In the postoperative intensive care setting after cardiac surgery with cardiopulmonary bypass (CPB), particularly in patients requiring continued anticoagulation for prosthetic devices such as mechanical valves or extracorporeal support, unfractionated heparin (UFH) is commonly monitored using the activated partial thromboplastin time (aPTT). However, it may inadequately reflect anticoagulant activity in critically ill patients. The TEG6s system is a viscoelastic testing platform that enables bedside coagulation monitoring. This proof-of-concept study aimed to evaluate the utility of TEG6s-derived indices for estimating the anti-Xa level following CPB.

We prospectively enrolled 131 adult patients undergoing elective cardiac or aortic surgery with CPB. Paired blood samples were collected at intensive care unit admission and 4 h later. Heparin concentrations were measured using a chromogenic anti-Xa assay. Thromboelastography (TEG) parameters including the reaction times for the citrated kaolin without and with heparinase assays and their ratio (R ratio) and difference were obtained. Their correlations with the anti-Xa level were assessed using Pearson’s and Spearman’s analyses. Linear and polynomial regression models were used to evaluate predictive performance.

The R ratio and reaction time difference showed strong correlations with the plasma anti-Xa level (r = 0.738 and r = 0.733, respectively; p < 0.001). These correlations were comparable to that of the aPTT ratio (r = 0.813). The second-order polynomial regression model using the R ratio had the highest predictive accuracy (R² = 0.584, root mean square error = 0.134).

The R ratio may be a supplementary indicator of a residual heparin effect, particularly in cases with discordant aPTT findings.