Thromboelastography as a perioperative measure of anticoagulation resulting from low molecular weight heparin: A comparison with anti-Xa concentrations

Low molecular weight heparin (LMWH) is commonly used to prevent postoperati ve thromboembolism. Currently, there is no convenient test to measure the d egree of anticoagulation from LMWH. This prospective study examines the rel ationship of thromboelastography and serum anti-Xa concentration in patient s treated with enoxaparin. Twenty-four adult patients scheduled for orthope dic surgery using epidural anesthesia were enrolled. Epidural catheters wer e removed the morning after surgery before the commencement of subcutaneous enoxaparin 30 mg twice daily. Venous blood samples were obtained at 1) the induction of anesthesia (baseline), 2) immediately before the third dose o f enoxaparin postoperatively (Day 2-trough), 3) 4 h after the third dose po stoperatively (Day 2-peak), and 4) immediately before the fifth dose postop eratively (Day 3-trough). Whole blood samples were obtained for thromboelas tography, activated clotting time, and anti-Xa level analyses at each of th e four time intervals. At the four sample intervals, the r time (mean +/- S EM). (20 +/- 1, 25 +/- 2, 51 +/- 6, 31 +/- 3 mm) and the k time (9 +/- 0.7, 12 +/- 1, 27 +/- 5, 14 +/- 2 mm) of the thromboelastograph were significan tly correlated with the expected peak and trough levels of LMWH and serum a nti-Xa levels (P < 0.05). At the Day 3-trough, thromboelastograph r times e xceeded the normal range in 6 of 25 patients (25%). Prolongation of r time and k time on postoperative Day 3 may indicate an exaggerated response to L MWH. Thromboelastography is a test that could potentially correlate with th e degree of anticoagulation produced by low molecular weight heparin.