Sensitivity of Viscoelastic Tests to Platelet Function

Abstract

Viscoelastic tests provide a dynamic assessment of coagulation, by exploring the time to clot formation and the clot strength. Using specific activators or inhibitors, additional factors can be explored, like the fibrinogen contribution to clot strength. Since the early days, various attempts have been done to measure platelet function with viscoelastic test. In general, the difference between the maximum clot strength and the fibrinogen contribution is considered an index of platelet contribution. However, this parameter does not clearly split platelet count from function; additionally, the extensive thrombin generation of standard activated viscoelastic tests activates platelet through the protease activated receptors, bypassing the other pathways. For this reason, standard viscoelastic tests cannot be used to assess platelet reactivity under the effects of aspirin or P2Y₁₂ inhibitors. To overcome this limitation, a specific test was developed (thromboelastography platelet mapping). This test has been compared with the gold standard of light transmission aggregometry and with other point-of-care tests, with conflicting results. In general, the use of viscoelastic tests to assess the effects of antiplatelet agents is still limited. Conversely, platelet contribution to clot strength in the setting of coagulopathic bleeding is considered an important parameter to trigger platelet transfusion or desmopressin.

Keywords: platelet function tests; platelets; viscoelastic methods.

Figure 1

A standard viscoelastic test tracing. ROTEM: rotational thromboelastometry; TEG: thromboelastography.

Figure 2

Platelet contribution to clot strength as the difference between amplitudes in standard thromboelastography (TEG, white trace) and fibrinogen contribution (green trace).

Figure 3

Non-linear relationship between clot amplitude and shear modulus.

Figure 4

Thromboelastography (TEG) platelet mapping. White trace is standard TEG activated with kaolin; green trace is a platelet-free clot obtained by adding heparin and reptilase; orange tracing is the platelet activity, adding a specific platelet activator (arachidonic acid or ADP).