Remember Virchow's triad? • Stasis, vessel (endothelial) injury, and hypercoagulability.
Other random points:
- the superficial femoral vein is in the DEEP system and clinical significant
- don't be afraid to use a treatment dose of heparin / lovenox / fragmin if you have a clinical suspicion of DVT / PTE, and their is a delay in the diagnostic workup
- high clinical suspicion and negative lower extremity doppler ultrasound should prompt a repeat ultrasound in 3-5 days (see the table from AFP 2004 / IHI & guideline.gov
DVT isolated to the thigh and pelvic veins, without involvement of the calf veins, also may occur. Prior studies reported a 1% to 4% incidence of DVTs isolated to the iliac veins.  ,  With the advent of contrast enhanced CT and MR imaging that can detect venous thrombosis within the pelvic veins more readily, iliac vein thrombosis has been found to be more common than previously thought. In a study of 769 patients, Spritzer and colleagues reported an acute DVT isolated to the iliac vein or inferior vena cava in 20% of the patients in their study.
47 Carpenter J.P., Holland G.A., Baum R.A., et al: Magnetic resonance venography for the detection of deep venous thrombosis: comparison with contrast venography and duplex Doppler ultrasonography. J Vasc Surg 18. 734-741.1993; Abstract
48 Nicolaides A.N., Kakkar W., Field E.S., et al: The origin of deep venous thrombosis: a venographic study. Br J Radiol 44. 653-663.1971;
49 Spritzer C.E., Arata M.A., Freed K.S.: Isolated pelvic deep venous thrombosis: frequency as detected with MR Imaging. Radiology 219. 521-525.2001; Abstract