Residual Anti-Xa Activity After Last Treatment Dose of Enoxaparin

Enoxaparin is a factor Xa inhibitor commonly used for both prophylaxis against and treatment
of thromboembolism. It is also frequently used off-label as a perioperative bridge for
patients that are chronically anticoagulated prior to surgery, such as those taking Warfarin.
It is an attractive option for perioperative use secondary to its predictable pharmacologic
profile and the lack of recommended routine blood monitoring. Therefore, it is common to
encounter a patient who has recently received a treatment dose of Enoxaparin prior to
presenting for surgery. For these patients, and those on other anticoagulant medications,
published guidelines have been developed to help guide clinical decision-making when the
anesthetic/analgesic plan includes regional anesthesia.1 Currently, these guidelines
recommend that a minimum of 24-hours should elapse following the last treatment dose of
Enoxaparin before a neuraxial procedure is performed. However, a recently completed quality
improvement project conducted at Wake Forest Baptist Medical Center found that almost 60% of
patients presenting for surgery while on treatment dose enoxaparin still had significant
anticoagulant activity 24-hours following their last dose, as demonstrated by anti-Xa level
assay testing. Given that the risk of epidural hematoma formation is increased in the setting
of abnormal coagulation parameters2, the significance of this finding is that the risk of
bleeding complications following a neuraxial procedure may still be increased 24-hours after
the last treatment dose of enoxaparin.

While the routine use of anti-Xa level testing may be a viable option to determine when
residual enoxaparin activity is present before proceeding with a neuraxial procedure on a
patient-by-patient basis, it is not universally available at all hospitals. Therefore, it is
important to determine the time interval following the last enoxaparin dose at which the
likelihood that a clinically relevant amount of residual anti-Xa level activity no longer
persists, so that providers can confidently proceed with a neuraxial procedure when anti-Xa
level testing is not available.

Inclusion Criteria:

Eligible patients need:

- to be on treatment dose (1mg/kg twice daily or 1.5mg/kg daily) enoxaparin at the time
of presentation for elective surgery

- and must be able to accurately report the timing of their last dose and the
administered dosage.

- Patients must also be between the ages of 18-100 years of age

- and must be able to give written consent to participate.

Exclusion Criteria:

- Patients with severe renal insufficiency (creatinine clearance <30ml/min) will be
excluded from the study, as the elimination of enoxaparin is known to be affected in
this patient population.

- Pregnant patients will also be excluded, as the elimination and metabolism of
enoxaparin is known to be altered in this patient population, and dose adjustments are
recommended if treatment dose enoxaparin is used during pregnancy.

- Patients who are receiving enoxaparin as a bridge from another anti-Xa inhibiting
medication will be excluded as this could unpredictably affect the results of anti-Xa
testing. These medications include: Apixaban, Edoxaban, Fondaparinux, and Xarelto.