The process of native blood coagulation was studied with the resonant-acoustic method using the technology of low-frequency piezothromboelastography. It is shown that the experimental curve of time dependence of the piezoelectric sensor signal amplitude reflects the change in the rheological properties of blood during coagulation. A formula is obtained that relates the change in the concentration of thrombin to the rate of change in the complex coefficient of blood viscosity. A method has been developed for assessing thrombin concentration during fibrinogenesis based on the technology of piezothromboelastography using a resonant-acoustic method to determine the viscoelastic properties of whole blood. The results of calculating the thrombin concentration by this method are compared with the results of the thrombin generation test.