Consider biochemical predispositions in particular when unexplained DVT: protein S or C deficiency, Anti-thrombin 3 defic, Prothrombin mutation, factor 5 leiden resistance, anti-phospholipid antibody APL (arterial and venous clots)
Acquired predispositions often linked to clots in unusual places: renal vein (nephrotic syndrome); hepatic vein, portal vein, splanchnic veins (Polycythemia Vera, PNH, essential thrombocytosis; protein S, protein C, and antithrombin deficiency; factor V Leiden and prothrombin gene mutations, APL); Arterial and venous unprovoked clots (HIT, ET); hyperhomocystinemia
Upper extremity DVT in particular related to cancer and/or central venous catheters