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The effect of blood adhesion on the amplitude-frequency characteristics of a piezoelectric sensor in the method of low-frequency piezothromboelastography is studied. As a result of adhesion, a layer of blood cells can accumulate on the surface of the resonator needle immersed in blood, which changes its configuration and mass and, as a consequence, its amplitude-frequency characteristics. Experiments showed that during coagulation, the free frequency of the resonator needle oscillations decreases in direct proportion to the increase in the mass of adhered blood. To confirm this regularity, we carried out a numerical experiment on the basis of 3D-mathematical model of a piezoelectric sensor used in the piezotromboelastograph ARP-01M Mednord. The experiment was performed with the help of the COMSOL Multiphysics® 4.2 software package and the finite element method. We calculated the change in the free frequency of the resonator needle oscillations as a function of the adhered blood mass and their time dependence in the blood coagulation process. It is shown that during coagulation, the mass of adhered blood increases starting from the third minute and reaches its maximum of 0.42 milligram in 15 minutes, which leads to a noticeable shift in the resonance frequency of the piezoelectric sensor towards lower frequencies.