500px Blood has a natural tendency to clot when blood vessels are damaged (hemostasis) to minimize blood loss. Clotting is activated by the coagulation cascade and the clearing of clots that are no longer needed is accomplished by the process of fibrinolysis. Reductions in fibrinolysis or increases in coagulation can increase the risk of DVT. DVT often develops in the calf veins and "grows" in the direction of venous flow, towards the heart. DVT most frequently affects veins in the leg or pelvis including the popliteal vein (behind the knee), femoral vein (of the thigh), and iliac veins of the pelvis. Extensive lower-extremity DVT can even reach into the inferior vena cava (in the abdomen). Upper extremity DVT most commonly affects the subclavian, axillary, and jugular veins. The process of fibrinolysis, where DVT clots can be dissolved back into the blood, acts to temper the process of thrombus growth. This is the preferred process. Aside from the potentially deadly process of embolization, a clot can resolve through organization, which can damage the valves of veins, cause vein fibrosis, and result in non-compliant veins. Organization of a thrombus into the vein can occur at the third stage of its pathological development, in which collagen becomes the characteristic component. The first pathological stage is marked by red blood cells, and the second is characterized by medium-textured fibrin. jugular and brachiocephalic veins (not pictured). The cephalic and basilic veins, however, are superficial veins.|alt=An image showing major arm veins In arterial thrombosis, blood vessel wall damage is required, as it initiates coagulation, but clotting in the veins mostly occurs without any such mechanical damage. The beginning of venous thrombosis is thought to arise from "activation of endothelial cells, platelets, and leukocytes, with initiation of inflammation and formation of microparticles that trigger the coagulation system" via tissue factor. Vein wall inflammation is likely the inciting event. Importantly, the activated endothelium of veins interacts with circulating white blood cells (leukocytes). While leukocytes normally help prevent blood from clotting (as does normal endothelium), upon stimulation, leukocytes facilitate clotting. Neutrophils are recruited early in the process of venous thrombi formation. They release pro-coagulant granules and neutrophil extracellular traps (NETs) or their components, which play a role in venous thrombi formation. NET components are pro-thrombotic through both the intrinsic and extrinsic coagulation pathways. NETs provide "a scaffold for adhesion" of platelets, red blood cells, and multiple factors that potentiate platelet activation. In addition to the pro-coagulant activities of neutrophils, multiple stimuli cause monocytes to release tissue factor. Monocytes are also recruited early in the process. Tissue factor, via the tissue factor–factor VIIa complex, activates the extrinsic pathway of coagulation and leads to conversion of prothrombin to thrombin, followed by fibrin deposition. Fresh venous clots are red blood cell and fibrin rich. Platelets and white blood cells are also components. Platelets are not as prominent in venous clots as they are in arterial ones, but they can play a role. In cancer, tissue factor is produced by cancer cells. Cancer also produces unique substances that stimulate factor Xa, cytokines that promote endothelial dysfunction, and plasminogen activator inhibitor-1, which inhibits the breakdown of clots (fibrinolysis). alt=Image depicting D-dimer production Often, DVT begins in the valves of veins. The blood flow pattern in the valves can cause low oxygen concentrations in the blood (hypoxemia) of a valve sinus. Hypoxemia, which is worsened by venous stasis, activates pathways—ones that include hypoxia-inducible factor-1 and early-growth-response protein 1. Hypoxemia also results in the production of reactive oxygen species, which can activate these pathways, as well as nuclear factor-κB, which regulates hypoxia-inducible factor-1 transcription. Hypoxia-inducible factor-1 and early-growth-response protein 1 contribute to monocyte association with endothelial proteins, such as P-selectin, prompting monocytes to release tissue factor-filled microvesicles, which presumably begin clotting after binding to the endothelial surface. D-dimers are a fibrin degradation product, a natural byproduct of fibrinolysis that is typically found in the blood. An elevated level can result from plasmin dissolving a clot—or other conditions. Hospitalized patients often have elevated levels for multiple reasons. Anticoagulation, the standard treatment for DVT, prevents further clot growth and PE, but does not act directly on existing clots. Diagnosis A clinical probability assessment using the Wells score (see dedicated column in the table below) to determine if a potential DVT is "likely" or "unlikely" is typically the first step of the diagnostic process. The score is used in suspected first lower extremity DVT (without any PE symptoms) in primary care and outpatient settings, including the emergency department. The numerical result (possible score −2 to 9) is most commonly grouped into either "unlikely" or "likely" categories. A Wells score of two or more means DVT is considered "likely" (about a 28% chance), while those with a lower score are considered "unlikely" to have DVT (about a 6% chance). In those unlikely to have DVT, a diagnosis is excluded by a negative D-dimer blood test. In people with likely DVT, ultrasound is the standard imaging used to confirm or exclude a diagnosis. Imaging is also needed for hospital inpatients with suspected DVT and those initially categorized as unlikely to have DVT but who have a positive D-dimer test. While the Wells score is the predominant and most studied clinical prediction rule for DVT, it does have drawbacks. The Wells score requires a subjective assessment regarding the likelihood of an alternate diagnosis and performs less well in the elderly and those with a prior DVT. The Dutch Primary Care Rule has also been validated for use. It contains only objective criteria but requires obtaining a D-dimer value. With this prediction rule, three points or less means a person is at low risk for DVT. A result of four or more points indicates an ultrasound is needed. Instead of using a prediction rule, experienced physicians can make a DVT pre-test probability assessment using clinical assessment and gestalt, but prediction rules are more reliable. Criteria Wells score for DVT Dutch Primary Care Rule Active cancer (treatment within last 6 months or palliative) +1 point +1 point Calf swelling ≥ 3 cm compared to asymptomatic calf (measured 10 cm below tibial tuberosity) +1 point +2 points Swollen unilateral superficial veins (non-varicose, in symptomatic leg) +1 point +1 point Unilateral pitting<unk>  (in symptomatic leg) +1 point — Previous documented DVT +1 point — Swelling of entire leg +1 point — Localized tenderness along the deep venous system +1 point — Paralysis, paresis, or recent cast immobilization of lower extremities +1 point — Recently bedridden ≥ 3 days, or major surgery requiring regional or general anesthetic in the past 12 weeks +1 point +1 point Alternative diagnosis at least as likely −2 points — Positive D-dimer (≥ 0.5 mcg/mL or 1.7 nmol/L) — +6 points Absence of leg trauma — +1 point Male sex — +1 point Use of oral contraceptives — +1 point Compression ultrasonography for suspected deep vein thrombosis is the standard diagnostic method, and it is highly sensitive for detecting an initial DVT. A compression ultrasound is considered positive when the vein walls of normally compressible veins do not collapse under gentle pressure. Clot visualization is sometimes possible, but is not required. Three compression ultrasound scanning techniques can be used, with two of the three methods requiring a second ultrasound some days later to rule out the diagnosis. Whole-leg ultrasound is the option that does not require a repeat ultrasound, but proximal compression ultrasound is frequently used because distal DVT is only rarely clinically significant. Ultrasound methods including duplex and color flow Doppler can be used to further characterize the clot and Doppler ultrasound is especially helpful in the non-compressible iliac veins. CT scan venography, MRI venography, or a non-contrast MRI are also diagnostic possibilities. The gold standard for judging imaging methods is contrast venography, which involves injecting a peripheral vein of the affected limb with a contrast agent and taking X-rays, to reveal whether the venous supply has been obstructed. Because of its cost, invasiveness, availability, and other limitations, this test is rarely performed. DVTUS.An ultrasound with a blood clot visible in the left common femoral vein. (The common femoral vein is distal to the external iliac vein.) Ultrasonography of deep vein thrombosis of the femoral vein -annotated.Doppler ultrasonography showing absence of flow and hyperechogenic content in a clotted femoral vein (labeled subsartorial) distal to the branching point of the deep femoral vein. When compared to this clot, clots that instead obstruct the common femoral vein (proximal to this branching point) cause more severe effects due to impacting a significantly larger portion of the leg. Iliac vein deep vein thrombosis.An abdominal CT scan demonstrating an iliofemoral DVT, with the clot in the right common iliac vein of the pelvis
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