Deep Vein Thrombosis

• Venous Stasis and Hemodynamic Factors: Venous blood flow is slower than arterial flow and depends on active muscular contraction, respiration, and venous valve function. Immobility (prolonged bed rest, long flights, paralysis) reduces calf muscle pump activation, causing blood to pool in distal calf veins where shear stress is lowest. In pooled blood, platelets and coagulation factors have prolonged residence time in the vessel, increasing collision probability and allowing cascade amplification. Venous obstruction (malignancy, external compression) increases hydrostatic pressure and further compromises flow. Valve incompetence or destruction allows reflux, perpetuating stasis. The soleal and gastrocnemius veins of the calf are particularly vulnerable due to their small caliber and low-flow characteristics; this is why most DVTs originate in the calf and why calf vein thrombi have higher recurrence rates than proximal DVTs.

• Endothelial Injury and Tissue Factor Exposure: Mechanical trauma to the venous endothelium (central venous catheters, orthopedic surgery, previous thrombosis) causes denudation and exposes the subendothelial matrix, including tissue factor (TF), which initiates the extrinsic coagulation cascade. The TF-Factor VII complex activates Factor X, bypassing the need for Factor VIII and IX and rapidly initiating thrombin generation. Surgery, especially orthopedic procedures, causes direct vessel wall trauma and releases thromboplastic substances (tissue factor, phosphatidylserine from damaged cells). Chemotherapy agents can directly injure endothelium. Infection and inflammatory states (sepsis, inflammatory bowel disease, malignancy) upregulate TF expression on monocytes and endothelial cells, increasing the thrombotic potential.

• Hypercoagulability and Thrombophilic States: Hypercoagulability encompasses both acquired and inherited abnormalities that shift the hemostatic balance toward thrombin generation. Acquired hypercoagulability results from increased synthesis of procoagulant factors (Factor V, II, VII, X increase during malignancy and estrogen use; fibrinogen elevation with acute inflammation), decreased synthesis of anticoagulants (liver disease reduces protein C, S, and antithrombin production), increased platelet activation (malignancy, estrogen, smoking), and impaired fibrinolysis (elevated plasminogen activator inhibitor-1 in obesity, diabetes, acute illness). Inherited thrombophilias include Factor V Leiden (G1691A mutation preventing protein C-mediated cleavage; present in 5% of Caucasians, 3-4% increased relative risk for VTE), prothrombin G20210A mutation (increased Factor II levels; 2-3% increased relative risk), antithrombin deficiency (both Type I [reduced quantity] and Type II [dysfunctional]; rare but high penetrance—50% lifetime VTE risk), protein C deficiency (Type I or II; increased risk of warfarin-induced skin necrosis when anticoagulation initiated without bridging), and protein S deficiency (Type I, II, or III; protein S is a cofactor for protein C-mediated Factor V and VIII inactivation). Patients with multiple thrombophilic defects or homozygosity have exponentially higher thrombotic risk.

• Molecular Cascade and Thrombin Generation: Once tissue factor is exposed or circulating procoagulants are elevated, the coagulation cascade amplifies rapidly. The extrinsic pathway (TF + FVII) activates Factor X to Factor Xa. Factor Xa combines with Factor V (as the "prothrombinase complex") to convert Factor II (prothrombin) to Factor IIa (thrombin). Thrombin is the central protease of coagulation—it cleaves fibrinogen into fibrin monomers that polymerize into a clot network, activates Factor XIII (transglutaminase) to cross-link fibrin, and provides positive feedback by activating Factors V, VIII, XI, and platelets, creating an amplification loop. Thrombin also activates platelets through protease-activated receptors (PAR-1, PAR-4), promoting granule secretion of ADP and thromboxane A2, which further recruit platelets. The intrinsic pathway (Factor XII contact activation via negatively charged surfaces) contributes to thrombin generation but is less critical than the extrinsic pathway in venous thrombosis; this is why Factor XII deficiency does not cause thrombosis but prolonged aPTT. The tissue factor pathway inhibitor (TFPI) and antithrombin are the primary physiologic brakes on this cascade. Antithrombin (a serpin protease inhibitor) is dramatically accelerated by heparin and low-molecular-weight heparin (LMWH), which provide rapid anticoagulation. Protein C and protein S inactivate Factors V and VIII, providing slower but essential long-term anticoagulation (mechanism of warfarin).

• Fibrin Clot Formation and Organization: As thrombin generates fibrin, the clot initially forms in the area of lowest blood flow (typically at venous valve cusps in the calf), where local thrombin concentration reaches threshold. The clot propagates proximally as more fibrin is deposited and additional thrombin is generated, potentially extending into the popliteal, femoral, and iliac veins (proximal DVT). The clot may partially or completely occlude the vein, dramatically reducing venous return from that extremity. Over days to weeks, the clot becomes organized: platelets retract the clot, endothelial cells at the clot margins proliferate, and tissue factor-bearing cells (leukocytes, fibroblasts) invade the thrombus. Smooth muscle cells migrate into the thrombus and synthesize extracellular matrix. Eventually, the thrombus may recanalize, with new venous channels forming within the clot, or it may remain organized and fibrotic, causing permanent venous valve damage and chronic obstruction.

• Immobility and Prolonged Stasis: Prolonged bed rest (acute illness, paralysis, ICU admission), long-distance travel (>4 hours air, train, or car travel—"economy class syndrome"), and post-operative immobilization are among the most common precipitants. Any condition reducing calf muscle pump function increases risk: paralysis (stroke, spinal cord injury), coma, sedation, or neurologic disease. The risk of symptomatic VTE after orthopedic surgery (particularly hip replacement, hip fracture repair, knee replacement) is 40-60% without prophylaxis, among the highest of any patient population. Extended bed rest in ICU patients increases VTE risk by 10-fold.

• Surgery and Trauma: All major surgery increases VTE risk through multiple mechanisms (tissue factor release, endothelial injury, immobility, inflammation). Orthopedic surgery (hip and knee replacement, femur fracture repair) carries the highest thrombotic risk (40-60% symptomatic VTE without prophylaxis); abdominal and pelvic surgery (especially malignancy-related), thoracic surgery, and neurosurgery also carry substantial risk (10-40%). Trauma, especially lower extremity fractures and spinal cord injury, causes both direct endothelial injury and prolonged immobilization.

• Malignancy: Patients with active cancer have a 4-7 fold increased VTE risk compared to age-matched controls; the risk is highest in the first 3-6 months after diagnosis. Adenocarcinomas (lung, gastric, ovarian, pancreatic) carry higher risk than squamous cell carcinomas. The mechanism involves tumor expression of tissue factor and cancer procoagulant (direct Factor X activator), production of pro-inflammatory cytokines (IL-1, TNF), tumor cell-induced platelet aggregation, and often advanced disease with immobility. Chemotherapy further increases risk, particularly gemcitabine and cisplatin. Cancer-associated thrombosis often presents as unprovoked VTE, may be recurrent despite anticoagulation, and is associated with mortality rates of 7-30% when PE occurs.

• Venous Catheters and Central Lines: Central venous catheters (CVCs) injure the endothelium directly and cause inflammation; the risk increases with catheter duration, catheter diameter, and chemical irritation from infusates (potassium, calcium, chemotherapy). Peripherally inserted central catheters (PICCs) and jugular CVCs carry lower thrombotic risk than subclavian CVCs. Thrombosis can occur around the catheter (catheter-related thrombosis) or as extensive proximal vein thrombosis; incidence is 5-20% depending on catheter type and duration.

• Estrogen and Hormone Therapy: Combined oral contraceptives and hormone replacement therapy (HRT) containing estrogen increase VTE risk 3-4 fold due to estrogen-induced increases in Factors II, VII, IX, X and fibrinogen, decreased anticoagulant synthesis (protein S), and impaired fibrinolysis. The risk is highest with newer progestins (desogestrel, gestodene, drospirenone—up to 2-fold higher risk than older progestins like levonorgestrel) and with higher estrogen doses. The absolute annual VTE incidence in healthy women on combined oral contraceptives is 3-4 per 10,000, compared to 1-2 per 10,000 in non-users. Transdermal estrogen carries lower risk than oral (first-pass metabolism avoidance reduces hepatic factor synthesis), and progestin-only methods have minimal VTE risk.

• Inherited Thrombophilias: Factor V Leiden and prothrombin G20210A are the most common, present in ~5% and 2% of the general population respectively, but account for only 50% of familial thrombosis clusters. Antithrombin deficiency (rare, 1 in 2000-5000), protein C deficiency (1 in 200-300; often clinically silent until provoked), and protein S deficiency (1 in 500; distinguish Type I quantitative deficiency from Type III with low free but normal total S levels) confer higher individual risk but are rarer. Lupus anticoagulant and anticardiolipin antibodies (antiphospholipid syndrome) are acquired thrombophilias strongly associated with VTE and arterial thrombosis; diagnosis requires repeat positive testing 12 weeks apart. Patients with inherited thrombophilias typically require indefinite anticoagulation only if they have unprovoked VTE or recurrent VTE despite anticoagulation; those with provoked VTE (surgery, immobility) usually require 3 months of anticoagulation only.

• Acute Medical Illness: Acute infection (sepsis, pneumonia, UTI), acute coronary syndrome, congestive heart failure, acute stroke, and acute respiratory failure all increase VTE risk through inflammation, immobility, and endothelial activation. Inflammatory bowel disease and other chronic inflammatory states increase baseline VTE risk. Nephrotic syndrome increases risk through urinary loss of anticoagulant proteins and increased Factor V synthesis.

• Prior Venous Thromboembolism: History of prior DVT or PE increases risk of recurrence; approximately 30% of patients with unprovoked VTE have recurrence within 10 years off anticoagulation, compared to 3% with provoked (surgery, immobility) VTE. Patients with proximal DVT or PE have higher recurrence risk than those with isolated distal calf DVT.

• Obesity and Metabolic Syndrome: Obesity (BMI >30) increases VTE risk by 1.5-2.5 fold. Obesity is associated with increased Factor VIII, Factor II, and fibrinogen; decreased anticoagulant response (impaired protein C response); and increased PAI-1 (impaired fibrinolysis). Diabetes mellitus also increases risk through endothelial dysfunction and impaired fibrinolysis.

• Smoking: Active smoking increases VTE risk 1.5-2 fold through effects on endothelial function, increased von Willebrand factor, and platelet activation. The risk is dose-dependent and declines after smoking cessation.

• Pregnancy and Postpartum Period: Pregnancy increases VTE risk 4-5 fold (physiologic hypercoagulability with increases in Factors VII, VIII, IX, X; decreased anticoagulant response; venous stasis due to uterine compression and progesterone-induced venous dilation). The risk is highest in the immediate postpartum period (first 6 weeks), where risk is 10-20 fold elevated compared to non-pregnant women. Cesarean delivery increases risk further. Maternal VTE accounts for ~1-2% of maternal deaths.

• Leg Swelling (Edema): The cardinal symptom of DVT, resulting from increased venous pressure distal to the occlusion preventing normal venous drainage and causing fluid accumulation in interstitial spaces. Swelling is typically unilateral (unless bilateral DVT or IVC thrombosis), usually begins at the ankle and progresses proximally, and is worse at the end of the day. The calf is most commonly affected since 80-90% of DVTs originate there. In proximal DVT (popliteal, femoral, iliac), swelling may extend to the hip and groin. Calf circumference measured 10 cm below the tibial tuberosity can be compared to the contralateral side; a difference of >3 cm is significant. Some patients with DVT may have minimal edema if collateral circulation develops rapidly or if the thrombus is occlusive.

• Leg Pain and Tenderness: Pain typically has an aching, cramping quality, is aggravated by standing or walking, and is relieved by leg elevation (due to reduced venous pressure). Calf pain with walking is classic. Tenderness may be elicited over the calf muscles (soleal and gastrocnemius veins) or along venous tracts. The pain in DVT is generally less acute than in compartment syndrome or acute arterial insufficiency, and limb perfusion (skin color, temperature, capillary refill, pulses) is preserved.

• Erythema and Skin Changes: The overlying skin may appear reddish or cyanotic (from venous stasis and increased deoxygenation), particularly in the lower calf and ankle. Superficial veins may become distended and visible. In phlegmasia cerulea dolens (extensive ileofemoral thrombosis causing severe venous obstruction), the leg becomes mottled blue-purple, extremely painful, and cool—a medical emergency with risk of tissue necrosis and limb loss if not treated urgently. In phlegmasia alba dolens (less common, associated with malignancy), the leg becomes white and edematous due to arterial compression from massive venous engorgement.

• Homans' Sign: Calf pain elicited by passive dorsiflexion of the foot; historically taught as a clinical sign of DVT but actually insensitive (present in only 25-30% of DVT) and non-specific (can occur with muscle strain, contusion, cell

Step 1 — Risk stratify with the Wells score for DVT

• Components: active cancer, paralysis/recent immobilization of the leg, recent bed rest >3 days or major surgery, localized tenderness along the deep venous system, entire-leg swelling, calf circumference difference >3 cm, pitting edema confined to the symptomatic leg, collateral superficial veins, prior documented DVT (each +1); subtract 2 points if an alternative diagnosis is at least as likely.
• Interpretation: a score of 2 or more is DVT likely; 1 or less is DVT unlikely.

Step 2 — D-dimer in the low-probability patient

• High-sensitivity D-dimer: a fibrin degradation product; sensitive but not specific (elevated in pregnancy, malignancy, sepsis, surgery, advanced age). Its value is a negative result, which in a DVT unlikely patient effectively excludes DVT and ends the workup. A positive D-dimer never establishes the diagnosis — it mandates imaging.

Step 3 — Imaging

• Compression ultrasonography with Doppler: the initial confirmatory test in essentially all patients and the test of choice if DVT is likely (skip D-dimer). The diagnostic finding is failure of the vein to collapse under probe pressure; supportive findings include visible echogenic intraluminal thrombus, absent or blunted respiratory phasicity, and loss of color flow.
• Serial ultrasound in 5–7 days: used when an initial study is negative but suspicion remains high, or for isolated distal (calf) DVT managed without anticoagulation, to detect proximal propagation.
• Contrast venography: the historical gold standard (intraluminal filling defect), now essentially replaced by ultrasound; reserved for equivocal cases.
• CT or MR venography: preferred when iliac/IVC thrombus or pelvic pathology is suspected, a territory ultrasound images poorly.

Ancillary testing

• Thrombophilia panels: do not send during acute thrombosis or on anticoagulation — acute clot consumes antithrombin/protein C and S, and heparin, warfarin, and DOACs all distort assays, generating false positives.

Immediate assessment

• Screen for limb- or life-threatening presentation: phlegmasia cerulea dolens, suspected concurrent PE with hemodynamic compromise, or evidence of major bleeding all change the pathway before any drug is chosen.
• Anticoagulate empirically while awaiting imaging if suspicion is high and bleeding risk is low.

First-line anticoagulation (CHEST/ACCP antithrombotic guideline; also ASH 2020 VTE treatment guideline)

• Direct oral anticoagulants (DOACs) are preferred over warfarin for most patients with acute proximal DVT.
• Factor Xa inhibitors: apixaban and rivaroxaban are started as monotherapy with a higher-intensity loading phase — no parenteral lead-in.
• Edoxaban and dabigatran (direct thrombin inhibitor) require 5 days of parenteral lead-in with low-molecular-weight heparin such as enoxaparin.
• LMWH or fondaparinux bridging to warfarin (target INR 2–3, overlap at least 5 days and until INR therapeutic twice) remains appropriate when a DOAC is unsuitable.

Special populations

• Pregnancy: LMWH only — warfarin is teratogenic and DOACs cross the placenta and are not recommended (ACOG).
• Cancer-associated VTE: an oral factor Xa inhibitor or LMWH; ASCO cautions against oral Xa inhibitors with luminal GI or genitourinary tumors because of bleeding.
• Antiphospholipid syndrome, especially triple-positive: warfarin, not a DOAC — DOACs performed worse in trials.
• Severe renal impairment or need for rapid reversibility: unfractionated heparin, which is monitored by aPTT or anti-Xa.

Duration

• Provoked by a transient major risk factor: 3 months.
• Unprovoked, recurrent, or persistent risk factor (active cancer, antiphospholipid syndrome): extended/indefinite therapy with periodic bleeding-risk reassessment.
• Isolated distal DVT with mild symptoms and low risk: serial imaging surveillance is an accepted alternative to anticoagulation.

Escalation and procedures

• Catheter-directed thrombolysis or thrombectomy: reserved for phlegmasia cerulea dolens and threatened limb, not for routine iliofemoral DVT to prevent post-thrombotic syndrome.
• IVC filter: only when anticoagulation is absolutely contraindicated (active major bleeding); retrieve once anticoagulation can resume. A filter is not a substitute for anticoagulation.
• Graduated compression stockings: for symptom control, not routinely to prevent post-thrombotic syndrome.

Complications of the disease

• Pulmonary embolism — emergency: thrombus embolizes from the proximal deep veins to the pulmonary arteries, causing dead-space ventilation, hypoxemia, and acute RV pressure overload. Signaled by sudden dyspnea, pleuritic chest pain, tachycardia, hypoxemia, or syncope; hypotension defines massive/high-risk PE and prompts consideration of systemic thrombolysis.
• Phlegmasia cerulea dolens and venous gangrene — emergency: near-total iliofemoral outflow obstruction raises interstitial pressure above capillary perfusion pressure, producing a cyanotic, tensely swollen, exquisitely painful limb with compromised pulses. Requires urgent anticoagulation plus catheter-directed thrombolysis or thrombectomy.
• Post-thrombotic syndrome: thrombus organization destroys venous valves, producing ambulatory venous hypertension. Presents months to years later with chronic limb heaviness, edema, hyperpigmentation over the medial malleolus (hemosiderin staining), lipodermatosclerosis, and venous stasis ulceration.
• Recurrent VTE: highest in unprovoked events and active malignancy; recurrence while therapeutically anticoagulated should raise suspicion for occult cancer or antiphospholipid syndrome.
• Chronic thromboembolic pulmonary hypertension: failure of thrombus resolution in the pulmonary vasculature; signaled by progressive exertional dyspnea with a loud P2 and RV failure after a prior PE.

Complications of treatment

• Major hemorrhage — emergency when intracranial or retroperitoneal: sudden headache with focal deficit, or flank/back pain with unexplained hemoglobin drop and hypotension. Reverse warfarin with 4-factor PCC plus vitamin K, dabigatran with idarucizumab, and oral factor Xa inhibitors with andexanet alfa; protamine reverses unfractionated heparin (only partially reverses LMWH).
• Heparin-induced thrombocytopenia (type II) — emergency: IgG antibodies against platelet factor 4–heparin complexes cross-link platelet FcγRIIa, causing platelet activation and paradoxical thrombosis. Signaled by a platelet fall of more than 50% at roughly day 5–10 (4Ts score). Stop all heparin and start a non-heparin anticoagulant — argatroban, bivalirudin, or fondaparinux. Never give warfarin alone acutely: protein C depletion precipitates venous limb gangrene.
• Warfarin-induced skin necrosis: same mechanism of early protein C depletion, classically in protein C or S deficiency, with painful skin lesions over fatty areas in the first days of therapy.
• IVC filter complications: filter thrombosis, migration, strut fracture, IVC perforation, and increased long-term recurrent DVT.

• The algorithm is the answer: Wells score first. If DVT is unlikely, a negative high-sensitivity D-dimer rules it out. If DVT is likely, go straight to compression ultrasound — ordering a D-dimer in a high-probability patient is the classic wrong answer.
• The diagnostic ultrasound finding is non-compressibility of the vein, not visualization of clot. Contrast venography is the historical gold standard but is almost never the correct next step.
• Best next step for suspected DVT with high suspicion and delayed imaging: start empiric anticoagulation. Do not wait for the scan if bleeding risk is low.
• Apixaban and rivaroxaban need no parenteral lead-in; dabigatran and edoxaban do (5 days of LMWH first). This distinction is heavily tested.
• Pregnancy = LMWH. Warfarin is teratogenic and DOACs are not recommended — a stem describing a pregnant patient started on warfarin or a DOAC is testing this.
• Antiphospholipid syndrome, particularly triple-positive, gets warfarin, not a DOAC — the single most tested "exception to DOAC-first."
• Platelet count falling >50% around day 5–10 of heparin with new thrombosis is HIT: stop heparin and start argatroban, bivalirudin, or fondaparinux. Simply stopping heparin, or starting warfarin alone, are the two seeded distractors — warfarin alone causes venous limb gangrene.
• IVC filter is only for absolute contraindication to anticoagulation (active major bleeding). It is not an adjunct to anticoagulation and does not treat the clot.
• Do not send a thrombophilia panel during the acute event or while anticoagulated — acute thrombosis and the drugs themselves produce spurious results.
• Distinguish the phlegmasias: phlegmasia cerulea dolens is a blue, tense, painful limb from massive iliofemoral occlusion and is a limb-threatening emergency warranting thrombolysis/thrombectomy; Homans' sign is a historical curiosity that is neither sensitive nor specific and should never drive management.