In this chapter, various tests have been discussed in the diagnosis of DVT and have been classified according to various patient categories. To summarize, the following guidelines may be of clinical use in the management of patients with suspected DVT. Acute, First Event of Suspected DVT These patients often suffer from occluding, proximal thrombi. Therefore, noninvasive tests such as CUS or IPG are most suitable for these patients. If an abnormal CUS or IPG result is found, the diagnosis is virtually proven, and this may serve as a basis to treat the patient with anticoagulants. If a normal CUS or IPG result is obtained, serial testing is indicated to detect extending calf vein thrombi or nonoccluding DVT, which becomes occlusive at follow up. Anticoagulants may be withheld safely if the test remains normal within 1 week. Acute, Recurrent Suspected DVT These patients may have residual thrombi present, which makes the noninvasive tests (CUS/IPG) less useful. However, if a normal noninvasive test was documented previous to the acute recurrent event, this test may be used. If an abnormal test result is found in the presence of a documented, normal previous-test outcome, this may serve as a basis for anticoagulant therapy. Although no formal studies have been performed to evaluate the safety of withholding anticoagulants if a normal CUS or IPG result is obtained, serial testing is likely to be adequate in these circumstances. Phlebography is the only truly evaluated approach, and this could be considered in all suspected recurrent DVT. Furthermore, contrast venography is the test of choice to discern acute from old thrombi. Asymptomatic DVT in High-Risk Patients The majority of these thrombi are mostly localized in the calf veins only and are often nonocclusive. This makes noninvasive tests unreliable for their detection. Therefore, only contrast venography should be used in this patient category.