Hemostasis

Primary hemostasis (platelet plug)

• Step 1, Adhesion: vascular injury exposes subendothelial collagen and von Willebrand factor (vWF). vWF binds collagen and platelet glycoprotein Ib (GP1b), so the platelet adheres.
• Step 2, Activation: agonists (ADP, collagen, thrombin, epinephrine, TXA2) bind platelet receptors → shape change, granule release.
• α-granules release: vWF, fibrinogen, factor V, PDGF, β-thromboglobulin, P-selectin (mediates leukocyte recruitment).
• Dense granules release: ADP, ATP, calcium, serotonin.
• Step 3, Aggregation: activated GPIIb/IIIa receptor binds fibrinogen, cross-linking adjacent platelets.

Secondary hemostasis (coagulation cascade)

• Extrinsic pathway (PT/INR): tissue factor (TF, exposed on subendothelium) plus factor VII → activates factor X.
• Intrinsic pathway (aPTT): factor XII → XI → IX → activates factor X (with cofactor VIII).
• Common pathway: factor X (with cofactor V) → prothrombin (II) → thrombin (IIa); thrombin cleaves fibrinogen (I) → fibrin (Ia); factor XIII cross-links fibrin.
• Cell-based model (modern): tissue factor on subendothelial cells → small thrombin burst (initiation) → activation of platelets and cofactors V, VIII, XI (amplification) → tenase and prothrombinase assembly on the activated platelet surface with a large thrombin burst (propagation).

Cascade logic and mnemonics

• Intrinsic (aPTT) pathway: XII → XI → IX, then IXa with cofactor VIIIa activates factor X. Mnemonic TENET (12, 11, 9, 8); VIII is a cofactor (activated by thrombin), not a sequential enzyme. Normal aPTT 25 to 35 sec.
• Extrinsic (PT) pathway: factor VII (the short pathway). Normal PT ~11 to 13 sec.
• Common pathway: factors X, V, II (prothrombin), I (fibrinogen). Mnemonic: the factors less than a $20 bill (10, 5, 2, 1). Factor X with cofactor V converts prothrombin (II) to thrombin (IIa); thrombin cleaves fibrinogen (I) to fibrin (Ia).
• Thrombin feedback: thrombin also activates FVIII (thrombin burst) and FXI.
• Clinical correlation: F8 deficiency = hemophilia A; F9 deficiency = hemophilia B; F11 deficiency = hemophilia C (minimal to mild bleeding, may use tranexamic acid around surgery). FXII, HMWK, and prekallikrein deficiencies prolong the aPTT but cause NO bleeding (John Hageman died of pulmonary embolus, not bleeding).

Coagulation factors, quick reference

• Vitamin K-dependent factors: II, VII, IX, X plus proteins C, S, Z (mnemonic: 1972 plus CSZ). Carboxylation of glutamate residues at the γ-position is needed for Ca2+ binding to phospholipid surfaces.
• Half-lives (relevant for warfarin and reversal): factor VII shortest (~6 hours), first to fall when warfarin is started. Protein C is also short, so early warfarin imbalance can cause skin necrosis (esp. in protein C deficiency). Factor II (prothrombin) longest (~60 hours).

Coagulation testing (PT, aPTT, TT, INR)

• PT: assesses the extrinsic and common pathways, normal ~11 to 13 sec. The "quaternary complex" requires patient plasma factors (enzymes), tissue factor (the cofactor absent from the aPTT, which is why aPTT is "partial"), phospholipid (negative charge and surface for complex assembly), and calcium.
• aPTT: assesses the intrinsic and common pathways, normal 25 to 35 sec. Requires patient plasma factors, a contact activator (not tissue factor), phospholipid, and calcium.
• TT (thrombin time): tests for fibrinogen deficiency or dysfunction, and detects thrombin-inhibiting drugs (dabigatran, argatroban, bivalirudin). More often a fibrinogen level is ordered directly (normal 200 to 400 mg/dL).
• INR: was standardized for warfarin monitoring and is also used in prognostic scores (MELD, DIC/SIC). It does not predict procedural bleeding in cirrhosis and does not measure DOAC activity, so a normal INR does not exclude anticoagulant effect.

Mixing study

• Patient plasma is mixed 50:50 with normal plasma.
• Corrects (normalizes) = factor deficiency (the added normal plasma supplies the missing factor).
• Does not correct = inhibitor (heparin, autoimmune factor inhibitor, lupus anticoagulant/APLS) or drug.
• A true correction should normalize completely (≤ 35 sec); in real-world labs a value of 35 plus 4 sec may still be called corrected.
• Report PT/aPTT at time 0 (immediately after mixing) and again after 1 to 2 hr incubation at 37°. Different patterns of correction at different times help differentiate inhibitor types.
• Corrects (or slightly corrects) at time 0 then re-prolongs after 1 to 2 hr incubation at 37° = characteristic of a time-dependent FVIII inhibitor (acquired hemophilia A, or an alloantibody in congenital hemophilia A); confirm with FVIII activity and a Bethesda inhibitor assay.

Factor inhibitors

• Specific factor inhibitors: the majority are directed against FVIII. Seen in congenital hemophilia A after exposure to replacement factor products, or spontaneously (mostly in the elderly, that is, acquired hemophilia). Rare spontaneous inhibitors can form against factors II, V, VII, IX, X.
• Non-specific factor inhibitors: mostly lupus anticoagulants, named "anticoagulant" because they prolong the aPTT in vitro. Present in 2 to 4% of the general population and 15 to 30% of patients with SLE. Usually a lab artifact when transient or isolated, but persistent lupus anticoagulant is a high-risk antiphospholipid profile associated with first thrombosis and obstetric morbidity, even before an APS-defining event.

Isolated prolongation differential

Anticoagulant drugs and their factor targets

Natural anticoagulant systems

• Antithrombin (AT, formerly AT-III): serine protease inhibitor. Inhibits thrombin (IIa) and factor Xa primarily; also IXa, XIa, XIIa. Heparin (UFH/LMWH/fondaparinux) and endogenous heparan sulfate accelerate AT activity 1000-fold.
• Protein C / Protein S system: thrombin binds endothelial thrombomodulin → activates protein C. Activated protein C (APC), with protein S as cofactor, cleaves factors Va and VIIIa, inactivating them.
• Tissue factor pathway inhibitor (TFPI): inhibits the TF, factor VIIa, factor Xa complex.
• Plasmin (fibrinolytic system): from plasminogen via tissue plasminogen activator (tPA, from endothelium) or urokinase. Plasmin degrades fibrin and fibrinogen → fibrin(ogen) degradation products; only FXIIIa-cross-linked fibrin breakdown yields D-dimer (a marker of cross-linked fibrin turnover, not specific for intravascular thrombosis).
• Plasmin inhibitors: α2-antiplasmin (rapid inactivator), PAI-1 (inhibits tPA/uPA).

Endothelial regulators

• Healthy endothelium is anticoagulant: thrombomodulin (activates protein C), heparan sulfate (potentiates AT), TFPI, prostacyclin (PGI2, inhibits platelets), nitric oxide (vasodilator and platelet inhibitor), tPA (fibrinolysis).
• Injured endothelium / vWF release: from Weibel-Palade bodies on activation. vWF circulates as ultra-large multimers cleaved by ADAMTS13 (a disintegrin and metalloproteinase with thrombospondin type 1 motif, member 13). ADAMTS13 deficiency → uncleaved ultra-large vWF → spontaneous platelet thrombi → TTP.

High yield

• Vit K-dependent factors: II, VII, IX, X plus protein C, S, Z. Both pro- and anti-coagulant factors are vitamin K-dependent, which explains the warfarin transition.
• Factor VIII is NOT made by the liver alone: endothelium is a major source, therefore factor VIII can be normal or elevated even in advanced liver disease.
• Factor XII deficiency: prolonged aPTT but NO bleeding (Mr. Hageman, the original patient, died of pulmonary embolus, not bleeding).
• Factor XIII deficiency: delayed umbilical bleeding in newborns; normal PT/aPTT; quantitative FXIII activity assay is the recommended initial test (urea clot solubility is insensitive, abnormal only in severe deficiency, and a normal result does not exclude it).
• Liver disease: all factors except VIII. PT prolongs first (factor VII shortest half-life).
• DIC labs: ↓ fibrinogen, ↓ platelets, ↑ D-dimer, ↑ PT/aPTT, schistocytes, ↑ FDPs.
• Vitamin K deficiency → prolonged PT (and aPTT in severe cases). Reverses with vitamin K (PO or IV); 4F-PCC (with concurrent vitamin K) for major or life-threatening bleeding (off-label in nutritional deficiency; FDA indication is VKA reversal).
• Dilutional coagulopathy (massive transfusion): fibrinogen falls to a critical level first (before platelets and other factors, including labile V and VIII); replace with FFP, cryoprecipitate or fibrinogen concentrate, and platelets.