Postoperative bleeding following cardiopulmonary bypass (CPB) remains a significant challenge. Although viscoelastic testing is increasingly used, the relative contributions of fibrinogen, platelet count and clot firmness to blood loss remain debated. We evaluated the diagnostic accuracy of thromboelastometry (ROTEM) compared with platelet aggregometry (PA) and standard tests, using the Hb/kg index to quantify blood loss.

In this prospective observational study conducted at the University Hospital (Kaunas, Lithuania) we enrolled 79 patients undergoing elective cardiac surgery. Simultaneous assessments using ROTEM (EXTEM, INTEM, FIBTEM, PLTEM), PA, and standard coagulation tests were performed. The primary endpoint was the correlation between haemostatic parameters and the Hb/kg Index. Diagnostic accuracy for hypofibrinogenaemia (<2.5 g/L) and thrombocytopenia (<150 × 10⁹/L) was assessed using Receiver Operating Characteristic (ROC) analysis.

Post-CPB platelet count and fibrinogen decreased significantly (p < 0.001). However, a notable dissociation was found: neither platelet count, PLTEM, PA parameters nor standard clotting times correlated with the Hb/kg Index (p > 0.05). In contrast, viscoelastic measures of clot firmness (FIBTEM A10) and fibrinogen levels significantly predicted blood loss. FIBTEM A10 (12 mm) demonstrated excellent accuracy for hypofibrinogenaemia (AUC = 0.888), providing a sensitivity of 96% and a negative predictive value of 97.4%.

Post-CPB bleeding is primarily driven by reduced clot firmness and fibrinogen deficiency rather than by platelet count or aggregation defects. FIBTEM A10 is a superior rapid detector of hypofibrinogenaemia. Transfusion algorithms should prioritize the maintenance of functional clot firmness over the correction of static platelet numbers.