Strong risk factors (odds ratio > 10)

Fracture (hip or leg)

Hip or knee replacement

Major general surgery

Major trauma

Spinal cord injury

Intermediate risk factors (odds ratio 2 to 9)

Arthroscopic knee surgery

Central venous lines

Chemotherapy

Chronic heart or respiratory failure

Hormone therapy

Malignancy

Oral contraceptive therapy

Paralytic stroke

Pregnancy/postpartum

Previous venous thromboembolism

Thrombophilia

Weak risk factors (odds ratio < 2)

Bed rest longer than three days

Immobility due to sitting (e.g., car or air travel longer than eight hours)

Increasing age

Laparoscopic surgery

Obesity (body mass index greater than 40 kg per m2)

Pregnancy/antepartum

Varicose veins