High dose thrombin time versus the activated clotting time during cardiopulmonary bypass.

In this study we compared the High Dose Thrombin Time (HiTT) with the Activated Clotting Time (ACT) during cardiopulmonary bypass (CPB) in non-aprotinin treated patients. On the advice of the HiTT test manufacturer each institution should perform comparative ACT/HiTT assays in the cardiac surgery population. In previous tests our target ACT value of 480 seconds corresponds with a mean HiTT value of 190 seconds. Our results showed that after heparinization (300-400 IU/kg body weight) 8 out of 20 patients did not reach the target ACT of 480 seconds, while the HiTT results in those 8 patients were higher than our target time of 190 seconds. Four heparin pretreated patients who received 400 IU/kg heparin, had relatively low ACT values (467 ± 14 sec.) and high HiTT values (324 ± 47 sec.). Before and during CPB there was a poor correlation between the HiTT and ACT (r = 0.38). The results of this study show that for the individual patient the target HiTT of 190 seconds is no guarantee for reaching an adequate ACT of 480 seconds. Although the HiTT may be a very useful assay for monitoring heparin effects during CPB, the determination of the target time can be a point of discussion. In contrast of the advice of the manufacturer we therefore suggest that comparative ACT/HiTT assay should be done in every individual patient to determine a safe target HiTT time, instead of the whole group of patients.