Unfractionated Heparin Dose Requirements Targeting Intermediate Intensity Antifactor Xa Concentration During Pregnancy

Study Objective. To describe unfractionated heparin (UFH) dosing requirements and monitoring parameters to achieve target antifactor Xa concentrations in pregnant women receiving intermediate-dose UFH therapy. Design. Retrospective cohort analysis. Setting. Centralized anticoagulation service in an integrated health care delivery system. Patients. Twenty-five pregnant women (who had 27 pregnancies) who received intermediate-dose UFH between January 1, 1998, and March 31, 2005. Measurements and Main Results. Demographic and clinical data were retrieved by using an integrated electronic medical, pharmacy, and laboratory records system. The primary outcome was the required UFH dose at the time of first target antifactor Xa level achieved. Antifactor Xa levels were measured at the mid-dosing interval for UFH. An antifactor Xa assay concentration of 0.1-0.3 unit/ml was used as the target range for intermediate-dose UFH for antithrombotic management of pregnant women as described in the American College of Chest Physicians clinical practice guidelines. The mean UFH dose required to achieve this target antifactor Xa range was 236.9 units/kg/day. The UFH doses required to achieve target antifactor Xa levels correlated significantly with patient weight (r = 0.682, p<0.001). The final UFH dose/kg required at the end of pregnancy was similar to that at the first target level (p>0.05) when adjusted for weight. However, the total daily amount of UFH did increase by the end of pregnancy (21,889 vs 19,320 units, p=0.02). There was an average of 7.9 antifactor Xa determinations and 3.1 dosage modifications during the mean of 19 weeks of antenatal UFH therapy. Most (62%) antifactor Xa levels were within the target range. No thromboembolic events, heparin-induced thrombocytopenia, or osteopenic fracture occurred. There was one hemorrhagic complication that resolved with temporary withdrawal of UFH. Conclusions. Pregnant women required a mean UFH dose of 236.9 units/kg/day to achieve the targeted antifactor Xa level of 0.1-0.3 unit/ml. The required UFH doses correlated with patient weight, and most antifactor Xa levels were within the desired target range. These findings may assist clinicians in more precisely initiating and monitoring intermediate-dose UFH in pregnant patients.