PB0460 - Does close anti-Xa monitoring of LMWH dosing for pregnant women with inherited antithrombin deficiency improve clinical outcome?

Background: Women with inherited antithrombin deficiency (ATD) are at risk of venous thromboembolism (VTE) during pregnancy & the puerperium. The appropriate low molecular weight heparin (LMWH) dosing in pregnant women with ATD is difficult, due to (1) changing plasma volumes, and (2) because AT is the substrate for the heparin anticoagulant effect and therefore larger doses of LMWH are required than normal. In 2012 our institution changed practice to recommend anti-Xa-guided LMWH for all pregnant women with ATD- as we previously had a 10% rate of VTE in pregnancy.

Aims: To review anti-Xa guided LMWH dosing, and clinical outcomes during pregnant in women with ATD in our tertiary centre

Methods: Retrospective cohort study of pregnant women with ATD from 2012 - 2023.

Results: 17 women with 32 pregnancies were included. Baseline characteristics are in Table 1. Of the 32 pregnancies, 22 had a livebirth with 9 first-trimester miscarriages and one second-trimester pregnancy loss. Three had VTE– two occurred when receiving subtherapeutic LMWH doses prior to referral to our centre in the index pregnancy; the third had homozygous AT Budapest & was non-compliant. Median starting dose of LMWH was 6250 units (96 units/kg), with doses increasing to a median of 16750 units (247 units/kg) by the end of pregnancy. Peak and trough anti-Xa levels are shown in Figure 1. Antithrombin concentrate was given peripartum in 21 pregnancies (91% of those that progressed beyond the first trimester). There were four post-partum haemorrhages (12.5%) – 1 minor, 3 major. Postpartum LMWH was continued for six weeks at a median 8667 units dose, except those on long-term anticoagulation who changed to warfarin whilst breast feeding.

Conclusion(s): Close monitoring of LMWH dosing with anti-Xa activity measurement and use of peripartum antithrombin concentrate in 32 pregnancies in women with ATD, resulted in VTE-free pregnancy & puerperium.