Summary

Low molecular weight heparin (LMWH) is used increasingly forprophylaxis and treatment of venous thromboembolism duringpregnancy. However, the prophylactic dose for patients at moderaterisk varies between centers, and the recommendedLMWH dose for the non pregnant patient is frequently used inpregnant women. The aim of this study was to investigate theeffects of pregnancy on the pharmacokinetics of anti-Xa levelsduring moderate risk thromboprophylaxis with the LMWH,tinzaparin. In 24 pregnant women, one of three doses oftinzaparin (50, 75 or 100 IU/kg) were given according to thetreating physician’s assessment of their risk profile. Four-hourpeak anti-Xa levels were measured throughout pregnancy and24-hour profiles were measured at 28 and 36 weeks gestation.Doses were adjusted when peak anti-Xa levels fell below 0.1 IU/ml and, in some cases, when levels at 10 and 18 hourspost injection were undetectable (<0.05 IU/ml). Our resultsshowed that women receiving tinzaparin (50 IU/kg) frequentlyhad peak (4 hour) anti-Xa levels below 0.1IU/ml and that 46%of these patients required dose adjustment. Similarly anti-Xalevels were also found to be low over the 24-hour period. Astarting dose of 75 IU/kg, once daily, gave greater anti-Xa coverover the 24-hour period and may avoid the need for doseadjustment. The results suggest that the pharmacokinetics oftinzaparin are affected by pregnancy. Larger studies arerequired to determine whether an increased tinzaparin dose(75 IU/kg) would be more effective in the prevention of thrombosisduring pregnancy than 50 IU/kg