Comparison of High-Dose Thrombin Time with Activated Clotting Time for Monitoring of Anticoagulant Effects of Heparin in Cardiac Surgical Patients

Abstract

The activated clotting time (ACT) is routinely used for monitoring of heparin effects during cardiopulmonary bypass (CPB). However, ACT is not a specific assay for heparin and may be influenced by several other factors, which may be misleading with regard to the proper administration of heparin and protamine. In this pilot study, we compared a new test, the high-dose thrombin time (HiTT), with the conventional ACT test for both in vitro and in vivo heparin-induced anticoagulation. Our in vitro results showed that there were heparin dose-dependent increases in ACT and HiTT. Data on 30 adult cardiac patients indicated that HiTT correlated well with heparin concentration both after initial heparin administration and during CPB (r = 0.645 and 0.515). Hypothermia and hemodilution occurring during CPB did not alter HiTT results. ACT also correlated well with both heparin concentration and HiTT before CPB, but the linear relationship was lost during CPB. Our results suggest that HiTT is a useful assay for monitoring heparin effects during cardiac surgery, even during hypothermia and hemodilution.

Effect of Heparin, Platelets, Activated Platelets, Platelet Fragments, and Hematocrit on Activated Clotting Time

“…Andrew et al (10) showed that although there was a correlation between plasma heparin concentration and ACT values when blood of adult persons was previously drawn into syringes containing heparin, there was no correlation when the test was performed with blood of their pediatric patients. Wang et al (11) reported that although a linear response of ACT to heparin levels in the blood of their patients was observed in vitro, this correlation was lost during CPB due to hypothermia. Chan et al (12) observed that ACT values did not correlate to heparin concentration during cardiopulmonary bypass, likely due to activation of the clotting system and platelets, hypothermia, and hemodilution.…”

“…However, if the heparin level is increased to 1.0 U/ml, ACT most likely will be significantly higher than in a lower heparin level. This indicates that heparin is reflected by ACT only poorly and could explain why a correlation between ACT and heparin levels was not found in vivo (10,11).…”

“…The aim of this study was to gain a more complete picture of the role that heparin, platelet counts, platelet fragments, activated platelets, and hematocrit play in ACT determination (10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21).…”

Point‐of‐care testing in haemostasis

“…1 The High Dose Thrombin Time (HiTT, Hemochron) is more specific to heparin effects than the activated clotting time [ACT] (Wang et al, 1994) In contrast to the ACT, the HiTT is not altered by haemodilution or hypothermia. However, both before and during CPB there is poor correlation between the HiTT and ACT and for individual patients a target HiTT of 190 s may not equate to an adequate ACT of 480 s. In such cases, therefore, a comparative ACT/HiTT assay may be required to establish a therapeutic HiTT time (de Jong, 1999).…”

Activated clotting time as a screening test prior to catheter‐based cardiovascular procedures

“…Given the good correlation of the ACT to the anticoagulant effects of heparin, its role has principally been in monitoring this effect during both surgical and catheter-based procedures [9 -18]. The ACT, however, is not a specific assay for monitoring the effect of heparin and can be affected by factors other than heparin, such as hemodilution, hypothermia, disseminated intravascular coagulation, platelet dysfunction, or hypercoagulability [26,27]. As aspirin affects platelet function indirectly, it too, may alter the correlation between ACT value and coagulability.…”