In thromboelastometry (ROTEM^TM) the difference in amplitude between the EXTEM and the FIBTEM is considered an index of platelet contribution to clot strength (PCS_amp). The difference in elasticity (PCS_el) is rarely used. We investigated the ability of PCS_amp and PCS_el in reflecting platelet count and function in 103 patients undergoing cardiac surgery, simultaneously measuring ROTEM and platelet function tests (multiple electrode aggregometry ADPtest and TRAPtest, Multiplate^TM). PCS_amp and PCS_el were tested for association with platelet count and function. The PCS_amp showed a low (R coefficient 0.32–0.39) association with platelet count and function (ADPtest), whereas the PCS_el showed higher values of association (R coefficient 0.55–0.71) with the same variables. No association was found between PCS and TRAPtest. In a multivariable model, both the platelet count (R coefficient 0.60, P = 0.001) and the ADPtest (R coefficient 0.36, P = 0.001) were independently associated with the PCS_el. The discrimination properties of the PCS_el for the prediction of a low platelet count/function were very good (c-statistics 0.837). In clinical practice, the difference in elasticity between EXTEM and FIBTEM should replace the difference in amplitude.