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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.