Medical School · Year 2 · Hematology Oncology · includes a quiz and discussion video

Lecture 06: Thrombotic Disorders

Unit 2.9: Hematology and Oncology

Learning Objectives

By the end of this lecture, students will be able to:

1. Describe the pathophysiology of thrombosis (Virchow's triad)
2. Explain inherited thrombophilias
3. Describe acquired thrombophilic conditions
4. Explain the clinical presentation of venous thromboembolism
5. Describe anticoagulant medications and their mechanisms
6. Explain the approach to thrombophilia testing and management

Lecture Outline

I. Thrombosis Overview

Virchow's triad describes the three broad categories of factors predisposing to thrombosis, providing a framework for understanding both the pathophysiology and clinical risk assessment for venous thromboembolism. Stasis allows activated coagulation factors to accumulate locally without dilution by flowing blood, occurring with immobilization from surgery, hospitalization, paralysis, or long-distance travel. Endothelial injury exposes tissue factor and collagen that activate the coagulation cascade and platelets, occurring with trauma, indwelling catheters, surgery, and inflammation. Hypercoagulability encompasses inherited defects in natural anticoagulants, acquired prothrombotic conditions, and alterations in coagulation factor levels or function. Most thrombotic events result from interaction of multiple factors across these categories.

Arterial and venous thrombosis differ fundamentally in their composition, risk factors, and prevention strategies. Arterial thrombi form in high-flow environments where platelet adhesion and aggregation predominate, producing white clots rich in platelets bound by thin fibrin strands. These develop at sites of atherosclerotic plaque rupture or endothelial dysfunction, with risk factors including hypertension, diabetes, hyperlipidemia, and smoking. Prevention focuses on antiplatelet therapy. Venous thrombi form in low-flow environments where the coagulation cascade predominates, producing red clots rich in fibrin and trapped erythrocytes. These develop in areas of stasis, particularly the deep veins of the lower extremities, with risk factors including immobility, surgery, inherited thrombophilias, and hypercoagulable states. Prevention and treatment focus on anticoagulation.

Natural anticoagulant systems prevent excessive clot formation by regulating activated coagulation factors through several complementary mechanisms. Antithrombin is the major physiologic inhibitor of thrombin and factor Xa, with its activity dramatically enhanced by heparin, which explains heparin's mechanism of action. Protein C, activated by thrombin bound to endothelial thrombomodulin, proteolytically inactivates factors Va and VIIIa with protein S serving as an essential cofactor. Tissue factor pathway inhibitor limits the initiation of coagulation by inhibiting the tissue factor-factor VIIa complex. Deficiencies in these natural anticoagulants predispose to venous thrombosis and form the basis of several inherited thrombophilias.

The fibrinolytic system balances clot formation by mediating clot dissolution once vascular repair is complete. Tissue plasminogen activator released from endothelium converts plasminogen to plasmin, the active fibrinolytic enzyme. Plasmin cleaves fibrin into degradation products including D-dimer, which serves as a clinical marker for fibrinolysis and thus indirectly for thrombosis. Plasminogen activator inhibitor-1 inhibits tPA, while alpha-2-antiplasmin inhibits plasmin, providing regulation of fibrinolytic activity. Defects in the fibrinolytic system can contribute to thrombosis, though clinically significant fibrinolytic deficiencies are rare. Understanding this system informs the use of thrombolytic agents that activate plasminogen to treat life-threatening thrombosis.

<image>Panel A: Virchow's triad diagram showing stasis, endothelial injury, and hypercoagulability with clinical examples of each factor. Panel B: Comparison of arterial versus venous thrombus composition showing platelet-rich white clot in arteries and fibrin-rich red clot in veins. Panel C: Natural anticoagulant pathways showing antithrombin inhibiting thrombin and Xa, and protein C pathway inactivating Va and VIIIa. Panel D: Fibrinolytic system showing tPA converting plasminogen to plasmin with D-dimer as degradation product.</image>

II. Inherited Thrombophilias

Factor V Leiden represents the most common inherited thrombophilia, resulting from a point mutation substituting glutamine for arginine at position 506, which is normally the protein C cleavage site. This mutation renders factor V resistant to inactivation by activated protein C, prolonging factor Va activity and increasing thrombin generation. Approximately 5 percent of Caucasians carry the mutation in heterozygous form, with much lower prevalence in other populations. Heterozygous carriers have a 3 to 7-fold increased risk of venous thromboembolism, while homozygotes face 50 to 80-fold increased risk. The mutation does not affect arterial thrombosis risk. Combined with other risk factors such as oral contraceptives or other thrombophilias, the risk increases synergistically. Testing uses either functional activated protein C resistance assay or genetic testing.

Prothrombin G20210A mutation involves a guanine to adenine substitution in the 3-prime untranslated region of the prothrombin gene, resulting in elevated prothrombin levels that increase thrombin generation capacity. This mutation occurs in 2 to 3 percent of Caucasians with lower frequency in other populations. Heterozygous carriers have a 2 to 3-fold increased risk of venous thromboembolism, making it a moderate risk factor. The combination of prothrombin mutation with Factor V Leiden produces synergistically increased risk. Testing requires genetic analysis since prothrombin levels alone do not reliably identify carriers. Like Factor V Leiden, this mutation does not significantly increase arterial thrombosis risk.

Antithrombin deficiency represents one of the most thrombogenic inherited disorders, with affected individuals experiencing recurrent venous thrombosis often beginning at a young age. Antithrombin is the major physiologic inhibitor of thrombin and factor Xa, accounting for approximately 80 percent of thrombin inhibition. Type I deficiency involves quantitative reduction of normally functioning protein, while Type II involves qualitatively abnormal protein with reduced activity. Inheritance is autosomal dominant, with homozygous deficiency generally incompatible with life. Heterozygotes have a 10 to 50-fold increased lifetime risk of thrombosis, the highest among inherited thrombophilias. Importantly, patients with antithrombin deficiency may show reduced response to heparin, which requires antithrombin for its anticoagulant activity. Antithrombin concentrate can be used during acute events or high-risk situations.

Testing for inherited thrombophilias requires attention to factors that affect assay results and interpretation. Genetic tests for Factor V Leiden and prothrombin G20210A are unaffected by anticoagulation or acute thrombosis and can be performed at any time. Functional assays for antithrombin, protein C, and protein S are affected by multiple factors. Heparin depletes antithrombin, lowering measured levels. Warfarin reduces protein C and S synthesis as vitamin K-dependent proteins. Acute thrombosis consumes natural anticoagulants. Pregnancy and estrogen-containing contraceptives reduce protein S levels. Inflammation decreases free protein S by increasing C4b-binding protein. Testing should ideally occur 2 to 4 weeks after discontinuing anticoagulation, though clinical circumstances may not permit this delay.

<image>Panel A: Molecular mechanism of Factor V Leiden showing the mutation at the protein C cleavage site and resulting resistance to activated protein C inactivation. Panel B: Comparison of inherited thrombophilia prevalence and relative risk increase showing Factor V Leiden as most common but antithrombin deficiency as most thrombogenic. Panel C: Diagram of antithrombin mechanism of action and how deficiency produces hypercoagulability. Panel D: Testing considerations showing which assays are affected by anticoagulation, acute thrombosis, and pregnancy.</image>

III. Protein C and S Deficiency

Protein C deficiency impairs the inactivation of factors Va and VIIIa, allowing continued thrombin generation after coagulation is initiated. Activated protein C is generated when thrombin binds endothelial thrombomodulin, forming a complex that converts the inactive zymogen to active enzyme. Type I deficiency involves quantitative reduction with proportional decrease in antigen and activity, while Type II involves dysfunctional protein with disproportionate activity reduction. Inheritance is autosomal dominant with variable penetrance, meaning not all carriers experience thrombosis. Heterozygotes have approximately 10-fold increased thrombosis risk. Homozygous protein C deficiency is rare but produces neonatal purpura fulminans, a catastrophic thrombotic syndrome with skin necrosis and disseminated intravascular coagulation requiring urgent protein C replacement.

Protein S serves as an essential cofactor for activated protein C, enhancing its ability to inactivate factors Va and VIIIa several-fold. Protein S circulates in two forms: free protein S, which is the active form available to serve as cofactor, and bound protein S complexed with C4b-binding protein, which is inactive. Deficiency may be quantitative affecting total protein S (Type I), qualitative affecting function (Type II), or selective for free protein S (Type III). Conditions that increase C4b-binding protein including pregnancy, estrogen therapy, and inflammation shift the equilibrium toward bound protein S, decreasing free levels even with normal total protein S. This physiologic reduction complicates interpretation of testing during these conditions. Thrombosis risk with heterozygous deficiency is approximately 10-fold increased.

Warfarin-induced skin necrosis represents a dramatic complication related to protein C and S deficiency that must be anticipated and prevented. Warfarin inhibits vitamin K-dependent carboxylation of factors II, VII, IX, and X as well as proteins C and S. Protein C has the shortest half-life among these proteins at approximately 6 hours, causing its levels to decline faster than the procoagulant factors during warfarin initiation. This creates a transient hypercoagulable state during days 3 to 6 of therapy when protein C is markedly reduced but factors II and X retain significant activity. Microvascular thrombosis produces painful purpuric skin lesions that progress to necrosis, typically affecting fatty areas including breasts, buttocks, and thighs. Patients with underlying protein C or S deficiency face highest risk. Prevention requires overlapping warfarin with therapeutic heparin until INR is therapeutic and using low initial warfarin doses.

Clinical management of patients with protein C or S deficiency requires balancing thrombosis risk against treatment-related complications. Initial anticoagulation for acute thrombosis should use parenteral agents before transitioning to warfarin with prolonged overlap and low starting doses given the skin necrosis risk. Duration of anticoagulation follows similar principles as other thrombophilias, with provoked events typically treated for 3 months and unprovoked events often warranting extended therapy. Prophylaxis during high-risk situations such as surgery or prolonged immobilization is essential. Women should avoid estrogen-containing contraceptives and hormone therapy given the additive thrombosis risk. Genetic counseling should be offered to first-degree relatives, who have a 50 percent chance of carrying the deficiency. Protein C concentrate is available for acute situations in severe deficiency.

<image>Panel A: Protein C activation pathway showing thrombin binding to thrombomodulin, activation of protein C, and inactivation of factors Va and VIIIa with protein S as cofactor. Panel B: Protein S forms showing equilibrium between free active form and C4b-binding protein-bound inactive form with conditions shifting the balance. Panel C: Warfarin-induced skin necrosis timeline showing protein C decline before procoagulant factors and the vulnerable window during days 3-6. Panel D: Clinical photograph of warfarin-induced skin necrosis demonstrating purpuric lesions progressing to necrosis in fatty tissue areas.</image>

IV. Acquired Thrombophilias

Antiphospholipid syndrome is an autoimmune disorder characterized by antibodies against phospholipid-binding proteins that paradoxically cause thrombosis despite prolonging in vitro coagulation tests. The three relevant antibodies are lupus anticoagulant, anticardiolipin antibodies, and anti-beta-2-glycoprotein I antibodies. Lupus anticoagulant prolongs phospholipid-dependent coagulation tests including aPTT, yet patients experience thrombosis rather than bleeding because the in vivo effect is prothrombotic. Diagnosis requires both clinical criteria including vascular thrombosis or pregnancy morbidity, and laboratory criteria with antibody positivity confirmed on two occasions at least 12 weeks apart. Clinical manifestations include venous thromboembolism, arterial events including stroke and myocardial infarction, and obstetric complications including recurrent miscarriage, preeclampsia, and intrauterine growth restriction. Catastrophic antiphospholipid syndrome involves rapid multi-organ failure from widespread microvascular thrombosis.

Malignancy-associated thrombosis reflects the prothrombotic state induced by cancer through multiple mechanisms and significantly impacts patient morbidity and mortality. Cancer patients face 4 to 7-fold increased venous thromboembolism risk compared to the general population. Mechanisms include tumor-derived tissue factor activation of coagulation, inflammatory cytokines, direct vessel compression and invasion, central venous catheter placement, and chemotherapy-induced endothelial damage and immobility. Cancers with highest thrombosis risk include pancreatic, gastric, lung, brain, and hematologic malignancies. Trousseau syndrome describes migratory superficial thrombophlebitis associated with occult malignancy. Unprovoked venous thromboembolism may be the presenting manifestation of undiagnosed cancer in some patients, though routine extensive cancer screening after unprovoked VTE remains controversial.

Other acquired conditions creating hypercoagulable states deserve recognition for appropriate thromboprophylaxis and management. Surgery and trauma produce hypercoagulability through immobility, tissue factor release, and endothelial injury, making perioperative prophylaxis essential for moderate and high-risk patients. Pregnancy increases thrombosis risk 4 to 5-fold through estrogen-induced increases in coagulation factors and physiologic reduction in protein S, with risk highest in the postpartum period. Oral contraceptives and hormone replacement therapy similarly increase clotting factor levels and reduce protein S. Nephrotic syndrome causes antithrombin loss in urine, producing significant thrombosis risk. Paroxysmal nocturnal hemoglobinuria involves complement-mediated endothelial activation causing both hemolysis and thrombosis. Myeloproliferative neoplasms produce qualitatively abnormal platelets and hyperviscosity predisposing to both arterial and venous events.

Heparin-induced thrombocytopenia represents an acquired immune-mediated hypercoagulable state requiring urgent recognition and management. Antibodies form against complexes of platelet factor 4 and heparin, binding to and activating platelets through Fc receptor engagement. Activated platelets release more platelet factor 4, amplifying the process. The result is both platelet consumption causing thrombocytopenia and platelet activation causing thrombosis, predominantly venous but also arterial. Typical presentation occurs 5 to 10 days after heparin initiation with greater than 50 percent platelet count decline from baseline. The 4Ts scoring system assesses clinical probability. Diagnosis requires both clinical suspicion and laboratory confirmation with anti-PF4/heparin antibodies and functional platelet activation testing. Management demands immediate cessation of all heparin products including heparin flushes and initiation of alternative anticoagulation with argatroban, bivalirudin, or fondaparinux.

<image>Panel A: Antiphospholipid syndrome pathophysiology showing antibody binding to beta-2-glycoprotein I on cell surfaces and resulting in vivo prothrombotic effect. Panel B: Mechanisms of cancer-associated thrombosis including tissue factor expression, inflammatory cytokines, vessel compression, and treatment effects. Panel C: Timeline of pregnancy-related hypercoagulability showing increased risk throughout pregnancy with peak in postpartum period. Panel D: Heparin-induced thrombocytopenia mechanism showing PF4-heparin complex, antibody formation, platelet activation, and paradoxical thrombosis.</image>

V. Venous Thromboembolism Clinical Presentation

Deep vein thrombosis most commonly affects the lower extremities, with clinical presentation and prognosis determined by thrombus location and extent. Proximal DVT involving the femoral, popliteal, or iliac veins carries significant risk of pulmonary embolism and post-thrombotic syndrome, requiring anticoagulation treatment. Distal or calf vein DVT below the knee has lower but not negligible embolism risk, with management ranging from treatment to serial surveillance depending on clinical circumstances. Symptoms of lower extremity DVT include unilateral leg swelling, pain, warmth, and erythema, though many patients have minimal symptoms. The Homan's sign of calf pain with dorsiflexion has poor sensitivity and specificity and should not be relied upon for diagnosis. Upper extremity DVT, often catheter-associated, presents with arm swelling and pain and can also embolize to the lungs.

Pulmonary embolism occurs when thrombus from the deep venous system embolizes to the pulmonary arterial circulation, producing a spectrum of presentations from asymptomatic to cardiovascular collapse. Symptoms include dyspnea, which is most common, pleuritic chest pain from lung infarction, cough, and hemoptysis. Signs include tachypnea, tachycardia, hypoxia, and in massive PE, hypotension and signs of right ventricular failure. The classic triad of dyspnea, pleuritic chest pain, and hemoptysis is present in only a minority of patients. Risk stratification distinguishes massive PE with hemodynamic instability requiring aggressive intervention, submassive PE with right ventricular dysfunction but stable hemodynamics, and low-risk PE without these findings. The Pulmonary Embolism Severity Index and simplified PESI help predict mortality and guide management intensity.

Clinical probability assessment guides diagnostic testing and interpretation for suspected venous thromboembolism. The Wells score for DVT incorporates clinical features including active cancer, paralysis, recent immobilization, localized tenderness, leg swelling, pitting edema, collateral veins, and alternative diagnosis likelihood. The Wells score for PE considers clinical signs of DVT, PE as the most likely diagnosis, tachycardia, immobilization or recent surgery, prior VTE, hemoptysis, and malignancy. The PERC rule allows PE to be excluded without further testing if all eight criteria are negative in low-probability patients, avoiding unnecessary D-dimer testing that may prompt additional workup. High clinical probability warrants empiric anticoagulation while awaiting confirmatory imaging given the risks of untreated VTE.

Diagnostic testing follows clinical probability stratification to efficiently confirm or exclude venous thromboembolism. D-dimer, a fibrin degradation product, has high sensitivity but low specificity for VTE, making a negative result useful for excluding thrombosis in low to intermediate probability patients. Age-adjusted D-dimer thresholds improve specificity in elderly patients. Compression ultrasonography is the first-line imaging modality for suspected DVT, with inability to compress the vein indicating thrombus. CT pulmonary angiography is the gold standard for PE diagnosis, visualizing thrombus as filling defects within the pulmonary arteries. Ventilation-perfusion scanning provides an alternative when CT is contraindicated, with mismatch indicating PE. Echocardiography can identify right ventricular strain indicating hemodynamically significant PE but cannot directly visualize pulmonary thrombus.

<image>Panel A: Anatomic diagram of lower extremity deep veins distinguishing proximal from distal DVT with associated pulmonary embolism risk for each location. Panel B: Clinical presentation of pulmonary embolism showing spectrum from asymptomatic through massive PE with hemodynamic collapse and corresponding management implications. Panel C: Wells score calculation for DVT and PE probability assessment with interpretation categories. Panel D: Diagnostic algorithm integrating clinical probability, D-dimer testing, and imaging for suspected DVT and PE.</image>

VI. Anticoagulant Therapy

Unfractionated heparin provides rapid, titratable anticoagulation primarily through enhancement of antithrombin-mediated inhibition of thrombin and factor Xa. Administered intravenously as a continuous infusion or subcutaneously for prophylaxis, heparin achieves immediate anticoagulant effect. Monitoring uses the activated partial thromboplastin time with a target of 1.5 to 2.5 times the control value, or anti-Xa levels in certain situations. The short half-life of approximately 1 hour and availability of protamine as a reversal agent make heparin ideal when anticoagulation may need rapid discontinuation, such as perioperatively or with high bleeding risk. Complications include bleeding, heparin-induced thrombocytopenia requiring monitoring of platelet counts, and osteoporosis with prolonged use.

Low molecular weight heparins offer advantages over unfractionated heparin through more predictable pharmacokinetics and greater convenience. Produced by depolymerization of UFH, LMWH has relatively greater anti-Xa activity than anti-thrombin activity. Subcutaneous administration achieves reliable absorption with predictable dose-response, eliminating the need for routine monitoring in most patients. Renal excretion requires dose adjustment in kidney disease. Enoxaparin and dalteparin are commonly used agents with evidence supporting their efficacy in VTE treatment and prophylaxis, acute coronary syndromes, and cancer-associated thrombosis. Anti-Xa level monitoring is indicated in obesity, renal insufficiency, and pregnancy. Protamine partially reverses LMWH anticoagulation. HIT can still occur, though possibly at lower frequency than with UFH.

Fondaparinux is a synthetic pentasaccharide that selectively inhibits factor Xa through antithrombin enhancement without affecting thrombin. This targeted mechanism produces predictable anticoagulation with once-daily subcutaneous dosing and no need for monitoring. An important advantage is the absence of HIT risk since the molecule does not bind platelet factor 4. Renal excretion requires avoidance in severe kidney disease. No specific reversal agent exists, though recombinant factor VIIa and activated prothrombin complex concentrate have been used. Indications include VTE prophylaxis and treatment, and fondaparinux can substitute for heparin in patients with HIT history.

Warfarin remains widely used for long-term anticoagulation despite the availability of newer agents, particularly for mechanical heart valve anticoagulation where it remains the standard. As a vitamin K antagonist, warfarin inhibits the gamma-carboxylation required for activity of factors II, VII, IX, and X as well as proteins C and S. The delayed onset of action over 5 to 7 days necessitates bridging with parenteral anticoagulation for acute thrombosis treatment. Monitoring uses the international normalized ratio with typical targets of 2.0 to 3.0 or 2.5 to 3.5 for higher-risk indications. Extensive drug and food interactions complicate management, requiring patient education about vitamin K-containing foods and drug interactions. Bleeding is the major complication, reversible with vitamin K, fresh frozen plasma, or four-factor prothrombin complex concentrate depending on urgency.

<image>Panel A: Mechanism of heparin showing antithrombin enhancement and relative inhibition of thrombin versus factor Xa for UFH, LMWH, and fondaparinux. Panel B: Comparison of unfractionated heparin and LMWH characteristics including administration, monitoring, reversal, and clinical situations favoring each. Panel C: Warfarin mechanism showing vitamin K cycle inhibition and affected coagulation factors. Panel D: Warfarin management considerations including drug interactions, food interactions, INR monitoring, and reversal strategies.</image>

VII. Direct Oral Anticoagulants

Direct oral anticoagulants have transformed anticoagulation therapy through their convenience, predictability, and favorable safety profile compared to warfarin for most indications. These agents directly inhibit specific coagulation factors without requiring antithrombin as a cofactor. Dabigatran directly inhibits thrombin, while rivaroxaban, apixaban, and edoxaban directly inhibit factor Xa. Rapid onset of action within hours eliminates the need for bridging with parenteral anticoagulation for most indications. Fixed dosing without routine monitoring simplifies management. Fewer drug and food interactions than warfarin improve convenience and reliability. Large randomized trials have demonstrated non-inferiority or superiority to warfarin for stroke prevention in atrial fibrillation and VTE treatment, with consistently lower rates of intracranial hemorrhage.

Individual DOAC characteristics guide agent selection based on patient factors and clinical indication. Dabigatran is taken twice daily with food, requires intact renal function for excretion, and has dyspepsia as a common side effect. Rivaroxaban can be dosed once daily for most indications when taken with food to optimize absorption. Apixaban is taken twice daily and has the least renal dependence for clearance, making it preferable in moderate renal impairment. Edoxaban is taken once daily but requires initial parenteral anticoagulation and has the unique characteristic of reduced efficacy in patients with very good renal function, limiting its use in that population. All require dose adjustment for renal function with specific thresholds varying by agent.

Despite their advantages, DOACs have important limitations requiring consideration in clinical decision-making. Higher acquisition cost compared to warfarin may affect access for some patients. The lack of routine monitoring prevents confirmation of therapeutic effect or compliance, potentially problematic in patients with suspected treatment failure. Contraindications include mechanical heart valves where DOACs showed increased thrombosis and bleeding compared to warfarin, and severe renal impairment where accumulation increases bleeding risk. Moderate hepatic impairment requires caution or avoidance. The short half-life means missed doses quickly result in subtherapeutic anticoagulation, making adherence critical. Drug interactions exist but are fewer and generally more manageable than warfarin interactions.

Reversal of DOAC anticoagulation is now possible with specific and nonspecific agents, addressing an earlier limitation. Idarucizumab is a monoclonal antibody fragment that specifically binds dabigatran with high affinity, immediately and completely reversing anticoagulation. Andexanet alfa is a recombinant modified factor Xa that binds and sequesters factor Xa inhibitors including rivaroxaban and apixaban. Four-factor prothrombin complex concentrate provides nonspecific reversal by supplying coagulation factors that can overwhelm the inhibited pathway. Activated charcoal may reduce absorption if administered within 2 hours of DOAC ingestion. For minor bleeding, drug discontinuation and supportive care often suffice given the relatively short half-lives. Severe or life-threatening bleeding warrants specific reversal agents when available.

<image>Panel A: DOAC mechanisms showing dabigatran directly inhibiting thrombin and factor Xa inhibitors blocking factor Xa without antithrombin requirement. Panel B: Comparison of individual DOAC characteristics including dosing frequency, renal clearance, food requirements, and specific considerations. Panel C: DOAC advantages versus warfarin including rapid onset, fixed dosing, fewer interactions, and lower intracranial hemorrhage rates. Panel D: Reversal strategies for DOAC bleeding showing idarucizumab for dabigatran, andexanet alfa for Xa inhibitors, and supportive measures.</image>

VIII. VTE Treatment and Duration

Initial treatment of venous thromboembolism requires prompt therapeutic anticoagulation to prevent thrombus extension and embolization. For most patients, treatment can begin with rivaroxaban or apixaban as monotherapy without need for parenteral lead-in. Alternatively, initial treatment with LMWH, UFH, or fondaparinux can transition to warfarin, requiring at least 5 days of overlap and therapeutic INR before discontinuing the parenteral agent. Edoxaban and dabigatran require at least 5 days of parenteral anticoagulation before initiation. In patients with extensive iliofemoral DVT, catheter-directed thrombolysis may be considered to reduce post-thrombotic syndrome risk, though this remains a selective approach. Massive PE with hemodynamic instability warrants systemic thrombolysis or surgical embolectomy in addition to anticoagulation.

Duration of anticoagulation depends on whether the thrombotic event was provoked by a reversible factor, as this determines recurrence risk after treatment discontinuation. Provoked VTE associated with major transient risk factors such as major surgery or significant trauma has low recurrence risk after the provoking factor resolves, supporting 3 months of treatment as standard duration. Provoked VTE associated with minor transient factors such as hospitalization, estrogen therapy, or long-distance travel has intermediate recurrence risk. Unprovoked VTE without identifiable provoking factor carries approximately 10 percent annual recurrence risk after stopping anticoagulation, prompting consideration of extended therapy. Cancer-associated thrombosis warrants anticoagulation for the duration of active cancer or at least 6 months, whichever is longer, with preference for LMWH or DOACs over warfarin based on trial evidence.

Extended anticoagulation decisions balance recurrence risk reduction against bleeding risk from continued therapy. Risk assessment tools help quantify bleeding risk, incorporating factors such as age, prior bleeding, anemia, renal or hepatic disease, and concomitant antiplatelet therapy. For patients with unprovoked VTE and low bleeding risk, extended anticoagulation significantly reduces recurrence with acceptable bleeding risk. Reduced-dose regimens using apixaban 2.5 mg twice daily or rivaroxaban 10 mg daily after completing initial treatment provide continued protection with potentially lower bleeding than full-dose anticoagulation. Aspirin provides modest recurrence reduction for patients who discontinue anticoagulation. Patient preference and values play an important role in these decisions requiring shared decision-making.

Special situations require modified approaches to VTE management. Subsegmental PE, detected increasingly with high-resolution CT, may warrant surveillance rather than anticoagulation in patients without proximal DVT, cancer, or high recurrence risk, as the clinical significance of these small emboli remains uncertain. Superficial vein thrombosis near the saphenofemoral junction carries significant risk of extension to deep veins and may warrant anticoagulation with fondaparinux or LMWH for 45 days. Upper extremity DVT, often catheter-related, requires anticoagulation with consideration of catheter removal versus retention based on catheter necessity and function. IVC filter placement is reserved for patients with acute VTE who have absolute contraindication to anticoagulation, with retrievable filters removed once anticoagulation becomes safe.

<image>Panel A: Initial VTE treatment options showing DOAC monotherapy pathway versus parenteral lead-in pathway with transition to various oral agents. Panel B: Duration of anticoagulation algorithm based on provoked versus unprovoked VTE and transient versus persistent risk factors. Panel C: Extended therapy decision framework incorporating recurrence risk, bleeding risk, patient preference, and reduced-dose options. Panel D: Special situations flowchart addressing subsegmental PE, superficial thrombosis, upper extremity DVT, and IVC filter indications.</image>

IX. Thrombophilia Testing

The role of thrombophilia testing remains debated, with decisions to test guided by potential impact on management rather than academic interest in diagnosis. Testing may inform anticoagulation duration when results would change the decision, counsel family members about inherited risk, guide decisions about estrogen-containing contraceptives or hormone therapy, and provide prognostic information for patient counseling. However, testing has limitations: many patients with thrombophilia never experience thrombosis, most thrombotic events occur without identifiable thrombophilia, and positive results may not change management if extended anticoagulation is already indicated. The decision to test should consider how results would influence care.

Appropriate indications for thrombophilia testing include unprovoked VTE in patients younger than 50 years where results may guide duration of therapy, recurrent VTE suggesting underlying predisposition, VTE in unusual sites such as cerebral or splanchnic veins, strong family history of VTE at young ages, warfarin-induced skin necrosis suggesting protein C or S deficiency, and recurrent pregnancy loss or other obstetric complications suggesting antiphospholipid syndrome. Testing is generally not indicated for VTE clearly provoked by major transient risk factors, advanced age where results rarely change management, or when extended anticoagulation is already planned regardless of results.

A comprehensive thrombophilia panel includes genetic tests unaffected by anticoagulation and functional assays requiring attention to timing. Factor V Leiden and prothrombin G20210A mutation testing uses PCR-based genetic analysis unaffected by clinical circumstances or anticoagulation. Antithrombin activity is reduced by heparin and acute thrombosis. Protein C activity is reduced by warfarin, acute thrombosis, and liver disease. Protein S, measured as free protein S for activity, is reduced by warfarin, pregnancy, estrogen therapy, inflammation, and acute thrombosis. Lupus anticoagulant testing is affected by anticoagulation, requiring careful interpretation or testing during bridging interruption. Anticardiolipin and anti-beta-2-glycoprotein I antibodies can be tested during anticoagulation. Homocysteine, a mild risk factor, can be measured anytime.

Interpretation of results requires understanding the clinical significance of different abnormalities and their management implications. Isolated heterozygous Factor V Leiden or prothrombin mutation modestly increases risk but may not warrant extended anticoagulation for a first provoked event. Antithrombin, protein C, or protein S deficiency represents more significant risk often supporting extended therapy. Homozygous mutations or compound heterozygosity markedly increase risk. Antiphospholipid syndrome diagnosed by clinical criteria plus persistent antibody positivity generally warrants indefinite anticoagulation. For family counseling, first-degree relatives of patients with inherited thrombophilia should be informed of testing availability and implications, particularly for women considering oral contraceptives. Genetic counseling supports informed decision-making about testing.

<image>Panel A: Decision framework for when to consider thrombophilia testing based on potential management impact versus situations where testing adds little value. Panel B: Thrombophilia panel components distinguishing genetic tests from functional assays with factors affecting each. Panel C: Testing timing recommendations showing optimal circumstances for each assay relative to anticoagulation and acute events. Panel D: Interpretation guide showing how different positive results may influence management decisions for anticoagulation duration and family counseling.</image>

X. Prevention and Special Populations

VTE prophylaxis in hospitalized patients significantly reduces thrombotic events when appropriately applied based on risk assessment. Medical patients should receive pharmacologic prophylaxis with LMWH, UFH, or fondaparinux when risk factors such as reduced mobility, acute illness, or cancer are present and bleeding risk is acceptable. Surgical patients require risk stratification based on procedure type and patient factors, with options ranging from early ambulation alone for very low risk to extended pharmacologic prophylaxis for high-risk procedures. Mechanical prophylaxis with sequential compression devices provides alternative when bleeding risk precludes pharmacologic agents. Extended prophylaxis beyond hospitalization is indicated after major orthopedic surgery including hip and knee replacement, continuing for 35 days. Risk assessment tools help identify medical patients who benefit from extended prophylaxis.

Pregnancy requires specialized approaches to VTE prevention and treatment given the contraindication to warfarin and DOACs. Pregnancy increases VTE risk 4 to 5-fold through physiologic increases in coagulation factors and protein S reduction, with risk highest in the postpartum period. Women with prior VTE or known thrombophilia may require antepartum prophylaxis based on risk stratification. Treatment of acute VTE during pregnancy uses LMWH, with dose adjustment based on anti-Xa levels given pregnancy-related pharmacokinetic changes. LMWH is held for delivery, typically 24 hours before planned delivery or epidural placement. Postpartum anticoagulation can transition to warfarin during breastfeeding, though DOACs are not recommended. The risk-benefit of anticoagulation and the significant risk of untreated VTE must be carefully communicated.

Cancer-associated thrombosis management has evolved with evidence supporting DOACs as alternatives to LMWH for many patients. Traditional management used LMWH for the duration of cancer treatment given evidence of superiority over warfarin in preventing recurrence. Recent trials demonstrate that edoxaban, rivaroxaban, and apixaban are non-inferior to LMWH with similar or lower recurrence rates. However, increased gastrointestinal and genitourinary bleeding occurs with DOACs in patients with GI or GU malignancies, particularly with luminal involvement, making LMWH preferred in these populations. Duration of anticoagulation typically continues throughout active cancer or for a minimum of 6 months. Patients with cancer who complete treatment and have no evidence of active disease can be managed similarly to patients without cancer.

Perioperative management of anticoagulation balances thrombotic risk during interruption against bleeding risk with continuation. For elective procedures with low bleeding risk, anticoagulation can often continue or be briefly held. High bleeding risk procedures require interruption, with timing determined by drug half-life and renal function. Warfarin is held 5 days before procedures; DOACs are held 1 to 3 days depending on renal function and agent. Bridging with therapeutic LMWH during warfarin interruption is reserved for patients at highest thrombotic risk, such as recent VTE within 3 months, mechanical mitral valve, or atrial fibrillation with prior stroke. Most patients do not require bridging, which increases bleeding without clearly reducing thrombosis in moderate-risk patients. Anticoagulation resumes once hemostasis is achieved, typically 24 to 48 hours postoperatively for low bleeding risk procedures.

<image>Panel A: Hospital VTE prophylaxis algorithm for medical and surgical patients incorporating risk assessment and prophylaxis options. Panel B: Pregnancy VTE management showing risk factors, antepartum prophylaxis indications, treatment approach, and peripartum management. Panel C: Cancer-associated thrombosis treatment algorithm distinguishing patients appropriate for DOAC versus LMWH based on cancer type and location. Panel D: Perioperative anticoagulation management showing interruption timing, bridging indications, and resumption guidelines.</image>

Summary

• Virchow's triad of stasis, endothelial injury, and hypercoagulability provides the framework for understanding thrombosis risk
• Factor V Leiden is the most common inherited thrombophilia, causing activated protein C resistance with 3 to 7-fold increased VTE risk in heterozygotes
• Antithrombin deficiency carries the highest thrombosis risk among inherited thrombophilias with 10 to 50-fold increased risk
• Antiphospholipid syndrome causes both venous and arterial thrombosis plus pregnancy complications despite prolonging aPTT
• DVT presents with unilateral leg swelling and pain; PE presents with dyspnea, pleuritic chest pain, and hypoxia
• Clinical probability assessment guides appropriate use of D-dimer and imaging for VTE diagnosis
• DOACs offer advantages over warfarin for most VTE treatment including fixed dosing, fewer interactions, and lower intracranial hemorrhage
• VTE treatment duration depends on whether the event was provoked, with unprovoked VTE potentially warranting extended therapy
• Thrombophilia testing should be guided by potential management impact rather than performed routinely
• Perioperative anticoagulation management balances thrombotic and bleeding risks with bridging reserved for highest-risk patients

Key Terms

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