Abstract： Objective This study was designed to determine the in vitro sensitivity of LMWH caused by different reagents,and to explore whether the ACT can be used to monitor LMWH.Methods This study was performed in vitro.ACT was measured with different reagents(glass beads,celite,and kaolin)on volunteer(n =30)blood samples spiked with increasing concentrations of LMWH(datleparin,0.2-1.8IU/ml).Linear regression analysis was performed to establish a regression equation from different concentration of datleparin and corresponding ACT values.Results Analysis of dose-response curves obtained in vitro,an excellent linear relationship was observed between the ACT and dalteparin concentrations for all three reagents(p less than 0.01).Differences in slope of the regression curves of ACT were observed with all the reagents tested(glass beads 249.7s/IU,celite 77.7s/IU,and kaolin 59.3s/IU,p less than 0.01).Reagents vary widely in their in-vitro sensitivity related to dalteparin.In the concentration range of 0.2-1.8 IU/ml,the gaolin reagent was insensitive to dalteparin,and glass beads was the most suitable reagent for monitoring the anticoagulant effect of dalteparin.Conclusions Glass beads,celite,and kaolin.Glass beads were the most suitable reagent for monitoring the anticoagulant effect of dalteparin.Vary widely in their in-vitro sensitivity related to datleparin.