In this episode, I’ll discuss anti-Xa monitoring of enoxaparin in VTE prophylaxis.
An advantage of using enoxaparin for VTE prophylaxis is that it has predictable pharmacokinetics and pharmacodyamics which allow for either fixed dosing of 40 mg subQ once daily or 0.5 mg/kg in the case of obese patients. However many patient characteristics exist that can reasonably call into doubt the predictability of enoxaparin dosing such as renal impairment, critical illness, and pregnancy. Given that the ability to monitor anti-Xa levels in response to enoxaparin dosing has become widespread, many clinicians opt to use anti-Xa monitoring for patients on enoxaparin who have one or more of these characteristics. However the benefits of this practice are largely unknown.
To evaluate patient outcomes from anti-Xa monitoring of enoxaparin for venous thromboembolism prophylaxis a group of authors published a single-center retrospective review in Pharmacotherapy. The authors used a composite primary outcome of any venous thromboembolism, any bleeding, and mortality in the 30 days after starting enoxaparin.
Over 6,600 patients who received enoxaparin for VTE prophylaxis were analyzed, just under 5% of which received anti-Xa monitoring. The adjusted odds ratio was 1.26 for the primary outcome comparing monitored to unmonitored patients with a 95% confidence interval ranging from 0.75-2.11. This indicates no statistically significant benefit that can be attributed to anti-Xa monitoring. When each of the parts of the composite outcome were analyzed separately, there was no benefit found for anti-Xa monitoring.
The authors concluded:
The results of this study support current guidelines, which recommend against anti-Xa-guided dosing in patients receiving LMWH for VTE prophylaxis, as the small likelihood of benefit associated with this practice does not seem clinically significant enough to justify the potential cost and associated effort.
In their discussion of the results, the authors highlight that lack of benefit was also noted in subgroups of patients that monitoring is typically recommended for such as those with obesity and active cancer, although they note that patients who were pregnant or had severe renal insufficiency were under-represented.
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