The relationship between anti-factor Xa level and clinical outcome in patients receiving enoxaparine low molecular weight heparin to prevent deep vein thrombosis after hip replacement.

Studies in experimental animals have demonstrated that there is a relationship between levels of low molecular weight (LMW) heparin and both bleeding and inhibition of thrombosis. The relationship between these outcomes and ex vivo anti-factor Xa levels has been examined in 163 patients undergoing total hip replacement who were given prophylaxis once daily with a LMW heparin (enoxaparine). Fifty patients received 60 mg of enoxaparine and 113 received 40 mg, both regimens being administered subcutaneously once daily. Blood samples for anti-factor Xa levels were collected 12 hours after the injection on the day of surgery and on days 1, 3 and 6, postoperatively. The incidence of wound hematoma was 5.3% when the maximum anti-factor Xa level was less than or equal to 0.2 units per ml, but increased to 24.5% when the anti-factor Xa level exceeded 0.2 units per ml, P = 0.0008. The incidence of postoperative thrombosis was low (6.3%) if the minimum anti-factor Xa level exceeded 0.1 units per ml, but increased to 14.6% when less than or equal to 0.1 units per ml, and to 18.8% if the anti-factor Xa level was less than or equal to 0.05 units per ml. Regression analysis revealed that there was a statistically significant relationship between anti-factor Xa level and wound hematoma, P = 0.002 and anti-factor Xa level and thrombosis, P = 0.03. These findings suggest that when enoxaparine is administered as a once daily subcutaneous injection, the 12 hour anti-factor Xa level should not exceed 0.2 units per ml to minimize bleeding and levels greater than 0.05 units per ml should be obtained to optimize efficacy.