A filter is most clearly indicated in patients who have: Proven DVT and contraindications to anticoagulation Recurrent DVT (or emboli) despite adequate anticoagulation Undergone pulmonary thromboendarterectomy Marginal cardiopulmonary function, causing concern for their ability to tolerate additional small emboli (occasionally) Because venous collaterals can develop, providing a pathway for emboli to circumvent the IVCF, and because filters occasionally thrombose, patients with recurrent DVT or nonmodifiable risk factors for DVT may still require anticoagulation.