EFFORT.

Methods	Randomised, double‐blind, pilot trial
Participants	Total number of participants: 198 Number of participants allocated to each group: fondaparinux group: 100, enoxaparin group: 98 Number of participants excluded and/or lost to follow‐up: Of the 198 randomised participants, 7 in the fondaparinux group and 7 in the enoxaparin group did not receive treatment according to protocol, but all randomised participants were analysed. Inclusion: Patients were eligible for the study if they were 18 years of age or older with a body mass index (BMI) of 35 to 59 kg/m2, and were undergoing laparoscopic vertical sleeve gastrectomy (VSG) or laparoscopic Roux‐en Y gastric bypass (LRYGB) Exclusion: Patients with BMI > 60 were excluded, as they may have required extended DVT prophylaxis. Patients with contraindications to low molecular weight heparin or selective antithrombin III agonists, previous history of DVT or PE, documented clotting/coagulation disorders, history of treatment for cancer within the past year, history of venous stasis or superficial thrombophlebitis, vein stripping or ligation, obesity hypoventilation syndrome or recent history of smoking (within the past year) were excluded. Patients with severe hepatic impairment, creatinine clearance < 30 mL per minute or platelet count < 100,000 per cubic millimetre were also excluded, as were women of childbearing age if they were pregnant or were taking oestrogen‐based birth control medication within 1 month of surgery
Interventions	FX: The fondaparinux group received a placebo on call to the operating room. Six hours after surgery stop time, participants were given 5 mg fondaparinux subcutaneously. Beginning on postoperative day 1, participants received 5 mg of fondaparinux subcutaneously once daily in the morning and placebo (saline) injections subcutaneously once daily in the evening for the duration of their hospital stay. Enoxaparin: In accordance with current practice, the enoxaparin group received a dose of enoxaparin 40 mg subcutaneously on call to the operating room. To maintain blinding, participants randomised to enoxaparin received placebo (saline) injection 6 hours after surgery stop time. Beginning on postoperative day 1, 40 mg of enoxaparin was administered subcutaneously twice daily for the duration of the participant's hospital stay.
Outcomes	Primary outcome was the effect of preoperative enoxaparin vs postoperative fondaparinux prophylaxis on antifactor Xa concentrations in participants undergoing bariatric surgery. Attainment of a target antifactor Xa level was determined on the basis of blood samples drawn 3 hours after the drug was received on postoperative day 1. This cutoff was the standard for adequate prophylaxis used by our in‐patient haematology lab (Z 0.20 IU/mL for enoxaparin and Z 0.39 mg/L for fondaparinux). Secondary outcomes were asymptomatic DVT, defined as a positive MRV within 2 weeks after surgery, and symptomatic DVT. Safety outcomes included perioperative bleeding, perioperative complications and death.
Notes	Use of adjunctive anticoagulative methods: All participants had sequential compression devices and antiembolic stockings placed before induction of anaesthesia; 4 to 6 hours after surgery stop time, participants were ambulated in the hallways. Sequential compression devices were removed during ambulation. Use of aspirin, non‐steroidal anti‐inflammatory drugs and other antiplatelet agents was prohibited during participants' hospital stay.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Quote: "using a computer‐generated randomization scheme (Microsoft Excel 2007 data analysis tool pack)" Comment: probably done
Allocation concealment (selection bias)	Low risk	Quote: "Allocation was performed by the pharmacy and was concealed from patients and study personnel" Comment: probably done
Blinding of participants and personnel (performance bias) All outcomes	Low risk	Quote: "placebo doses were prepared to maintain the blind. Active and placebo syringes were prepared by our inpatient pharmacy and were not identifiable by external appearance" Comment: probably done
Blinding of outcome assessment (detection bias) All outcomes	Low risk	Quote: "asymptomatic DVT, defined as a positive MRV within 2 weeks following surgery, and symptomatic DVT" Comment: probably done
Incomplete outcome data (attrition bias) All outcomes	Low risk	83 of 98 in the enoxaparin group, 94 of 100 in the fondaparinux group had MRV results. Comment: low risk of bias; most participants finished the study
Selective reporting (reporting bias)	Low risk	All primary efficacy and safety outcomes listed in the Methods section were reported. Comment: low risk of bias
Other bias	Unclear risk	Company provided study material and additional financial support but was not involved in the design and procedure of the study.