OBJECTIVE Coagulation point-of-care tests for the diagnosis and management of bleeding commonly are used in cardiac surgery. Recently, a new viscoelastic point-of-care device, Quantra (HemoSonics, Charlottesville, VA), which is based on sonic estimation of elasticity via resonance sonorheometry, entered the market. Prior studies have compared the Quantra parameters with conventional coagulation tests and thromboelastography or rotational thromboelastography, but no study has assessed the correlation of the platelet-derived Quantra parameter platelet contribution to stiffness (PCS) with platelet function assays. The present study compared the Quantra-derived coagulation parameters with conventional coagulation tests, rotational thromboelastography-derived parameters, and platelet function measured using multiplate aggregometry. METHODS Prospective cohort study. METHODS University research hospital. METHODS The study comprised 30 cardiac surgery patients before and after cardiopulmonary bypass. METHODS None. RESULTS Quantra clot stiffness showed a very strong correlation with the EXTEM maximum clot firmness before and after cardiopulmonary bypass and in the overall samples (r values 0.94-0.96). Quantra fibrinogen contribution to stiffness was compared with the corresponding FIBTEM and Clauss fibrinogen levels, which also showed a strong level of correlation (r values between 0.74 and 0.87). Quantra PCS strongly correlated with platelet count (r = 0.71) in the overall samples and moderately with adenosine diphosphate-dependent platelet function (r = 0.67). In a multivariable model, both the adenosine diphosphate test value and the platelet count remained independently associated with Quantra PCS. CONCLUSIONS Fibrinogen-dependent clot stiffness properties are well-reflected by the Quantra fibrinogen contribution to stiffness parameter, and PCS incorporates platelet count and function.