Enoxaparin Dosing and Target anti-Factor Xa Achievement in Infants by Postmenstrual Age

Currently, there is little literature delineating the optimal doses of enoxaparin required within the premature and infant populations. This study aims to assess the enoxaparin dose needed to achieve therapeutic anti-factor Xa concentrations through stratifying infants by postmenstrual age (PMA).

This is a retrospective, single-center, cohort study of infants who received at least 1 dose of treatment enoxaparin between September 2020 and September 2023. The primary endpoint was dose required to achieve therapeutic anti-factor Xa concentrations stratified by PMA group. Secondary endpoints were time in therapeutic range, average dose throughout course, number of dose adjustments and rates of bleeding and acute kidney injury.

Eighty-four patient encounters met inclusion criteria in which there were 108 unique courses of enoxaparin. Subjects were primarily of term gestation (median 37.5 weeks) and admitted for congenital heart disease (79.8%). The dose needed to achieve the first therapeutic anti-factor Xa concentration by course was significantly different based on PMA (p < 0.001): median 1.80 mg/kg (IQR 1.53-2.20 mg/kg) with PMA < 40 weeks, 1.64 mg/kg (IQR 1.55-1.77 mg/kg) with PMA 40 to 44 weeks and 1.49 mg/kg (IQR 1.19-1.74 mg/kg) with PMA >44 weeks. The median time in therapeutic range was 71% (IQR 53%-90%) per course. Overall, there were 6 cases of bleeding (5 were classified as minimal and none as severe).

Subjects with lower postmenstrual ages required significantly higher enoxaparin doses to achieve therapeutic anti-factor Xa concentrations. Postmenstrual age may provide a more appropriate basis for dosing enoxaparin compared with gestational age and postnatal age.