Hypercoagulable States — Thrombophilia

Key Mechanism 1: Resistance to Activated Protein C (APC Resistance) — Factor V Leiden

Key Mechanism 2: Prothrombin G20210A Mutation

Key Mechanism 3: Natural Anticoagulant Deficiencies (Protein C, Protein S, Antithrombin)

• Antithrombin (AT) deficiency: Antithrombin is a serine protease inhibitor that forms covalent complexes with thrombin, Factor Xa, and other coagulation enzymes, irreversibly inactivating them. AT function is potentiated 1000-fold by unfractionated heparin and LMWH, which bind to the heparin-binding site on AT. Hereditary AT deficiency (Type I: reduced quantity; Type II: dysfunctional protein) results in unopposed thrombin and Factor Xa activity, creating the highest thrombotic risk among inherited thrombophilias with approximately 50-60% lifetime VTE risk.

• Protein C deficiency: Protein C is a vitamin K-dependent serine protease activated on the endothelial surface by thrombin in complex with thrombomodulin. Activated protein C (APC) proteolytically inactivates Factors Va and VIIIa, thereby limiting thrombin generation. Protein C deficiency (Types I and II) impairs this feedback inhibition of coagulation amplification, allowing unchecked propagation of the coagulation cascade. Type II protein C deficiency (dysfunctional protein) may present with warfarin-induced skin necrosis due to rapid depletion of Protein C (which has a short 8-hour half-life) before depletion of vitamin K-dependent procoagulants (which have longer half-lives).

• Protein S deficiency: Protein S acts as a cofactor for activated protein C and as a direct inhibitor of Factor Xa and Factor IXa. Total Protein S consists of free Protein S (biologically active, ~40%) and C4b-binding protein-bound Protein S (inactive, ~60%). Deficiency impairs both the APC-mediated inactivation of Factor Va/VIIIa and direct factor Xa inhibition. Diagnosis is complicated by the fact that estrogen therapy and pregnancy increase C4b-binding protein levels, thereby reducing free Protein S levels without true deficiency.

Key Mechanism 4: Lipoprotein(a) Elevation

Key Mechanism 5: Lupus Anticoagulant and Antiphospholipid Antibodies (Acquired)

• Direct inhibition of protein C and protein S anticoagulant activity
• Tissue factor upregulation on endothelial cells and monocytes
• Platelet activation and aggregation through β2-glycoprotein I binding
• Impaired fibrinolysis through anti-tPA/plasminogen effects
• Endothelial cell activation and increased expression of adhesion molecules

Key Mechanism 6: Elevated Coagulation Factors (VIII, IX, X, XI)

Inherited (Primary) Thrombophilias

• Factor V Leiden (G1691A mutation): The most common inherited thrombophilia in Caucasians (5-7% prevalence), accounting for 50% of familial thrombosis cases. Presents with first VTE typically in second to third decade, though penetrance is incomplete (~10% of heterozygotes develop clinical thrombosis over lifetime). Particularly significant when combined with other risk factors (oral contraceptives, pregnancy, surgery).

• Prothrombin G20210A mutation: Occurs in 2-3% of Caucasians, accounts for 5-10% of familial thrombosis. Confers intermediate VTE risk between FVL and natural anticoagulant deficiencies. May present slightly later in life than FVL (third to fourth decade).

• Antithrombin deficiency: Rarest inherited thrombophilia (~1:2000-5000) but carries highest thrombotic burden with 50-60% lifetime VTE risk. Type I deficiency (reduced quantity and function) accounts for ~80% of cases; Type II deficiency (dysfunctional protein) has variable thrombotic risk depending on mutation. Clinical presentations often occur in late adolescence or early adulthood, sometimes precipitated by surgery, immobilization, or estrogen use.

• Protein C deficiency: Prevalence of ~1:200-400 in general population but only ~1:20,000 with symptomatic thrombosis. Heterozygous deficiency associated with 10-15% lifetime VTE risk. Homozygous or compound heterozygous deficiency causes neonatal purpura fulminans — a life-threatening condition presenting within 24-48 hours of birth with disseminated intravascular coagulation-like picture and microvascular thrombosis. Type II Protein C deficiency may paradoxically present with warfarin-induced skin necrosis due to the short Protein C half-life compared to longer-lived vitamin K-dependent procoagulants.

• Protein S deficiency: Prevalence of functional deficiency is ~1:500-700. Heterozygous deficiency associated with 10% VTE risk. Diagnosis challenging because pregnancy, estrogen use, and systemic lupus erythematosus all decrease free Protein S levels; repeat testing off estrogens and outside of acute thrombotic events is essential.

• Elevated Factor VIII: Occurs in approximately 10-15% of first VTE cases and 25% of patients with recurrent thrombosis. Factor VIII is an acute phase reactant, so levels must be repeated after resolution of acute thrombosis (at least 3 months later) to distinguish from transient elevation. Persistent Factor VIII >150 IU/dL in multiple testing occasions suggests genuine predisposition.

• Elevated Factor IX, X, XI: Less well-characterized inherited predispositions. Often detected as incidental findings during hypercoagulability workup; clinical significance less clear than Factor VIII elevation.

• Lipoprotein(a) elevation: Genetically determined with Lp(a) >50 mg/dL considered elevated. Accounts for 5-10% of VTE cases in some populations. Interaction with other thrombophilias (especially FVL or prothrombin mutation) may further increase risk.

• Fibrinogen abnormalities: Hereditary dysfibrinogenemia or hypofibrinogenemia can impair fibrinolysis and promote thrombosis in some families, though most dysfibrinogenemias are clinically silent.

• Methylenetetrahydrofolate reductase (MTHFR) deficiency: Causes elevated homocysteine through impaired folate metabolism. MTHFR polymorphisms are not independent thrombotic risk factors despite popular belief; however, true MTHFR deficiency causes severe hyperhomocysteinemia with thrombotic risk.

Acquired (Secondary) Thrombophilias

• Antiphospholipid syndrome (APS): Defined by presence of aPLs (lupus anticoagulant, anticardiolipin IgG/IgM >40 GPL/MPL units, or anti-β2-glycoprotein-I IgG/IgM >99th percentile) plus clinical thrombosis (venous or arterial) or pregnancy morbidity (≥3 unexplained consecutive first-trimester losses or ≥1 unexplained fetal death >10 weeks or premature birth <34 weeks due to placental insufficiency). Occurs in 1-2% of general population but in 2-3% of VTE patients. Most common cause of acquired thrombophilia.

• Malignancy: Solid tumors (particularly adenocarcinomas — lung, gastric, pancreatic, ovarian) and hematologic malignancies produce tissue factor-expressing microparticles, secrete cancer procoagulant, upregulate P-selectin, and induce inflammatory cytokines (IL-1, TNF-α). Cancer-associated VTE has 10-fold higher recurrence rate and requires indefinite anticoagulation (or until cancer is cured).

• Myeloproliferative neoplasms: Essential thrombocythemia (ET), polycythemia vera (PV), and primary myelofibrosis create acquired hypercoagulability through increased leukocyte-derived microparticles, elevated von Willebrand factor, platelet dysfunction (despite thrombocytosis), and JAK2/CALR/MPL mutations driving endothelial inflammation. Thrombotic risk in PV and ET is 20-25% over 10 years; Budd-Chiari syndrome (hepatic vein thrombosis) is a characteristic presentation in myeloproliferative disorders, particularly PV.

• Nephrotic syndrome: Loss of anticoagulant proteins (protein S, protein C, antithrombin, plasminogen) in urine combined with increased hepatic synthesis of procoagulant factors creates profound hypercoagulability. VTE risk in nephrotic syndrome is 20-25%, with membranous nephropathy carrying highest risk.

• Oral contraceptives and hormone replacement therapy: Estrogen increases hepatic synthesis of Factors II, VII, IX, X, fibrinogen, and von Willebrand factor while decreasing antithrombin, protein C, and protein S levels. Risk is dose-dependent (higher with older, higher-dose formulations) and increases 3-4 fold with combined oral contraceptives, approximately 2 fold with progestin-only methods. Risk further increases in presence of inherited thrombophilia, with FVL heterozygotes on OCPs having ~35-fold increased VTE risk compared to general population baseline. Third-generation progestins (desogestrel, gestodene) may carry higher thrombotic risk than second-generation (levonorgestrel).

• Pregnancy and postpartum period: Pregnancy induces physiologic changes that amplify coagulation (increased Factors II, VII, VIII, IX, X, XII; decreased anticoagulant activity) and impair fibrinolysis. VTE risk increases 5-fold in pregnancy and 10-20 fold in immediate postpartum period (highest within 2 weeks of delivery). Combined with inherited thrombophilia, risk is substantially magnified.

• Surgery and immobilization: Trauma initiates tissue factor release, induces endothelial injury, causes blood stasis, and triggers inflammatory response. Orthopedic surgeries (hip fracture repair, knee replacement) carry particularly high VTE risk (40-80% without prophylaxis). Immobilization from any cause (stroke, spinal cord injury, prolonged bed rest) impairs calf muscle pumping, causing venous stasis.

• Acute medical illness: Sepsis, acute myocardial infarction, acute stroke, and pneumonia trigger inflammatory cytokine release, increase tissue factor expression, and cause systemic activation of coagulation. Sepsis carries highest risk.

• Central venous catheters and pacemakers: Foreign body in vein causes mechanical endothelial injury and blood stasis; risk is catheter-duration dependent and increased with infection.

• Smoking: Increases Factors II, VII, VIII, IX, X; impairs fibrinolysis; activates platelets; and damages endothelium. Each pack-year increases VTE risk modestly.

• Obesity: Multiple mechanisms including adipokine-induced inflammation, increased Factors II, VII, VIII, IX, X; decreased anticoagulant protein levels; and impaired fibrinolysis.

• Hyperhomocysteinemia: Homocysteine >15 μmol/L increases VTE risk 2-3 fold through direct endothelial injury, impaired APC resistance, platelet activation,

Venous manifestations (most common)

• Deep vein thrombosis: unilateral calf/thigh swelling, warmth, erythema, and tenderness along the deep venous course; asymmetric calf circumference is more reliable than Homans sign. Obstruction of outflow with preserved arterial inflow produces the edema.
• Pulmonary embolism: acute pleuritic chest pain, dyspnea, tachycardia, hypoxemia; V/Q mismatch and shunt physiology explain the hypoxemia, while increased alveolar dead space explains the ventilatory inefficiency and hypocapnia; acute RV pressure overload explains the JVD and loud P2. Syncope or hypotension signals hemodynamically significant (massive) PE.
• Unusual-site thrombosis: cerebral venous sinus thrombosis (progressive headache, papilledema, seizure), Budd-Chiari (RUQ pain, tender hepatomegaly, ascites — think myeloproliferative neoplasm/PNH), portal or mesenteric vein thrombosis (pain out of proportion), and upper-extremity thrombosis around a central catheter.

Findings that name the specific disorder

• Antiphospholipid syndrome: livedo reticularis, arterial events (stroke or TIA in a young adult), recurrent fetal loss, mild thrombocytopenia, and an incidentally prolonged aPTT that does not correct on mixing study.
• Protein C deficiency: warfarin-induced skin necrosis — painful, sharply demarcated hemorrhagic bullae over breast, thigh, or buttock a few days after starting warfarin without heparin overlap.
• Homozygous protein C or S deficiency: neonatal purpura fulminans within the first days of life, with retiform purpura and DIC-like laboratory findings.
• Antithrombin deficiency: thrombosis plus apparent heparin resistance — the aPTT fails to rise despite escalating unfractionated heparin, because heparin works by potentiating antithrombin.

Step 1 — diagnose the clot, not the thrombophilia

• Risk stratify first: the Wells score for DVT and for PE, or the PERC rule to exclude PE in low-risk patients, per ACEP and ASH 2018 diagnostic guidance.
• D-dimer: high sensitivity, poor specificity — useful only to rule out in low/intermediate probability. Age-adjusted cutoffs are endorsed for patients over 50.
• Confirmatory imaging: compression ultrasonography with loss of vein compressibility for DVT; CT pulmonary angiography for PE (V/Q scan when contrast or radiation is contraindicated, as in pregnancy). Doppler or MR venography for cerebral sinus and abdominal venous thrombosis.

Step 2 — targeted thrombophilia testing, and only when it will change management

• Factor V Leiden: screen with a functional activated protein C resistance assay; confirm with PCR-based genotyping for the G1691A mutation, which is unaffected by anticoagulants or acute illness.
• Prothrombin G20210A: genotyping only — there is no functional assay.
• Antithrombin, protein C, protein S: functional (activity) assays first; if low, quantify antigen to separate type I (low quantity) from type II (dysfunctional).
• Antiphospholipid syndrome: requires persistent positivity of at least ONE of: lupus anticoagulant (a phospholipid-dependent clotting test such as dRVVT, with a mixing study failing to correct plus phospholipid-dependent confirmation), anticardiolipin IgG/IgM at medium-to-high titer, or anti-β2-glycoprotein I IgG/IgM — demonstrated on two occasions ≥12 weeks apart — plus a clinical criterion (thrombosis or defined pregnancy morbidity), per the revised Sapporo/Sydney criteria, now updated by the 2023 ACR/EULAR classification criteria. Triple positivity is a risk-stratifier that predicts recurrence and drives anticoagulant choice, not a diagnostic requirement.

Timing pitfalls that examiners love

• Acute thrombosis consumes antithrombin, protein C, and protein S — levels are falsely low.
• Warfarin lowers proteins C and S (vitamin K–dependent); heparin lowers antithrombin; DOACs falsely normalize APC resistance assays and cause false-positive lupus anticoagulant results.
• Estrogen and pregnancy raise C4b-binding protein and lower free protein S.
• Therefore defer functional testing until the patient is off anticoagulation, typically weeks after the acute event.

First-line anticoagulation (ASH 2020 and CHEST guidelines)

• Direct oral anticoagulants are preferred for most acute VTE: a factor Xa inhibitor (apixaban or rivaroxaban, both with an initial higher-dose lead-in) or *dabigatran*/*edoxaban* after 5 days of parenteral heparin.
• Low-molecular-weight heparin (enoxaparin) remains the parenteral agent of choice for initiation, for pregnancy, and historically for malignancy — though DOACs are now acceptable in cancer-associated VTE except with luminal GI or genitourinary tumors, where bleeding risk favors LMWH.
• Warfarin with heparin bridge for at least 5 days and until INR is therapeutic on two measurements.

Situation-specific escalation

• Antiphospholipid syndrome, particularly triple-positive or arterial events: vitamin K antagonist, not a DOAC — rivaroxaban performed worse in randomized comparison, and ISTH guidance advises against DOACs here.
• Antithrombin deficiency with heparin resistance: antithrombin concentrate, or transition to a direct thrombin inhibitor.
• Protein C deficiency starting warfarin: overlap with heparin and avoid loading doses to prevent skin necrosis; severe neonatal deficiency needs protein C concentrate or fresh frozen plasma.
• Pregnancy: LMWH throughout; ACOG recommends against warfarin and DOACs.

Complications of the disease

• Pulmonary embolism with RV failure: acute afterload rise dilates the RV, bows the septum leftward, and drops LV preload — hypotension, elevated troponin/BNP, and RV strain on echocardiography. Emergency.
• Post-thrombotic syndrome: valvular destruction and residual obstruction after DVT cause chronic venous hypertension — limb heaviness, edema, hyperpigmentation, and medial malleolar ulceration. Graduated compression may help symptoms.
• Chronic thromboembolic pulmonary hypertension: unresolved organized thrombus produces fixed vascular obstruction with progressive dyspnea and a mismatched V/Q scan; potentially curable by pulmonary thromboendarterectomy.
• Catastrophic antiphospholipid syndrome: multiorgan small-vessel thrombosis over days with renal failure, ARDS, and encephalopathy. Emergency — treated with anticoagulation, corticosteroids, plasma exchange, and/or IVIG.
• Cerebral venous sinus thrombosis and Budd-Chiari syndrome: venous outflow obstruction producing raised intracranial pressure or hepatic congestion with ascites; both are emergencies.
• Obstetric morbidity: placental thrombosis causing recurrent fetal loss, preeclampsia, and growth restriction.
• Neonatal purpura fulminans in homozygous protein C or S deficiency: retiform purpura with microvascular thrombosis, an emergency requiring factor replacement.

Complications of treatment

• Major hemorrhage, including intracranial bleeding — the dominant risk of indefinite anticoagulation. Reversal: vitamin K plus 4-factor PCC for warfarin, idarucizumab for dabigatran, andexanet alfa for factor Xa inhibitors.
• Warfarin-induced skin necrosis: protein C (half-life ~8 hours) falls before factors II and X, producing transient hypercoagulability and dermal microvascular thrombosis on days 3–5, classically over fatty tissue. Prevented by heparin overlap.
• Heparin-induced thrombocytopenia: PF4-heparin IgG antibodies activate platelets; platelet count falls ~50% at days 5–10 with thrombosis, not bleeding. Stop all heparin, start a non-heparin anticoagulant (argatroban), and do not start warfarin until platelets recover — doing so risks venous limb gangrene.
• Heparin-induced osteoporosis and hyperkalemia with prolonged unfractionated heparin.

• Factor V Leiden is the answer to "most common inherited thrombophilia" in a Caucasian patient; the mechanism is activated protein C resistance, and the confirmatory test is PCR genotyping for G1691A — unaffected by heparin, warfarin, or acute clot.
• Antithrombin deficiency = heparin resistance. A patient whose aPTT will not rise despite escalating unfractionated heparin has the highest-risk inherited thrombophilia; heparin's entire mechanism is potentiating antithrombin. Next step: antithrombin concentrate or a direct thrombin inhibitor.
• Warfarin-induced skin necrosis on the breast, buttock, or thigh days after starting warfarin = protein C deficiency. Protein C's short half-life creates a transient procoagulant window; prevention is heparin overlap, not avoidance of warfarin forever.
• Prolonged aPTT that fails to correct on mixing study, in a patient who clots rather than bleeds, is the lupus anticoagulant. Requires positivity on two occasions ≥12 weeks apart per the revised Sapporo/Sydney criteria to diagnose antiphospholipid syndrome.
• The one association examiners test: triple-positive or arterial APS is treated with warfarin, not a DOAC — the ISTH advises against factor Xa inhibitors here after randomized data showed excess thrombotic events.
• Young woman with DVT weeks after starting a combined oral contraceptive — the OCP is the trigger, the inherited defect is the substrate; the single best next step is diagnostic imaging plus anticoagulation, and estrogen-containing contraception is thereafter contraindicated.
• Common distractor — MTHFR. MTHFR polymorphism testing is not recommended and the variant is not an independent VTE risk factor; do not select it as the cause of thrombosis.
• Second common distractor — testing during the acute event or on anticoagulation. Acute thrombosis and warfarin lower protein C and S, heparin lowers antithrombin, and DOACs corrupt both APC-resistance and lupus anticoagulant assays. Defer functional testing.
• Unprovoked VTE plus Budd-Chiari or portal vein thrombosis should prompt evaluation for a myeloproliferative neoplasm (JAK2) or paroxysmal nocturnal hemoglobinuria.