Dose and Time Dependent Elevations of Initial Unfractionated Heparin Anti-Xa Levels Associated With Low Molecular...

Intravenous unfractionated heparin (UFH) is routinely utilized for anticoagulation in acute care patients. Appropriate laboratory monitoring is an essential component of UFH therapy, including minimizing the effect of variables that interfere with the monitoring assays. While alterations in anti-factor Xa (anti-Xa) values due to factor Xa inhibitors have been described, the dose-dependent influence of low molecular weight heparins (LMWHs) has not been widely reported.

The purpose of this study was to evaluate the impact of LMWH on initial UFH anti-Xa values in patients with recent LMWH exposure.

This was a retrospective cohort study using national Veterans Affairs data from 2014 to 2025 with UFH anti-Xa monitoring to assess the proportion of elevated anti-Xa levels (>0.7 IU/mL) among 3 groups, defined as recipients of treatment-dose LMWH, no LMWH, and prophylactic-dose LMWH within 24 hours prior to initiation of UFH therapy.

The proportion of initial elevated anti-Xa values with treatment-dose LMWH 24 hours prior to UFH was significantly greater at 48.3% compared to no LMWH (18.0%) or prophylactic-dose LMWH (24.0%; P = .0002), with the treatment-dose median (interquartile range [IQR]) anti-Xa significantly elevated at 1.21 (0.69-1.71) compared to no LMWH (0.43 [0.21-0.83]) or prophylactic-dose LMWH (0.48 [0.28-0.83]; P < .0001). The proportion of initial elevated anti-Xa levels with treatment-dose LMWH 48 hours prior to UFH was significantly greater at 37.0% compared to no LMWH (17.8%) or prophylactic-dose LMWH (25.8%; P = .002), with the treatment-dose median (IQR) anti-Xa significantly elevated at 0.96 (0.36-1.82) compared to no LMWH (0.42 [0.21-0.83]) or prophylactic-dose LMWH (0.49 [0.28-0.84]; P = .0002). The median duration of elevated anti-Xa levels was significantly longer with treatment-dose LMWH administered within both 24 and 48 hours prior to UFH (14.0 and 13.6 hours, respectively) compared to no LMWH (8.3 hours) or prophylactic-dose LMWH (7.5 and 7.6 hours; P = .02 and P = .03, respectively).

When LMWHs are administered prior to UFH, there is a dose- and time-dependent association with a significantly increased risk of initial elevated UFH anti-Xa levels, as well as an extended duration of these elevated values. Institutions should assess the impact of LMWH therapies on anti-Xa monitoring to guide local anticoagulation practices.