The effect of heparin on three whole blood activated clotting tests and thrombin time

Whole blood activated clotting time (ACT) can be determined by many different methods that use a variety of clotting cascade activators and end-points. This study compared the results of three whole blood ACT instruments at equivalent concentrations of heparin. Whole blood (9.8 ml) from 10 healthy adult volunteers without coagulation abnormalities was added to 0.2 ml of heparin solution producing heparin concentrations of 0, 0.1, 0.2, 0.4, 0.6, 0.8, and 1.0 U/ml. Coagulation status was determined in duplicate with the Hemochron 400 System (HC), the HemoTec Automated Coagulation Timer (HT), and the TriMed ACTivator (TM). Thrombin times or dilutions (TT) were also determined for each sample. Baseline values did not differ (p<0.05); however, the HT and TM ACT values were significantly longer (p<0.05) than the HC ACT values at predicted heparin concentrations >0.2 U/ml. Results from the HT and TM instruments both provided a greater ACT range over the heparin concentrations tested. Of the tests studied to monitor heparin therapy, mean scaled TTs showed the best correlation with predicted heparin concentrations.