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Measurement of the activated clotting time (ACT) represents a standard method for coagulatory assessments. The test employs specific agents to trigger the coagulation process. The present study aimed to compare kaolin (Hemotec®) versus a combination of silica, kaolin and phospholipid (Hemochron® Jr) ACTs.Hemotec® and Hemochron® Jr ACT monitors were compared by simultaneous measurement of paired arterial blood samples (n=114) with respect to precision and bias during clinical conditions of cardiopulmonary bypass (CPB). The influence of haemodilution on the ACT was tested in an ex-vivo model.The precision of Hemotec® and Hemochron® Jr ACT measurements attained 21±2.6 s versus 27.0±2.6 s (p=0.126) during CPB and 2.5±2.2 s versus 9.4±6.9 s (p=0.000) after protamine administration, respectively. The Hemochron® Jr monitor was associated with a bias of −102±13.7 s compared to the Hemotec® ACT monitor (p=0.000) during CPB and −6.9±2.9 s after protamine (p=0.025). Linear regression analysis of ACT readings between monitors reached r=0.526 (p=0.000). Hemochron® Jr ACT values correlated with the erythrocyte volume fraction r=0.379 (p=0.000). Ex-vivo data indicated that the Hemotec® ACT monitor was associated with relatively higher ACT readings after haemodilution.The ACT is not a standardized measure. Test results are strongly associated with the specific compounds used to initiate the coagulation process.