Background: Low molecular weight heparins(LMWH) are used for the prophylaxis and treatment of venous thromboembolism and peri-procedure bridging of patients on anticoagulation. Anti-Xa activity is widely used for monitoring select groups of patients on LMWH despite the lack of consensus for its use.

Aims: To determine reasons for ordering anti-Xa test for patients on LMWH. Secondary objectives:To assess the appropriateness of anti-Xa testing and sample collection and their impact on patient care.

Methods: Retrospective,single-centre observational study which included hospitalised patients ≥21 years old,with anti-Xa testing from October 2018 to September 2019. Patients were identified using hospital electronic records with results and outcomes assessed through laboratory and clinical records.

Results: 292 anti-Xa tests were performed on 183 patients on enoxaparin. Of the tests performed, 115(62.8%), 27(14.8%), 92(50.3%) and 59(32.2%) samples were from patients with severe renal impairment, moderate renal impairment extreme body weight and bleeding concerns(Figure 1).
Trough anti-Xa tests were taken 21.6h(IQR=11.6-23.7) after the last enoxaparin dose for renally impaired patients.
56(30.6%) of the 183 patients had more than one anti-Xa taken. 14(25%) had their enoxaparin dose adjusted thereafter. 3 of 14 had major or clinically relevant non-major bleeding(CRNMB) event during the follow up period.
26 patients(14.2%) had their enoxaparin doses adjusted after the first anti-Xa result. 14(53.8%) of them had a second anti-Xa performed, none of whom had their doses adjusted thereafter. 4 of the 14 patients had major or CRNMB during the follow up period.
29 of the 46 patients who had bleeding event(s) during enoxaparin therapy had anti-Xa monitoring performed before the bleeding event. 9 of the 29 patients had enoxaparin dose adjustments thereafter(Figure 2).

Conclusions: Anti-Xa monitoring is most commonly performed on patients with renal impairment followed by extremes of body weight. These patients had increased bleeding events despite monitoring, attesting to their higher risk. The value of monitoring remains uncertain.

[Reasons for checking anti-Xa activity]

[Patients who had a bleeding event during enoxaparin therapy and during the follow up period]

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ISTH Congress Abstracts - https://abstracts.isth.org/abstract/an-evaluation-of-the-basis-for-anti-xa-testing-in-patients-on-low-molecular-weight-heparin-and-its-impact-on-subsequent-management/