and its reversal. Nature. 1962;194:927-929.
2. Gresele P, et al. Platelets in Thrombotic and Non-thrombotic Disorders. Cambridge, UK: Cambridge University Press; 2002;473.
3. Topol EJ, Gum P, Kottke-Marchant K. Determination of the natural history of aspirin resistance among stable patients with cardiovascular disease. J Am Coll Cardiol. 2003;42(7):1336-1337.
4. Schaef-Jones G. Clopidogrel platelet function tests: caveats and controversies. Department of Laboratory Medicine and Pathology. Mayo Clinic, Jacksonville, FL. 2011.
5. Clinical and Laboratory Standards Institute. Platelet function testing by aggregometry. Approved Guideline H 58 A. 2008.
6. Holmes DR Jr, Dehmer GJ, Kaul S, Leifer D, O’Gara PT, Stein CM. ACCF/AHA clopidogrel clinical alert: approaches to the FDA “boxed warning.” J Am Coll Cardiol. 2010. doi:10.1016/j.jacc.2010.05.013.
7. Gurbel PA, et al. Platelet function during extended prasugrel and clopidogrel therapy for patients with ACS treated without revascularization. JAMA. 2012;306(17):1785-1794.
8. Lim GB. Antiplatelet therapy: ARCTIC leaves platelet testing out in the cold. Nature Reviews Cardiology. doi:10.1038/nrcardio.2012.165.
9. Guidance for Industry and FDA Staff. Small Entity Compliance Guide 2-3. Center for Devices and Radiologic Health. FDA. February 26, 2003.
10. Osterberg L, Blasche T. Adherence to medication. N Engl J Med. 2005;353(5):487-497.
11. Li SC, Jin J,  Sklar GE,  Min V,  Oh S. Factors affecting therapeutic compliance: a review from the patient’s perspective. Therapeutics and Clinical Risk Management. 2008:4(1):269-286.
12. Medication non-adherence is a $1.88 billion revenue gap for pharma, says Capgemni. Pharmaceutical Commerce. 2013;1:10.
13. Harris Interactive, on behalf of Schering-Plough Corporation. Cardiovascular unmet medical needs survey. March 2009.
14. Tjia J, Field TS, Garber LD. Development and pilot testing of guidelines to monitoring high-risk medications in the ambulatory setting. Am J Mang Care. 2010;16(7):489-496.
15. Swinderman A. ddn Market Research: CDx expected to become the standard of care within the next decade. Drug Discovery News. December 2012; 36.
16. Center for Medicare and Medicaid Services. 2012 Clinical Diagnostic Laboratory Fee Schedule. Includes all changes in CR7654. Revised for January 2012.
17. Center for Medicare and Medicaid Services. 2013 Clinical Diagnostic Laboratory Fee Schedule. Includes all changes in CR8132. Revised for January 2013.
18. Collar BS. Historical perspective and future directions in platelet research. J Thromb Haemost. 2011;9 (Suppl 1):374-395.
19. Price MJ, Montalescot G. Bringing platelet function in from the cold: platelet response redux. Medscape Education CME. 2012.

William M. Trolio, BS, MT, CLT, MBA, FBA, is Vice President and Chief Science Officer for Pennsylvania-based Bio/Data Corporation. Contact him at bill.trolio@ biodatacorp.com