Transfusion management during cardiac surgery is commonly guided by algorithms using viscoelastic tests such as Rotational Thromboelastometry (ROTEM). Recently, the ROTEM Sigma, a new device developed as a fully automated successor to the ROTEM Delta (which required multiple pipetting steps), was approved by the Food and Drug Administration (FDA).¹ At our institution, we transitioned from the ROTEM Delta to the ROTEM Sigma and assessed the interchangeability of these 2 devices in the context of a postprotamine reversal transfusion algorithm, where the cutoff value for CT EXTEM is set at 80 seconds.^2,3 However, other algorithms use ROTEM measurements during the “rewarming phase,” when the patient is heparinized, and apply higher cutoff values for CT EXTEM of 100 seconds, such as recommended by the Society of Cardiovascular Anesthesiologists.⁴ Previous studies have shown that CT EXTEM may be sensitive to heparin both in vitro⁵ and potentially in vivo.^6,7 During our analysis, we noted differences in CT EXTEM values between the Delta and Sigma devices during cardiac surgery, particularly in the presence of heparin, which are potentially clinically relevant. These findings are reported here.