Background: Patients undergoing cardiac surgery are at risk for excessive bleeding and associated complications. This bleeding often leads to transfusion of allogeneic blood and hemostatic blood components. The aim of study: It was to investigate the association between platelet function as determined by PFA-100® testing and blood loss and need for transfusion of blood products as well as PFA-100® method for assessing the effectiveness of platelet (PLT) transfusion in cardiac surgery patients. Methods: After Medical Ethics Committee of University Hospital Center Split approval and written consent, 126 patients undergoing cardiac surgery were investigated in this prospective observational study and 96 patients met the required criteria for all parameters of the study. For the study blood samples were collected and analyzed at three time points: (1) before operation, (2) 15 min after administration of protamine and neutralization of heparin, (3) 30-60 minute after platelet transfusion. In the first part of the study we investigated the effectiveness of the PFA-100® method with respect to the limit value of CADP-CT (118 s) and CEPI-CT (165 s) for assessment risk of bleeding and consumption of blood products in patients undergoing coronary artery bypass grafting (CABG) and patients with cardiac valve repair and replacement (non-CABG). In statistical analysis categorical variables are expressed as percentages and continuous variables as mean ± standard deviation (SD). Comparison between subgroups was made by using the t-test to test the differences between the two arithmetic means in subgroups. P value < 0.05 was considered statistically significant. In the second part of the study we compared the two methods of assessing the effectiveness of PLT transfusion: (1) PLT count > 200x109/L after PLT transfusion and (2) CADP-CT < 118 s after PLT transfusion. PLT transfusion was estimated effectively if the patient did not need PLT for 48 h after surgery. Sensitivity, specificity and predictive value are used to estimate the effectiveness of the methods. Results: Comparison of the variables between subgroups of CABG patients with CADP-CT ≥ 118 s and < 118 s before operation demonstrated significant differences in blood loss for 24 h (p = 0.002), blood loss for 25-48 h (p = 0.003), as well as red blood cells (RBCs) (p = 0.002) and fresh frozen plasma (FFP) (p = 0.003) units during operation, RBC (p = 0.002), FFP (p = 0.015), and PLT (p = 0.002) units given in the first 48 postoperative hours. Between subgroups of CABG patients with CADP-CT ≥ 118 s and < 118 s after operation demonstrated significant differences in blood loss for 24 h (p = 0.001), blood loss for 25-48 h (p = 0.003), as well as RBCs (p = 0.002), FFP (p = 0.015), and PLT (p < 0.001) units given in the first 48 postoperative hours. With regard to value of CEPI-CT before and after surgery, statistical significal differences were not found. Comparison of the variables between subgroups of non-CABG patients with CEPI-CT ≥ 165 s and < 165 s before operation demonstrated significant differences in RBCs (p=0.001) and FFP (p=0.001) units given during operation. With regard to value of CEPI-CT after surgery and CADP-CT before and after surgery, statistical significant differences were not found in any of outcome of the study. The method for estimating the effectiveness of PLT transfusion based on the value of CADP-CT showed a sensitivity 83 %, specificity 93 %, positive predictive value 86 % and negative predictive value 90 %. Ninety % patients with CADP-CT < 118 s after PLT transfusion did not need additionaly PLT units for 48 h after surgery. Conclusion: Value of CADP-CT ≥ 118 s before and after surgery may predict bleeding within 24 h and 25-48 h after surgery, as well as RBCs and FFP transfusion requirements during surgery and RBCs, FFP and PLT transfusions within 48 h after surgery in CABG patients. Normalization value of CADP-CT after PLT transfusion in the same patient is method of assessing the effectiveness of PLT transfusion.