Hemostasis — Primary and Secondary

Primary Hemostasis: Platelet Plug Formation

Platelet adhesion and activation

• Upon vascular endothelial injury, subendothelial collagen and von Willebrand factor (vWF) become exposed
• vWF binds to the GPIb-IX-V complex on platelet surface, initiating platelet adhesion and shape change from biconcave disc to spherical form
• Thrombin (factor IIa) and ADP released from activated platelets and damaged tissue further activate platelets via G-protein coupled receptors (PAR-1, PAR-4 for thrombin; P2Y12 for ADP)
• This triggers conformational change in integrin αIIbβ3, enhancing adhesion and promoting platelet aggregation via fibrinogen bridging
• Activated platelets undergo granule secretion: dense granules release ADP and calcium; alpha granules release fibrinogen, vWF, and P-selectin (CD62P)

Platelet plug stabilization

• Phosphatidylserine (PS) externalization on platelet membrane surface creates negatively charged microenvironment essential for coagulation factor assembly
• Morphologically, platelets lose their discoid shape, develop pseudopodia, and fuse into cohesive hemostatic plug visible on scanning electron microscopy

Secondary Hemostasis: Coagulation Cascade

Tissue Factor (TF) Pathway — Initiation Phase

• Vascular injury exposes Tissue Factor (TF, Factor III), a transmembrane glycoprotein normally sequestered in extravascular space
• TF binds Factor VII (only vitamin K-dependent factor activated by TF), forming the TF-Factor VIIa complex
• This complex activates Factor X → Factor Xa (the tenase complex)
• Initial Factor IIa (thrombin) generation is limited; thrombin further amplifies the cascade

Contact Activation Pathway — Historical Relevance

• Factor XII (Hageman factor), prekallikrein, and high-molecular-weight kininogen (HMWK) form contact factor complex
• Though important in vitro (prolongs aPTT), this pathway is not critical for hemostasis in vivo (Factor XII deficiency does not cause bleeding)

Common Pathway — Amplification/Propagation Phase

• Thrombin-mediated feedback amplification: generated thrombin activates Factors V and VIII (cofactors), as well as Factor XI (intrinsic tenase amplification)
• Intrinsic Tenase Complex: Factor IXa + Factor VIIIa + Factor X → massive Factor Xa generation on platelet surface
• Prothrombinase Complex: Factor Xa + Factor Va + prothrombin (Factor II) → thrombin (Factor IIa) burst production
• Fibrin formation: thrombin cleaves fibrinogen (Factor I) into fibrin monomers, which polymerize into loose network
• Factor XIII (fibrin-stabilizing factor) is activated by thrombin → cross-links fibrin polymers, creating mechanically stable clot resistant to fibrinolysis

Vitamin K-Dependent Factors (II, VII, IX, X) — Carboxylation

• Require γ-carboxylation of glutamic acid residues in the Gla domain by vitamin K-dependent carboxylase
• γ-carboxyl groups chelate calcium ions, enabling proper binding to phospholipid surfaces
• Essential for coagulation factor assembly on platelet membranes

Anticoagulant Mechanisms (Normal Regulation)

• Thrombomodulin on endothelial cells activates Protein C, which (with Protein S as cofactor) inactivates Factors Va and VIIIa, limiting thrombin generation
• Tissue Factor Pathway Inhibitor (TFPI) blocks TF-Factor VIIa-Factor Xa complex
• Antithrombin III inactivates thrombin, Factor Xa, and Factor IXa (activity enhanced 1000-fold by heparin)

Tertiary Hemostasis: Fibrinolysis

• Tissue plasminogen activator (tPA) and urokinase plasminogen activator (uPA) activate plasminogen → plasmin
• Plasmin degrades fibrin into fibrin degradation products (FDPs) and D-dimers (cross-linked FDPs)
• Balanced by plasminogen activator inhibitor-1 (PAI-1) and α2-antiplasmin, which prevent excessive clot dissolution
• Thrombin-activatable fibrinolysis inhibitor (TAFI) removes lysine residues from fibrin, reducing plasminogen binding and slowing fibrinolysis

Bleeding Diatheses (Disorders of Hemostasis)

Primary Hemostasis Disorders

• Platelet abnormalities: quantitative (thrombocytopenia from bone marrow failure, immune destruction, consumptive coagulopathy, sequestration) or qualitative (storage pool disease, Glanzmann thrombasthenia—absent GPIIb/IIIa)
• von Willebrand disease (vWD): quantitative deficiency (Type 1, 3) or qualitative defect (Type 2) of vWF protein; most common inherited bleeding disorder (~1% population)
• Aspirin and P2Y12 inhibitors: irreversibly inhibit platelet activation
• Uremia: impaired platelet function despite normal count
• Endothelial dysfunction: scurvy (vitamin C deficiency), hereditary hemorrhagic telangiectasia (HHT)

Secondary Hemostasis Disorders

• Factor VIII deficiency: Hemophilia A (X-linked recessive, ~1:5,000 males); ~80% due to intron 22 inversion
• Factor IX deficiency: Hemophilia B (Christmas disease) (X-linked recessive, ~1:30,000 males)
• Factor II, V, VII, X deficiency: rare autosomal recessive conditions
• Vitamin K deficiency: impairs carboxylation of Factors II, VII, IX, X (dietary insufficiency, antibiotics, malabsorption, liver disease)
• Fibrinogen disorders: quantitative (afibrinogenemia—autosomal recessive) or qualitative (dysfibrinogenemia)
• Factor XIII deficiency: rare; presents with delayed umbilical bleeding days after birth (unique clinical marker)

• Liver disease: synthesizes all coagulation factors except vWF; additionally has portal hypertension-related thrombocytopenia and impaired clearance of fibrin degradation products
• Disseminated intravascular coagulation (DIC): consumptive coagulopathy from sepsis, malignancy, obstetric emergencies (amniotic fluid embolism, placental abruption, HELLP syndrome); consumes all coagulation factors and platelets
• Massive transfusion/dilutional coagulopathy: excessive volume replacement without coagulation factor replacement
• Anticoagulation: warfarin (inhibits Factors II, VII, IX, X), heparin (enhances antithrombin), direct oral anticoagulants (DOACs)
• Inhibitor formation: anti-Factor VIII antibodies in ~20% of hemophilia A patients (alloimmunization), especially in severe disease
• Lupus anticoagulant: paradoxical prolonged aPTT in vitro but thrombophilic in vivo (due to antibodies against phospholipid-binding proteins like β2-glycoprotein I)

Thrombophilic States (Pathologic Clot Formation)

Inherited Thrombophilias

• Factor V Leiden mutation (R506Q): most common heritable thrombophilia (~5-7% population); resistance to activated Protein C, leading to unopposed Factor Va activity
• Prothrombin G20210A mutation: elevated Factor II levels; second most common heritable thrombophilia
• Antithrombin III deficiency: rare (1:2,000-5,000); significantly increases VTE risk
• Protein C and Protein S deficiency: impaired negative feedback inhibition of Factors Va and VIIIa
• Lipoprotein(a) elevation: interferes with plasminogen binding and fibrinolysis

Acquired Thrombophilias

• Antiphospholipid syndrome (APS): IgG/IgM antibodies against phospholipid-binding proteins; causes thrombosis + recurrent pregnancy loss (most common acquired thrombophilia, especially among younger patients with arterial/venous thrombosis)
• Cancer/malignancy: tissue factor expression, microparticle shedding, endothelial damage; hypercoagulable state
• Immobilization, surgery, trauma: endothelial injury, venous stasis
• Oral contraceptives and hormone replacement therapy: increase Factors II, VII, VIII, X and decrease Antithrombin III and Protein S
• Pregnancy and postpartum period: increase Factors I, II, VII, VIII, X, XII; decrease Protein S; venous stasis
• Smoking: endothelial injury, platelet activation
• Obesity: chronic inflammation, Factor XI elevation
• Polycythemia vera, essential thrombocythemia: elevated RBC/platelet production causing hyperviscosity and thrombosis
• Nephrotic syndrome: loss of Protein S and antithrombin in urine
• Heparin-induced thrombocytopenia (HIT): IgG antibodies against heparin-platelet factor 4 (PF4) complex; causes paradoxical thrombocytopenia + thrombosis

Bleeding Diatheses

Cardinal Features of Primary Hemostasis Disorders

• Mucocutaneous bleeding: petechiae, purpura (characteristically non-blanching), epistaxis, menorrhagia in women, bleeding from gums
• Immediate bleeding after injury (platelet plug formation occurs within seconds to minutes)
• Absence of deep hematomas and joint bleeding (hemarthrosis)—these suggest secondary hemostasis defects
• Petechiae are pathognomonic for thrombocytopenia or qualitative platelet defects; appear as 1-3 mm purple non-blanching macules in dependent areas and mucous membranes; represent capillary bleeds due to inadequate platelet plugs

Cardinal Features of Secondary Hemostasis Disorders

• Delayed bleeding (hours to days after injury)—coagulation cascade requires time for amplification
• Deep tissue hematomas: firm, fluctuant masses in muscles and soft tissues (distinguish by location from superficial petechiae)
• Hemarthrosis: bleeding into joints (especially knees, ankles, elbows in hemophilia); causes joint pain, swelling, erythema, and limitation of motion; chronic hemarthrosis leads to hemophilic arthropathy with cartilage erosion and synovial hypertrophy
• Gastrointestinal bleeding: melena or hematemesis
• Intracranial hemorrhage (ICH): spontaneous in severe deficiency; represents life-threatening complication
• Retroperitoneal hematoma: presents with abdominal pain, ileus, anemia; risk of compartment syndrome
• Post-operative bleeding: may be delayed 24-48 hours

Special Clinical Features

• Severity classification by Factor level:
• Severe: <1% (spontaneous bleeding, prolonged bleeding with minor trauma)
• Moderate: 1-5% (bleeding with minor trauma)
• Mild: 5-40% (bleeding with major trauma or surgery)
• Target joint phenomenon: repetitive hemarthrosis in same joint despite prophylaxis
• Inhibitor formation: development of anti-Factor VIII/IX alloantibodies manifests as inadequate response to factor infusion; occurs in ~20% hemophilia A, ~5% hemophilia B

• Type 1 (partial quantitative, ~75% cases): mild mucocutaneous bleeding, may worsen with stress/medications (decreased vWF release)
• Type 2 (qualitative defect): disproportionate prolongation of bleeding time relative to vWF antigen level
• Type 3 (complete deficiency, rare): severe bleeding similar to hemophilia A; presents in infancy with hemarthrosis and severe mucocutaneous bleeding

• Acute presentation (minutes to hours): bleeding from multiple sites (spontaneous petechiae, oozing from wounds, mucous membranes, GI tract); accompanied by thrombosis (acrocyanosis, digital necrosis, renal cortical necrosis)
• Microangiopathic hemolytic anemia (MAHA): schistocytes on blood smear from mechanical destruction of RBCs by fibrin strands in microvasculature
• Organ dysfunction: acute kidney injury (fibrin deposition), respiratory distress, altered mental status

• Pathognomonic delayed umbilical bleeding: bleeding from umbilical stump days after birth (while other newborns stop bleeding within hours)—unique feature distinguishing from other factor deficiencies

Thrombophilic States

Venous Thromboembolism (VTE)

• Deep vein thrombosis (DVT): unilateral calf/leg swelling, pain, warmth, erythema; Homan's sign and calf tenderness
• Pulmonary embolism (PE): chest pain, dyspnea, tachycardia, hypoxemia, syncope in massive PE; signs of right heart strain (elevated JVD, RV heave)
• Thrombosis in unusual sites: mesenteric vein, portal vein (Budd-Chiari syndrome), cerebral sinuses (cavernous sinus thrombosis)

Arterial Thrombosis

• Myocardial infarction, stroke, limb ischemia: in young patients without traditional atherosclerotic risk factors
• Recurrent thrombosis despite anticoagulation suggests anti-Factor Xa activity in antiphospholipid syndrome

Antiphospholipid Syndrome

• Obstetric manifestations: recurrent pregnancy loss (typically 2nd/3rd trimester), placental insufficiency, preeclampsia
• Thrombotic complications: DVT, PE, stroke, myocardial infarction; can occur spontaneously or with minor precipitants
• Catastrophic APS (1-5% APS patients): multi-organ thrombosis within days, high mortality

Heparin-Induced Thrombocytopenia (HIT)

• Thrombocytopenia: platelet count drop >50% from baseline 5-14 days after heparin initiation (or earlier if prior heparin exposure)

Initial screening panel (order these first)

• CBC with platelet count and peripheral smear: separates quantitative from qualitative platelet defects; look for schistocytes (DIC, TTP/HUS), giant platelets (Bernard–Soulier), or clumping (pseudothrombocytopenia from EDTA).
• PT/INR: assesses the tissue factor–VIIa pathway plus common pathway; isolated prolongation points to factor VII deficiency, early warfarin effect, or early liver disease.
• aPTT: assesses contact/intrinsic plus common pathway; isolated prolongation suggests hemophilia A/B, factor XI or XII deficiency, heparin, or a lupus anticoagulant.
• Both prolonged: common pathway (X, V, II, fibrinogen), severe liver disease, vitamin K deficiency, DIC, or supratherapeutic anticoagulation.
• Bleeding time is obsolete in U.S. practice; platelet function analyzers or light transmission aggregometry have replaced it.

Confirmatory testing by pattern

• Mixing study: 1:1 mix with normal plasma. Correction = factor deficiency; failure to correct = inhibitor (acquired factor VIII antibody, lupus anticoagulant). Lupus anticoagulant is confirmed when the prolongation corrects with excess phospholipid.
• Specific factor activity assays: define hemophilia severity (severe <1%, moderate 1–5%, mild 5–40% of normal activity).
• von Willebrand panel: vWF antigen, vWF platelet-binding/ristocetin cofactor activity, factor VIII activity, then multimer analysis and ristocetin-induced platelet aggregation to subtype — the sequence endorsed by the 2021 ASH/ISTH/NHF/WFH von Willebrand disease guideline.
• Factor XIII deficiency: normal PT and aPTT; diagnosed by urea clot solubility screen and confirmed with factor XIII activity.

Named scoring systems

• ISTH DIC score: platelet count, PT prolongation, fibrin-related marker (D-dimer), and fibrinogen; a score at or above the ISTH threshold defines overt DIC. Falling fibrinogen and rising D-dimer with consumption of all factors is the signature.
• 4Ts score (ASH 2018 HIT guideline): thrombocytopenia magnitude, timing (5–14 days), thrombosis, other causes. Low probability effectively excludes HIT; intermediate/high triggers anti-PF4/heparin immunoassay, confirmed by functional serotonin release assay.
• Wells criteria plus D-dimer for suspected DVT/PE, with compression ultrasound or CT pulmonary angiography as the confirmatory study.
• Revised Sapporo (Sydney) criteria for antiphospholipid syndrome: clinical thrombosis or defined pregnancy morbidity plus persistent antibody positivity (lupus anticoagulant, anticardiolipin, or anti-β2-glycoprotein I) on two occasions at least 12 weeks apart.

Immediate stabilization in active hemorrhage

• Airway, hemodynamics, and source control first; type and screen, and activate massive transfusion protocol for uncontrolled bleeding.
• Factor replacement before imaging in a hemophiliac with suspected intracranial or airway bleed — the World Federation of Hemophilia advises treating on clinical suspicion, not waiting for the CT.

Bleeding disorders — first-line therapy

• Recombinant factor concentrates: factor VIII for hemophilia A, factor IX for hemophilia B; prophylactic dosing rather than on-demand is the WFH standard of care.
• Vasopressin analogue (desmopressin/DDAVP): releases stored vWF and factor VIII from endothelial Weibel–Palade bodies; used for mild hemophilia A, type 1 vWD, and uremic platelet dysfunction. Give a test dose first.
• Antifibrinolytics (tranexamic acid, aminocaproic acid): adjuncts for mucosal, dental, and menstrual bleeding.
• vWF-containing concentrate for type 2 and type 3 vWD, per the 2021 ASH/ISTH/NHF/WFH management guideline.
• Corticosteroids (prednisone or dexamethasone) first line for ITP, with IVIG added when bleeding is severe and a rapid platelet rise is needed (ASH 2019 ITP guideline).

Escalation and second-line options

• Bispecific antibody (emicizumab): bridges factor IXa and X, mimicking factor VIIIa — prophylaxis for hemophilia A with or without inhibitors.
• Bypassing agents: recombinant factor VIIa or activated prothrombin complex concentrate for patients with inhibitors; immune tolerance induction for eradication.
• TPO receptor agonists (eltrombopag, romiplostim), rituximab, or splenectomy for steroid-refractory ITP.
• Anticoagulant reversal: vitamin K plus 4-factor PCC for warfarin, protamine for heparin, idarucizumab for dabigatran, andexanet alfa for factor Xa inhibitors.

DIC and thrombophilia

• Treat the precipitant (sepsis, obstetric emergency, malignancy); transfuse platelets, plasma, and cryoprecipitate only for bleeding or planned procedures, not for numbers alone.
• DOACs are preferred for most VTE (ASH 2020), but vitamin K antagonist warfarin is preferred over DOACs in triple-positive antiphospholipid syndrome.

Contraindicated

• Desmopressin in type 2B vWD — worsens thrombocytopenia; also avoid in young children and monitor sodium.
• Warfarin before platelet recovery in HIT — precipitates venous limb gangrene; all heparin must stop and a non-heparin agent (argatroban, bivalirudin, fondaparinux) started.
• Warfarin in pregnancy — teratogenic; use LMWH.

Complications of the underlying disorder

• Intracranial hemorrhage: leading cause of bleeding death in severe hemophilia and in profound thrombocytopenia; headache, vomiting, or altered mental status in a known bleeder is a true emergency — replace factor immediately, then image.
• Hemophilic arthropathy: repeated hemarthrosis drives iron-induced synovitis, synovial hypertrophy, and cartilage erosion, producing a fixed, contracted target joint.
• Compartment syndrome and Volkmann contracture: intramuscular bleeding (iliopsoas, forearm) raises compartment pressure; pain out of proportion, paresthesias, and a tense compartment signal an emergency.
• Airway obstruction from retropharyngeal or sublingual hematoma — emergency.
• Inhibitor development: alloantibodies against infused factor VIII/IX; the signal is failure of bleeding to respond to an adequate factor dose plus a mixing study that fails to correct.
• Purpura fulminans and digital/renal cortical necrosis in DIC: widespread microvascular fibrin deposition; retiform purpura with acral gangrene is the visible marker. Emergency.
• Catastrophic antiphospholipid syndrome: multiorgan small-vessel thrombosis over days, with high mortality. Emergency.
• HIT-associated thrombosis: PF4–heparin immune complexes activate platelets via FcγRIIa; the paradox is thrombosis despite a falling platelet count.

Complications of treatment

• Desmopressin-induced hyponatremia and seizures: V2-mediated free water retention, particularly with repeated dosing or unrestricted fluids in children.
• Warfarin-induced skin necrosis: protein C has a short half-life, so early warfarin creates transient hypercoagulability; dermal vessel thrombosis appears over fatty areas days after initiation, classically in protein C or S deficiency. Emergency.
• Venous limb gangrene if warfarin is started during active HIT.
• Heparin-related osteoporosis and hyperkalemia with prolonged unfractionated heparin (aldosterone suppression).
• Thrombotic risk with bypassing agents: activated PCC combined with emicizumab has caused thrombosis and thrombotic microangiopathy.
• Transfusion reactions: TRALI (hypoxemia and bilateral infiltrates within hours, non-cardiogenic) and TACO (volume overload); citrate-induced hypocalcemia in massive transfusion.
• Anaphylaxis and nephrotic syndrome with factor IX exposure in hemophilia B inhibitor patients.

• Pattern recognition beats memorizing factors: mucocutaneous bleeding, petechiae, immediate oozing = primary hemostasis; hemarthrosis, deep muscle hematoma, delayed rebleeding = secondary hemostasis. Stems are built on this split.
• The single best next step for an unexplained prolonged aPTT is a mixing study. Correction means deficiency; failure to correct means an inhibitor — acquired factor VIII autoantibody in an elderly patient or postpartum woman, or a lupus anticoagulant that paradoxically causes thrombosis, not bleeding.
• Factor XII deficiency: strikingly prolonged aPTT with no bleeding whatsoever — the classic distractor. The contact pathway matters in the test tube, not in vivo.
• von Willebrand disease: prolonged aPTT (vWF carries factor VIII) with a normal platelet count; confirm with reduced ristocetin cofactor activity. Desmopressin works in type 1 and is contraindicated in type 2B.
• Factor XIII deficiency: normal PT, normal aPTT, delayed umbilical stump bleeding, and a positive urea clot solubility test — the only screen-negative bleeding disorder examiners routinely use.
• DIC versus TTP: DIC has prolonged PT/aPTT, low fibrinogen, and high D-dimer; TTP has normal coagulation times with schistocytes, thrombocytopenia, and low ADAMTS13. Giving platelets in TTP is the trap.
• Warfarin prolongs PT first because factor VII has the shortest half-life; the transient hypercoagulable window reflects the equally short half-life of protein C and explains warfarin skin necrosis.
• HIT: platelets fall >50% around days 5–14 of heparin with new thrombosis. Stop all heparin (including flushes), start argatroban or fondaparinux, and do not start warfarin or transfuse platelets — per the ASH 2018 HIT guideline.
• Aspirin irreversibly acetylates COX-1 for the platelet's lifespan; uremia causes a qualitative defect with a normal count, and both are corrected functionally by desmopressin.