Real-time Anti-Factor Xa Measurements in Surgical Patients to Examine Enoxaparin Metabolism and Optimize Enoxaparin Dose

Venous thromboembolism (VTE) encompasses deep venous thrombosis and pulmonary embolus, and is
the proximate cause of death in over 100,000 hospitalized patients per year. To put this in
better context, VTE kills more people each year than the annual morbidity from motor vehicle
crashes and breast cancer combined. Surgeons commonly provide enoxaparin, a low molecular
weight heparin, for VTE prophylaxis. Enoxaparin's activity is quantified by anti-Factor Xa
(aFXa) levels. Studies of enoxaparin metabolism in patients with traumatic injury, thermal
injury, or those undergoing reconstructive surgery have shown that standard dosing can result
in inadequate aFXa levels, likely from the hypermetabolic state associated with significant
injury. Small studies have associated subtherapeutic aFXa levels with increased risk for life
or limb-threatening VTE events. Prior work from has shown that 2-10% of highest risk surgical
patients have a VTE event despite enoxaparin prophylaxis. The investigators believe that
surgical patients would benefit from an individualized dosing regimen for enoxaparin
prophylaxis and that individualized dosing will decrease observed rates of life or
limb-threatening post-operative VTE events.

This project will critically examine the pharmacokinetics of prophylactic doses of enoxaparin
in surgical patients, and will evaluate how alteration of enoxaparin dose magnitude and
frequency affects peak and trough aFXa levels as well as risk for re-operative hematoma. If
subtherapeutic aFXa levels are observed, the study will design, implement and test a clinical
protocol to optimize post-operative aFXa levels. Although not an explicit Aim, this study
will also provide important preliminary data on VTE rates in surgical patients with in range
and out of range aFXa levels.

Inclusion Criteria:

- Adults, (age≥18)

- Patients who have had surgery with general anesthesia.

- Post-operative stay will be ≥2 days

Exclusion Criteria:

- Contradiction to use enoxaparin

- History of intracranial bleeding/stroke, hematoma or bleeding disorder,
heparin-induced thrombocytopenia positive, and heparin-induced thrombocytopenia
positive

- Creatinine clearance ≤ 30mL/min

- Serum creatinine >1.6mg/dL

- Epidural anesthesia

- Patients placed on non-enoxaparin chemoprophylaxis regimens per their surgeon's
discretion.