Q&A column

Editor: Frederick L. Kiechle, MD, PhD

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Q. What are your recommendations for using viscoelastic assays to perform platelet mapping studies? What is the clinical value of obtaining these test results?

A. October 2019—Viscoelastic assays (VEA) measure clot kinetics using whole blood samples and a cup and pin mechanism. With the TEG (Haemonetics) or ROTEM (Instrumentation Laboratory) devices, citrated blood samples are placed in a cup at 37°C and oscillated via cup and pin. As the clot forms, oscillation becomes increasingly restricted. Oscillation is detected mechanically with TEG (thromboelastography) or optically with ROTEM (rotational thromboelastometry) and a characteristic tracing is generated, with a trace reflecting the changes in viscoelasticity across all stages of clot formation and resolution.

The VEA mimics the in vivo hemostatic processes described in the cell-based model of hemostasis. A modification of the TEG test, the TEG PlateletMapping assay (TEG PM), isolates the effect of platelet inhibitors on clot strength. Together, the TEG and TEG PM provide a snapshot of the overall hemostatic status (overall functionality of coagulation factors, platelet function, and fibrinolysis) of the patient that can be used to tailor anticoagulant and antiplatelet therapies. The British Society for Haematology and International Society on Thrombosis and Haemostasis have published guidance on the utility of viscoelastic assays in various clinical settings.^1,2 No such guidelines are available for TEG PM, but it has emerged as an attractive approach for evaluating hemostasis and monitoring antithrombotic therapy.

The TARGET-CABG (Time Based Strategy to Reduce Clopidogrel Associated Bleeding Related to Coronary Artery Bypass Graft) study has shown that a TEG PM–based strategy may reduce bleeding and preoperative wait time in clopidogrel-treated patients undergoing coronary artery bypass graft surgery.³ Preoperative platelet function testing in the context of a transfusion algorithm may reduce red cell and fresh frozen plasma transfusion and blood loss in cardiac surgery.⁴

Patients implanted with mechanical circulatory support (MCS) devices are subject to conditions that may cause fluctuations in platelet function. Hemolysis, anemia, reactive thrombocytosis, increase in acute phase reactants (fibrinogen, von Willebrand factor), concurrent drugs, and renal dysfunction potentially will contribute to variable responses when testing platelet function in MCS patients. In the immediate postoperative period, it may be necessary to evaluate antithrombotic therapy daily. VEA-based protocols may provide clinicians with more complete information about imbalances in the patient’s hemostasis and efficacy of their anticoagulation and antiplatelet therapy from one assay.

Berlin Heart guidelines recommend using TEG and TEG PM to monitor anticoagulation and platelet inhibition in children with a Berlin Heart.^5,6 In our institution, we have used the TEG PM–based protocol to effectively manage antiplatelet therapy in patients supported by a mechanical circulatory support device.⁷ Although some concerns about the design of the TEG PM assay have been raised recently,^8,9 at our institution TEG PM is an acceptable method to monitor antiplatelet effect in patients supported by a MCS device.

Based on our experience monitoring platelet function in patients implanted with MCS devices, the TEG PM provides reliable and reproducible results. However, achieving reliable and reproducible results requires proper training in operating TEG PM instruments, a standardized protocol for blood collection, and interpretation expertise. Our institution has a team of eight laboratory scientists who are trained by the manufacturer, and an extensive validation process was undertaken prior to clinical use of the TEG PM test.

In 2015, Haemonetics released its TEG 6s, a point-of-care VEA that uses resonance frequency viscoelasticity measurements and a disposable, multichannel microfluidic cartridge to assess hemostasis and response to antiplatelet therapy. The TEG 6s is easier to use than the TEG 5000 and provides precise results. Multiple validation studies show that the TEG 6s is a promising device,^10,11 but more clinical experience and trials are needed before it can replace the TEG 5000/TEG PM.

The ROTEM platelet assay, which is for research use only and not FDA cleared, measures platelet aggregation using impedance. It is run in conjunction with the ROTEM Delta. A recently published study showed that this device correlates only moderately with other platelet function assays.¹²

1. Curry NS, Davenport R, Pavord S, et al. The use of viscoelastic haemostatic assays in the management of major bleeding: a British Society for Haematology Guideline. Br J Haematol. 2018;182(6):789–806.
2. Thomas W, Samama CM, Greinacher A, Hunt BJ; Subcommittee on Perioperative and Critical Care. The utility of viscoelastic methods in the prevention and treatment of bleeding and hospital-associated venous thromboembolism in perioperative care: guidance from the SSC of the ISTH. J Thromb Haemost. 2018;16(11):2336–2340.
3. Mahla E, Suarez TA, Bliden KP, et al. Platelet function measurement-based strategy to reduce bleeding and waiting time in clopidogrel-treated patients undergoing coronary artery bypass graft surgery: the timing based on platelet function strategy to reduce clopidogrel-associated bleeding related to CABG (TARGET-CABG) study. Circ Cardiovasc Interv. 2012;5(2):261–269.
4. Agarwal S, Johnson RI, Shaw M. Preoperative point-of-care platelet function testing in cardiac surgery. J Cardiothorac Vasc Anesth. 2015;29(2):333–341.
5. Fraser CD Jr, Jaquiss RDB, Rosenthal DN, et al. Prospective trial of a pediatric ventricular assist device. N Engl J Med. 2012;367(6):532–541.
6. Rosenthal DN, Lancaster CA, McElhinney DB, et al. Impact of a modified anti-thrombotic guideline on stroke in children supported with a pediatric ventricular assist device. J Heart Lung Transplant. 2017;36(11):1250–1257.
7. Volod O, Lam LD, Lin G, et al. Role of throm-boelastography platelet mapping and international normalized ratio in defining “normocoagulability” during anticoagulation for mechanical circulatory support devices: a pilot retrospective study. ASAIO J. 2017;63(1):24–31.
8. Karon BS, Tolan NV, Koch CD, et al. Precision and reliability of 5 platelet function tests in healthy volunteers and donors on daily antiplatelet agent therapy. Clinical Chem. 2014;60(12):1524–1531.
9. Nelles NJ, Chandler WL. Platelet mapping assay interference due to platelet activation in heparinized samples. Am J Clin Pathol. 2014;142(3):331–338.
10. Gurbel PA, Bliden KP, Tantry US, et al. First report of the point-of-care TEG: a technical validation study of the TEG-6S system. Platelets. 2016;27(7):642–649.
11. Olechowski B, Dalton RT, Khanna V, et al. Detection of individual responses to clopidogrel: validation of a novel, rapid analysis using thrombelastography 6s. Cardio­vasc Ther. 2018;36(4):e12433. doi: 10.1111/1755-5922.12433.
12. Polzin A, Helten C, Dannenberg L, et al. Platelet reactivity in patients on aspirin and clopidogrel therapy measured by a new bedside whole-blood assay. J Cardiovasc Pharmacol. 2019;73(1):40–47.

Oksana Volod, MD
Director, Coagulation Consultative Service
Associate Professor of Pathology
Cedars-Sinai Medical Center
David Geffen School of Medicine at UCLA Los Angeles
Member, CAP Hemostasis and Thrombosis Committee

In this “Best of Q&A” series, we reprint select coagulation-related questions and answers. All have been chosen for their timeliness and relevance today. The following question and answer were published in June 2017.

Q. In thawing plasma specimens for routine coagulation studies (prothrombin time, partial thromboplastin time, D-dimer, fibrinogen) as well as for special coagulation studies (lupus, proteins C and S), I am aware that water bath (37°C) thawing is highly recommended. Would the results of these tests be affected if I thawed them using a dry heating block? Are we allowed to use dry heating blocks?

A. The reader correctly points out that the preferred manner in which to thaw frozen plasma samples for coagulation studies is a water bath. It is specifically recommended that frozen plasma samples are thawed rapidly in a 37°C water bath for approximately three to five minutes, depending on the size of the aliquot tube, the amount of plasma in the tube, and the number of tubes in the water bath. Samples should be completely thawed but not left to linger in the water bath after thawing is complete. Care must be taken to ensure that the water bath is consistently maintained at the correct temperature. Inadequate or excessive incubation at 37°C must be avoided as sample integrity may be compromised if samples are either not completely thawed or maintained too long at 37°C. Incomplete thawing may not allow proteins that precipitate in the cold, such as factor VIII, factor XIII, and von Willebrand factor, to come into solution, resulting in the potential to falsely report these factors as decreased. This could result in an incorrect diagnosis and serious patient mismanagement. Prolonged exposure to 37°C or exposure to higher temperatures may lead to deterioration of coagulation factor activities and the reporting of spuriously low coagulation factor activity results or spuriously elevated activated partial thromboplastin times and prothrombin times.

Once samples are thawed, they should be promptly removed from the water bath and thoroughly and adequately mixed before testing. The advantage of thawing samples in a water bath is that the heat source is indirect and the thawing process is gentle and gradual.

Dry heat blocks may be available in clinical laboratories since dry heat can be used for incubation and activation of microbial cultures, for enzyme reactions, and when performing molecular analysis. However, dry heat blocks are not recommended for thawing frozen plasma samples because they provide a direct source of heat to the plasma sample. To my knowledge, published studies are not available that compare the impact on the integrity of plasma samples of thawing frozen plasma samples on dry heat blocks versus thawing in a 37°C water bath.

Adcock Funk DM, Lippi G, Favaloro EJ. Quality standards for sample processing, transportation, and storage in hemostasis testing. Semin Thromb Hemost. 2012;38(6):576–585.

Dorothy M. Adcock, MD
Currently Chief Medical Officer,
Senior Vice President, LabCorp of America
Burlington, NC