Editorials
Clinical Chemistry 60:12 1469–1470 (2014)
Platelet Function Assays—Not All Are Created Equal Wayne L. Chandler*
Platelet function testing has undergone major changes in the last decade. In the past, these tests were primarily used to diagnose hereditary bleeding disorders, typically using a platelet aggregation assay. Although diagnosis of bleeding disorders is still important, measuring platelet function to monitor platelet inhibitor therapy has become the more common reason for ordering this type of testing. This has also led to a variety of new platelet function assays designed to assist platelet inhibitor monitoring. It is now clear that many patients show resistance to platelet inhibitor therapy. These patients show less platelet inhibition than expected for a given dose of medication, and an increased risk of thrombosis. The most common forms of resistance are related to clopidogrel and aspirin therapy. Several different resistance mechanisms have been described, but the most common is reduced metabolism of clopidogrel to its active form, leading to inadequate platelet inhibition. Reduced absorption of medications and receptor polymorphisms leading to reduced binding and inhibition by these drugs has also been reported. Two approaches have been taken to assess resistance to platelet inhibitors: genetic studies of specific polymorphisms associated with drug resistance and platelet function studies. Platelet function assays provide an overall measure of drug effect, in theory integrating assessment of all the different potential resistance mechanisms into a single assay. Platelet function assays have been used to assess platelet inhibitor therapy in several different ways. First, for patients on chronic platelet inhibitor therapy, platelet function assays have been used to assess whether the platelet inhibitor appears to be working appropriately so that, if necessary, the dose or medication may be changed. Speed of the assay is less important than the accuracy of the assessment, since the therapy is long-term. Recent evidence-based evaluations of this type of monitoring in the internal medicine literature have concluded that there is insufficient evidence to support routine monitoring of platelet inhibitor
Department of Pathology and Genomic Medicine, Houston Methodist Hospital, Houston, TX. * Address correspondence to the author at: Methodist Pathology Associates, 6565 Fannin, Suite B-490, Houston, TX 77030. Fax 713-441-3489; e-mail [email protected]. Received September 25, 2014; accepted September 29, 2014. Previously published online at DOI: 10.1373/clinchem.2014.233130 © 2014 American Association for Clinical Chemistry
therapy using platelet function or other assays (1 ). This being said, this type of monitoring is common in many hospital laboratories today. Another use of platelet function assays is rapid assessment of platelet inhibition after interventional procedures, typically angioplasty and stent placement (2 ). In this situation, a simple rapid assay of platelet function is required to help prevent near-term periprocedural thrombosis. A third use for functional assessment of platelet inhibitor therapy is prediction of bleeding risk and cause of bleeding. Typically, this type of assessment occurs before a surgical or interventional procedure to predict bleeding risk associated with platelet inhibitor drugs or in a patient with new-onset bleeding to determine whether a platelet inhibitor might be contributing to the bleeding. In both cases, repeat testing is often performed to determine whether the platelet inhibition has cleared before surgery or in patients with continued bleeding. Rapid assessment and 24-h availability of platelet function testing is desired for this type of evaluation, typically necessitating a simple, point-of-care type of assay. Evaluation in the surgical literature of this type of platelet function assessment has concluded that it may be useful, particularly if normal platelet function is found (negative predictive value) (3 ). The study by Karon et al. in this issue provides additional information to help select assays for platelet function assessment (4 ). The study was not designed to determine clinical efficacy; instead, it evaluated the precision and reliability of 5 different platelet function assays: light transmission aggregometry, multiplate impedance aggregometry, VerifyNow, thromboelastography platelet mapping, and flow cytometry. Four of the assays were designed for operation by a technologist (light transmission aggregometry, multiplate impedance aggregometry, thromboelastography platelet mapping, and flow cytometry). One assay has a simple point-of-care design: VerifyNow, suitable for rapid analysis at all hours. The VerifyNow assay is the one that has been used most often for quick-turnaround analysis associated with interventional procedures. Multiplate impedance aggregometry was best overall for evaluation of aspirin and clopidogrel effect for patients on chronic therapy. In the study of Karon et al. (4 ), 1 assay clearly demonstrated worse performance than all the others: thromboelastography platelet mapping. This assay 1469
Editorials demonstrated poor precision and poor reliability. The author’s final conclusion was that thromboelastography platelet mapping “is least suited to monitor effects of antiplatelet agents” (4 ). This finding is consistent with several recent studies which found that thromboelastography platelet mapping had serious design and operational issues, including the use of heparinized samples for platelet function analysis that lead to platelet activation in the sample, overestimation of platelet inhibition in patients not taking medications (5 ), lack of utility for predicting preoperative bleeding risk (6 ), and lack of utility for monitoring platelet inhibitor therapy (7 ). Whereas it is still open to debate whether platelet function monitoring with current assays shows overall improvement in patient outcome, clearly some assays, in particular thromboelastography platelet mapping, are not suited for this type of evaluation.
Author Contributions: All authors confirmed they have contributed to the intellectual content of this paper and have met the following 3 requirements: (a) significant contributions to the conception and design, acquisition of data, or analysis and interpretation of data; (b) drafting or revising the article for intellectual content; and (c) final approval of the published article.
1470 Clinical Chemistry 60:12 (2014)
Authors’ Disclosures or Potential Conflicts of Interest: No authors declared any potential conflicts of interest.
References 1. Douketis JD, Spyropoulos AC, Spencer FA, Mayr M, Jaffer AK, Eckman MH, et al. Perioperative management of antithrombotic therapy: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines. Chest 2012;141(2 Suppl): e326S– e350S. 2. Pakala R, Waksman R. Currently available methods for platelet function analysis: advantages and disadvantages. Cardiovasc Revasc Med 2011;12: 312–22. 3. Ferraris VA, Saha SP, Oestreich JH, Song HK, Rosengart T, Reece TB, et al.; Society of Thoracic Surgeons. 2012 update to the Society of Thoracic Surgeons guideline on use of antiplatelet drugs in patients having cardiac and noncardiac operations. Ann Thorac Surg 2012;94:1761– 81. 4. Karon BS, Tolan NV, Koch CD, Wockenfus AM, Miller RS, Lingineni RK, et al. Precision and reliability of 5 platelet function tests in healthy volunteers and donors on daily antiplatelet agent therapy. Clin Chem 2014; 60:1524 –31. 5. Nelles NJ, Chandler WL. Platelet mapping assay interference due to platelet activation in heparinized samples. Am J Clin Pathol 2014;142: 331– 8. 6. Alstrom U, Granath F, Oldgren J, Ståhle E, Tyde´n H, Siegbahn A. Platelet inhibition assessed with VerifyNow, flow cytometry and PlateletMapping in patients undergoing heart surgery. Thromb Res 2009;124:572–7. 7. Collyer TC, Gray DJ, Sandhu R, Berridge J, Lyons G. Assessment of platelet inhibition secondary to clopidogrel and aspirin therapy in preoperative acute surgical patients measured by Thromboelastography® Platelet mapping™. Br J Anaesth 2009;102:492– 8.
Clinical Chemistry 60:12 1469–1470 (2014)
Platelet Function Assays—Not All Are Created Equal Wayne L. Chandler*
Platelet function testing has undergone major changes in the last decade. In the past, these tests were primarily used to diagnose hereditary bleeding disorders, typically using a platelet aggregation assay. Although diagnosis of bleeding disorders is still important, measuring platelet function to monitor platelet inhibitor therapy has become the more common reason for ordering this type of testing. This has also led to a variety of new platelet function assays designed to assist platelet inhibitor monitoring. It is now clear that many patients show resistance to platelet inhibitor therapy. These patients show less platelet inhibition than expected for a given dose of medication, and an increased risk of thrombosis. The most common forms of resistance are related to clopidogrel and aspirin therapy. Several different resistance mechanisms have been described, but the most common is reduced metabolism of clopidogrel to its active form, leading to inadequate platelet inhibition. Reduced absorption of medications and receptor polymorphisms leading to reduced binding and inhibition by these drugs has also been reported. Two approaches have been taken to assess resistance to platelet inhibitors: genetic studies of specific polymorphisms associated with drug resistance and platelet function studies. Platelet function assays provide an overall measure of drug effect, in theory integrating assessment of all the different potential resistance mechanisms into a single assay. Platelet function assays have been used to assess platelet inhibitor therapy in several different ways. First, for patients on chronic platelet inhibitor therapy, platelet function assays have been used to assess whether the platelet inhibitor appears to be working appropriately so that, if necessary, the dose or medication may be changed. Speed of the assay is less important than the accuracy of the assessment, since the therapy is long-term. Recent evidence-based evaluations of this type of monitoring in the internal medicine literature have concluded that there is insufficient evidence to support routine monitoring of platelet inhibitor
Department of Pathology and Genomic Medicine, Houston Methodist Hospital, Houston, TX. * Address correspondence to the author at: Methodist Pathology Associates, 6565 Fannin, Suite B-490, Houston, TX 77030. Fax 713-441-3489; e-mail [email protected]. Received September 25, 2014; accepted September 29, 2014. Previously published online at DOI: 10.1373/clinchem.2014.233130 © 2014 American Association for Clinical Chemistry
therapy using platelet function or other assays (1 ). This being said, this type of monitoring is common in many hospital laboratories today. Another use of platelet function assays is rapid assessment of platelet inhibition after interventional procedures, typically angioplasty and stent placement (2 ). In this situation, a simple rapid assay of platelet function is required to help prevent near-term periprocedural thrombosis. A third use for functional assessment of platelet inhibitor therapy is prediction of bleeding risk and cause of bleeding. Typically, this type of assessment occurs before a surgical or interventional procedure to predict bleeding risk associated with platelet inhibitor drugs or in a patient with new-onset bleeding to determine whether a platelet inhibitor might be contributing to the bleeding. In both cases, repeat testing is often performed to determine whether the platelet inhibition has cleared before surgery or in patients with continued bleeding. Rapid assessment and 24-h availability of platelet function testing is desired for this type of evaluation, typically necessitating a simple, point-of-care type of assay. Evaluation in the surgical literature of this type of platelet function assessment has concluded that it may be useful, particularly if normal platelet function is found (negative predictive value) (3 ). The study by Karon et al. in this issue provides additional information to help select assays for platelet function assessment (4 ). The study was not designed to determine clinical efficacy; instead, it evaluated the precision and reliability of 5 different platelet function assays: light transmission aggregometry, multiplate impedance aggregometry, VerifyNow, thromboelastography platelet mapping, and flow cytometry. Four of the assays were designed for operation by a technologist (light transmission aggregometry, multiplate impedance aggregometry, thromboelastography platelet mapping, and flow cytometry). One assay has a simple point-of-care design: VerifyNow, suitable for rapid analysis at all hours. The VerifyNow assay is the one that has been used most often for quick-turnaround analysis associated with interventional procedures. Multiplate impedance aggregometry was best overall for evaluation of aspirin and clopidogrel effect for patients on chronic therapy. In the study of Karon et al. (4 ), 1 assay clearly demonstrated worse performance than all the others: thromboelastography platelet mapping. This assay 1469
Editorials demonstrated poor precision and poor reliability. The author’s final conclusion was that thromboelastography platelet mapping “is least suited to monitor effects of antiplatelet agents” (4 ). This finding is consistent with several recent studies which found that thromboelastography platelet mapping had serious design and operational issues, including the use of heparinized samples for platelet function analysis that lead to platelet activation in the sample, overestimation of platelet inhibition in patients not taking medications (5 ), lack of utility for predicting preoperative bleeding risk (6 ), and lack of utility for monitoring platelet inhibitor therapy (7 ). Whereas it is still open to debate whether platelet function monitoring with current assays shows overall improvement in patient outcome, clearly some assays, in particular thromboelastography platelet mapping, are not suited for this type of evaluation.
Author Contributions: All authors confirmed they have contributed to the intellectual content of this paper and have met the following 3 requirements: (a) significant contributions to the conception and design, acquisition of data, or analysis and interpretation of data; (b) drafting or revising the article for intellectual content; and (c) final approval of the published article.
1470 Clinical Chemistry 60:12 (2014)
Authors’ Disclosures or Potential Conflicts of Interest: No authors declared any potential conflicts of interest.
References 1. Douketis JD, Spyropoulos AC, Spencer FA, Mayr M, Jaffer AK, Eckman MH, et al. Perioperative management of antithrombotic therapy: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines. Chest 2012;141(2 Suppl): e326S– e350S. 2. Pakala R, Waksman R. Currently available methods for platelet function analysis: advantages and disadvantages. Cardiovasc Revasc Med 2011;12: 312–22. 3. Ferraris VA, Saha SP, Oestreich JH, Song HK, Rosengart T, Reece TB, et al.; Society of Thoracic Surgeons. 2012 update to the Society of Thoracic Surgeons guideline on use of antiplatelet drugs in patients having cardiac and noncardiac operations. Ann Thorac Surg 2012;94:1761– 81. 4. Karon BS, Tolan NV, Koch CD, Wockenfus AM, Miller RS, Lingineni RK, et al. Precision and reliability of 5 platelet function tests in healthy volunteers and donors on daily antiplatelet agent therapy. Clin Chem 2014; 60:1524 –31. 5. Nelles NJ, Chandler WL. Platelet mapping assay interference due to platelet activation in heparinized samples. Am J Clin Pathol 2014;142: 331– 8. 6. Alstrom U, Granath F, Oldgren J, Ståhle E, Tyde´n H, Siegbahn A. Platelet inhibition assessed with VerifyNow, flow cytometry and PlateletMapping in patients undergoing heart surgery. Thromb Res 2009;124:572–7. 7. Collyer TC, Gray DJ, Sandhu R, Berridge J, Lyons G. Assessment of platelet inhibition secondary to clopidogrel and aspirin therapy in preoperative acute surgical patients measured by Thromboelastography® Platelet mapping™. Br J Anaesth 2009;102:492– 8.
