11.2 Blood Typing (ABO/Rh) & Hemostasis

Blood Typing (ABO/Rh) & Hemostasis

The maintenance of cardiovascular integrity requires two critical physiological mechanisms: immunological self-recognition of circulating blood cells (blood typing) to prevent autoimmune destruction, and rapid, localized arrest of hemorrhage following mechanical vascular injury (hemostasis). Failure of blood compatibility mechanisms precipitates catastrophic intravascular hemolysis, whereas dysregulation of hemostasis leads either to fatal hemorrhage or pathological intravascular thrombosis.

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The ABO Blood Group System

The human erythrocyte plasma membrane contains genetically determined glycoproteins and glycolipids that project into the extracellular space. These surface markers function as antigens (specifically termed agglutinogens because they promote cell clumping). When foreign antigens are introduced into a recipient's circulation, the recipient's immune system recognizes them as non-self, triggering an immune attack.

Molecular Genetics and Antigens

The ABO blood group is determined by a single polymorphic gene locus on chromosome 9 that encodes specific glycosyltransferase enzymes. These enzymes add specific terminal sugar residues to a baseline carbohydrate precursor chain on the erythrocyte membrane (termed the H antigen):

• $I^A$ Allele: Encodes an enzyme that attaches N-acetylgalactosamine to the H antigen, creating Antigen A.
• $I^B$ Allele: Encodes an enzyme that attaches D-galactose to the H antigen, creating Antigen B.
• $i$ Allele: A non-functional (null) allele that produces no active transferase, leaving the baseline H antigen unmodified (Type O).
• Codominance: The $I^A$ and $I^B$ alleles are codominant with respect to each other, and both are completely dominant over the recessive $i$ allele.

Pre-Formed Plasma Antibodies (Agglutinins)

A defining, unique feature of the ABO blood system is the spontaneous presence of pre-formed antibodies (termed agglutinins) in blood plasma. Unlike typical adaptive immune responses that require direct exposure to a foreign human red blood cell, ABO agglutinins appear spontaneously in an infant's plasma within the first 2 to 6 months of life. These antibodies are produced in response to environmental food antigens and harmless gut bacteria that display carbohydrate structures nearly identical to A and B agglutinogens.

A fundamental immunological rule dictates: An individual's plasma naturally contains pre-formed antibodies against whatever ABO agglutinogens are ABSENT from their own red blood cells.

1. Type A Blood:
   • Genotypes: $I^A I^A$ (homozygous) or $I^A i$ (heterozygous).
   • Erythrocyte Surface Antigens: Antigen A.
   • Plasma Antibodies: Anti-B antibodies.
2. Type B Blood:
   • Genotypes: $I^B I^B$ (homozygous) or $I^B i$ (heterozygous).
   • Erythrocyte Surface Antigens: Antigen B.
   • Plasma Antibodies: Anti-A antibodies.
3. Type AB Blood:
   • Genotype: $I^A I^B$ (heterozygous codominant).
   • Erythrocyte Surface Antigens: Both Antigen A and Antigen B.
   • Plasma Antibodies: Neither Anti-A nor Anti-B antibodies.
4. Type O Blood:
   • Genotype: $ii$ (homozygous recessive).
   • Erythrocyte Surface Antigens: Neither Antigen A nor Antigen B (possesses only unmodified H antigen).
   • Plasma Antibodies: Both Anti-A and Anti-B antibodies.

Transfusion Compatibility: Universal Donor and Universal Recipient

In clinical transfusion medicine, the primary danger arises when recipient antibodies attack transfused donor red blood cells. (Because whole blood is routinely separated into packed red blood cells [PRBCs], the volume of donor plasma administered is minimal and diluted within the recipient's circulation, making the donor's antibodies a secondary concern compared to the recipient's circulating antibodies):

• Universal Donor (Type O Negative Packed RBCs):
  • Type O erythrocytes lack both A and B surface antigens. When Type O packed red cells are transfused into a Type A, B, AB, or O recipient, the recipient's anti-A or anti-B antibodies find no target antigens on the donor RBCs. Therefore, Type O packed red blood cells can be safely administered to any ABO blood type in emergency situations where typing and cross-matching cannot be completed in time.
• Universal Recipient (Type AB Positive):
  • Type AB individuals possess both A and B surface antigens on their erythrocytes, and consequently their plasma contains neither anti-A nor anti-B antibodies. Because their plasma lacks agglutinins to attack incoming donor red blood cells, Type AB patients can safely receive packed red blood cells of any ABO type (A, B, AB, or O).

Acute Hemolytic Transfusion Reactions

If a patient receives an incompatible blood transfusion (for example, transfusing Type A red blood cells into a Type B recipient who possesses circulating anti-A antibodies):

1. Agglutination (Clumping): Recipient IgM agglutinins cross-link the foreign antigens on adjacent donor erythrocytes, binding them into microscopic cellular clumps.
2. Capillary Occlusion: These agglutinated RBC clumps lodge in and obstruct microvascular capillary beds throughout the body, cutting off local tissue perfusion and triggering severe ischemic pain, particularly in the chest, abdomen, and lumbar spine.
3. Complement-Mediated Hemolysis: Bound antibodies activate the classical complement cascade, producing membrane attack complexes (C5b-9) that rupture donor erythrocyte membranes (intravascular hemolysis).
4. Hemoglobinemia and Renal Failure: The massive destruction of donor RBCs releases free hemoglobin into the bloodstream (hemoglobinemia). When free hemoglobin concentrations exceed the binding capacity of the plasma protein haptoglobin, free hemoglobin is filtered across the renal glomeruli into the renal tubules (hemoglobinuria). In the acidic environment of the distal nephron, free hemoglobin precipitates into dense intratubular casts, inducing severe tubular obstruction, acute tubular necrosis, and life-threatening acute renal failure.

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The Rh Blood Group System

The Rh blood group system is named after the Rhesus macaque monkey, in which the antigens were first identified. The Rh system is governed by a complex of genes encoding over 50 distinct antigens, among which five are clinically prominent: C, c, D, E, and e.

The D Antigen and Rh Status

Among all Rh antigens, Antigen D is by far the most potent immunogen. In clinical practice, an individual's "Rh status" refers specifically to the presence or absence of the D antigen on their erythrocyte membranes:

• Rh-Positive ($Rh^+$): Individuals whose red blood cells express Antigen D (roughly 85% of the North American population).
• Rh-Negative ($Rh^-$): Individuals whose red blood cells lack Antigen D (roughly 15% of the population).

Critical Difference: Rh Antibodies Are NOT Pre-Formed

A critical distinction between the ABO and Rh systems must be emphasized for anatomy examinations:

• Unlike ABO blood grouping, anti-Rh antibodies are NOT naturally pre-formed in human blood plasma.
• An $Rh^-$ individual is born with zero anti-Rh antibodies in their bloodstream.
• An $Rh^-$ person synthesizes anti-Rh antibodies only if they are directly exposed to $Rh^+$ red blood cells. This initial exposure is termed sensitization (alloimmunization), and can occur through an inadvertent transfusion of $Rh^+$ blood or through feto-maternal hemorrhage during pregnancy.

Hemolytic Disease of the Newborn (Erythroblastosis Fetalis)

Hemolytic Disease of the Newborn (HDN) is a severe alloimmune disorder that occurs when an $Rh^-$ mother carries an $Rh^+$ fetus (having inherited the paternal Rh gene):

1. First Pregnancy (Sensitization Phase):

   • Throughout normal gestation, maternal and fetal blood supplies are physically separated by the placental syncytiotrophoblast barrier, preventing cellular mixing.
   • During labor, delivery, or placental detachment, placental micro-vessels tear, allowing fetal $Rh^+$ erythrocytes to cross into the maternal pelvic circulation (feto-maternal hemorrhage).
   • The mother's immune system recognizes the foreign fetal D antigen and mounts an active primary immune response. Over the following weeks to months, maternal B-lymphocytes differentiate into plasma cells that synthesize anti-Rh antibodies and establish persistent immunological memory.
   • Because sensitization typically finishes weeks after delivery, the firstborn $Rh^+$ infant is delivered completely healthy and unaffected.

2. Subsequent Pregnancy with an $Rh^+$ Fetus (Hemolytic Phase):

   • If the sensitized $Rh^-$ mother becomes pregnant with a second $Rh^+$ fetus, fetal D antigens stimulate maternal memory B-cells, triggering massive synthesis of anti-Rh IgG antibodies.
   • Unlike large pentameric IgM antibodies (which cannot cross the placenta), IgG antibodies are small monomeric immunoglobulins that readily cross the placental barrier via syncytiotrophoblast Fc receptors.
   • Maternal anti-Rh IgG enters the fetal circulation and binds to fetal $Rh^+$ erythrocytes, targeting them for destruction by fetal splenic macrophages.

3. Clinical Manifestations in the Fetus / Neonate:

   • Severe Hemolytic Anemia: Destruction of fetal RBCs impairs oxygen transport, leading to fetal tissue hypoxia.
   • Erythroblastosis Fetalis: In an attempt to compensate for rapid erythrocyte destruction, fetal hematopoietic tissues (liver, spleen, and bone marrow) undergo accelerated hematopoiesis, flooding the fetal bloodstream with immature, nucleated red blood cells called erythroblasts.
   • Hydrops Fetalis: Severe fetal anemia leads to high-output heart failure, hepatic dysfunction, and profound hypoalbuminemia, causing generalized fetal edema, pleural effusions, ascites, and intrauterine fetal death.
   • Kernicterus: Following delivery, the neonate's immature liver cannot conjugate the massive amounts of unconjugated bilirubin released from lysed red blood cells. Severe neonatal jaundice develops. Unconjugated bilirubin crosses the immature blood-brain barrier, depositing in the basal ganglia and brainstem nuclei—a condition known as kernicterus—producing irreversible neurological damage, cerebral palsy, sensorineural deafness, or death.

4. Prophylaxis: Rho(D) Immune Globulin (RhoGAM):

   • HDN is entirely preventable through the timely administration of Rho(D) immune globulin (RhoGAM).
   • RhoGAM consists of purified, exogenous anti-Rh antibodies.
   • Dosing Schedule: Administered intramuscularly to an $Rh^-$ mother routinely at 28 weeks of gestation and within 72 hours post-delivery of an $Rh^+$ infant (or immediately following amniocentesis, miscarriage, ectopic pregnancy, or abdominal trauma).
   • Mechanism of Action: The injected exogenous antibodies bind, coat, and agglutinate any fetal $Rh^+$ erythrocytes that enter the maternal bloodstream, clearing them via hepatic reticuloendothelial macrophages before the maternal immune system can recognize the foreign D antigen and mount an active primary immune response.

Comprehensive ABO/Rh Blood Compatibility Matrix Table

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Overview of Hemostasis

Hemostasis (from Greek haima = blood, stasis = standing still) is the physiological sequence of rapid, localized, and strictly regulated responses that stops bleeding following blood vessel disruption. It must be distinguished from blood coagulation: coagulation is merely the third phase of the overall hemostatic process.

Healthy intact vascular endothelium continuously secretes chemical inhibitors that maintain blood in a fluid state and prevent unwanted clotting. When a vessel wall is compromised, hemostasis halts blood loss through three overlapping sequential phases:

1. Vascular Spasm (Vasoconstriction)
2. Platelet Plug Formation
3. Coagulation (Blood Clotting)

Following hemostasis, the vessel wall is restored through clot retraction, tissue repair, and eventual fibrinolysis.

The Three Sequential Phases of Hemostasis
1. Vascular Spasm
   └── Endothelial injury -> Smooth muscle contraction -> Lumen narrows -> Blood flow drops
       (Triggered by myogenic reflex, pain receptors, and local endothelin release)
2. Platelet Plug Formation
   └── Collagen exposure -> von Willebrand factor (vWF) adhesion -> Platelet activation
       └── Degranulation: ADP recruits platelets; TXA₂ enhances aggregation and vasospasm
3. Coagulation Cascade
   ├── Intrinsic Pathway (Collagen / Factor XII) ──┐
   │                                               ├──> Factor X activated (Xa) -> Prothrombinase
   └── Extrinsic Pathway (Tissue Factor / Factor III) ─┘        │
                                                                v
                                           Prothrombin (II) ---------> Thrombin
                                                                         │
                                           Fibrinogen (I) -----------> Insoluble Fibrin Mesh
                                                                         │ (Factor XIII cross-links)
                                                                         v
                                                                Definitive Stable Clot

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Phase 1: Vascular Spasm (Vasoconstriction)

Immediately upon physical laceration or rupture of an arteriole or artery, the circular smooth muscle within the tunica media contracts vigorously, causing immediate vasoconstriction:

• Initiating Triggers:
  1. Direct Myogenic Response: Direct mechanical trauma to vascular smooth muscle cells depolarizes their plasma membranes, inducing sustained muscular contraction.
  2. Nociceptive Reflexes: Pain receptors (nociceptors) in the vessel adventitia and surrounding perivascular tissues are activated by injury, sending sensory signals that provoke localized sympathetic vasoconstrictor reflexes.
  3. Endothelin Secretion: Damaged vascular endothelial cells release the potent peptide vasoconstrictor endothelin into the local microenvironment.
• Physiological Impact: Vascular spasm rapidly narrows the vessel lumen, dramatically reducing local blood flow and perfusion pressure. This response lasts for 20 to 30 minutes, buying valuable time for the platelet plug to form and the enzymatic coagulation cascade to generate a definitive fibrin clot.

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Phase 2: Platelet Plug Formation

Under normal physiological conditions, platelets do not adhere to each other or to the vessel wall because healthy, intact endothelial cells present a smooth, non-thrombogenic surface and continuously secrete two powerful platelet-inhibiting paracrine molecules:

• Prostacyclin ($PGI_2$): A prostaglandin that stimulates platelet adenylate cyclase, elevating intracellular cAMP and inhibiting platelet activation.
• Nitric Oxide (NO): A gas that diffuses into platelets to stimulate guanylate cyclase, relaxing smooth muscle and suppressing platelet adhesion.

When a blood vessel wall tears, the protective endothelial monolayer is stripped away, initiating platelet plug formation:

1. Platelet Adhesion

Endothelial disruption exposes the underlying subendothelial connective tissue matrix, which is rich in collagen fibers. Circulating platelets bind to this exposed collagen through a bridging plasma glycoprotein called von Willebrand factor (vWF). Synthesized by both endothelial cells and bone marrow megakaryocytes, vWF binds simultaneously to exposed collagen fibrils and to glycoprotein Ib (GPIb) surface receptors on passing platelets, anchoring the platelets to the injury site despite the shearing force of flowing blood.

2. Platelet Activation and Degranulation

Anchoring to collagen triggers intracellular signaling cascades that activate the platelet:

• Morphological Transformation: The resting discoid platelet changes shape dramatically, extending long, spiny, filamentous pseudopods that interlock with neighboring platelets.
• Degranulation (Release Reaction): Activated platelets exocytose the contents of their cytoplasmic storage granules into the local microenvironment:
  • Adenosine Diphosphate (ADP): A potent chemical recruiter that binds purinergic receptors on nearby passing platelets, activating them and causing them to adhere to the growing platelet cluster.
  • Thromboxane $A_2$ ($TXA_2$): Synthesized rapidly from platelet membrane arachidonic acid via the cyclooxygenase-1 (COX-1) enzyme pathway. $TXA_2$ is a powerful platelet aggregating agent and further reinforces local vascular spasm.
    • Clinical Pharmacology: Aspirin Mechanism: Low-dose aspirin irreversibly acetylates and inhibits the cyclooxygenase-1 (COX-1) enzyme within platelets. Because platelets are anucleate fragments, they cannot synthesize new COX-1 enzymes. Consequently, aspirin permanently suppresses $TXA_2$ synthesis for the entire 8- to 10-day lifespan of the platelet, significantly reducing platelet aggregation and providing therapeutic cardioprotection against myocardial infarction and stroke.
  • Serotonin: Released from dense granules to reinforce and sustain localized vascular smooth muscle spasm.
  • Platelet Factor 3 (PF3): A phospholipid component of the activated platelet plasma membrane that serves as an essential catalytic surface for assembling clotting factor complexes during coagulation.

3. Platelet Aggregation (Positive Feedback Loop)

As newly recruited platelets adhere to the initial layer of platelets, activation triggers a conformational change in their surface glycoprotein IIb/IIIa (GPIIb/IIIa) receptors. Soluble plasma fibrinogen molecules bind to these active GPIIb/IIIa receptors, cross-linking adjacent platelets into a cohesive, multicellular mass.

This self-reinforcing positive feedback cycle—where adhering platelets release ADP and $TXA_2$ to recruit more platelets—rapidly builds a platelet plug that mechanically seals small vascular punctures within 1 to 3 minutes. (The spreading of the plug to uninjured adjacent vessel walls is prevented by prostacyclin and NO released from neighboring healthy endothelial cells).

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Phase 3: Coagulation (Blood Clotting)

While a platelet plug can temporarily seal small microvascular breaks, larger vascular wounds require coagulation to transform liquid blood into a durable, gelatinous gel. Coagulation reinforces the temporary platelet plug with an interlocking network of insoluble fibrin threads, forming the definitive hemostatic clot.

Clotting Factors and Essential Cofactors

Coagulation involves over a dozen circulating plasma proteins called clotting factors (designated by Roman numerals I through XIII in order of their historical discovery, not their order in the cascade). Most clotting factors circulate as inactive proenzymes (zymogens) synthesized by the liver:

• Calcium Ions ($Ca^{2+}$, Factor IV): Ionized calcium is an indispensable cofactor required at virtually every step of both intrinsic and extrinsic pathways. Without $Ca^{2+}$, blood cannot clot. (Clinical blood collection tubes utilize calcium chelators like citrate or EDTA to prevent in vitro coagulation).
• Vitamin K: A fat-soluble vitamin absorbed in the gut with dietary lipids. Vitamin K is an essential cofactor for hepatic gamma-glutamyl carboxylase, the enzyme responsible for post-translational gamma-carboxylation of glutamic acid residues on four crucial clotting factors:
  • Factor II (Prothrombin)
  • Factor VII
  • Factor IX
  • Factor X
  • (Clinical Mnemonic: Remember the year 1972—Factors 10, 9, 7, and 2).
  • Without vitamin K, these factors cannot bind $Ca^{2+}$ or anchor to platelet phospholipid surfaces, rendering them non-functional.

The Coagulation Cascade Pathways

Coagulation proceeds through two initial pathways—the Intrinsic Pathway and the Extrinsic Pathway—both of which converge upon a shared Common Pathway:

1. The Intrinsic Pathway

• Trigger: Initiated by trauma occurring inside the blood vessel wall.
• Mechanism: Circulating inactive Factor XII (Hageman factor) comes into direct physical contact with negatively charged subendothelial collagen fibers or glass surfaces.
• Cascade Sequence: Activated Factor XII (XIIa) enzymatically activates Factor XI (XIa). Factor XIa, in the presence of $Ca^{2+}$, activates Factor IX (IXa). Factor IXa then complexes with activated Factor VIII (VIIIa), ionic $Ca^{2+}$, and platelet factor 3 (PF3) to form the tenase complex.
• Kinetics: The intrinsic pathway is slower, typically requiring several minutes to complete, but it produces a massive amplification of activated factors.

2. The Extrinsic Pathway

• Trigger: Initiated rapidly by trauma occurring to perivascular tissues outside the blood vessel.
• Mechanism: Damaged perivascular tissue cells release a transmembrane glycoprotein called Tissue Factor (TF / Factor III / Tissue Thromboplastin) into the bloodstream.
• Cascade Sequence: Tissue Factor complexes directly with circulating Factor VII in the presence of $Ca^{2+}$, rapidly activating it to Factor VIIa. The TF-VIIa complex directly cleaves and activates Factor X.
• Kinetics: The extrinsic pathway is an explosive, rapid shortcut, producing active Factor X within 10 to 15 seconds.

3. The Common Pathway

Both intrinsic and extrinsic pathways converge at the activation of Factor X:

1. Factor X Activation: The tenase complex (intrinsic) or the TF-VIIa complex (extrinsic) cleaves Factor X into active Factor Xa.
2. Prothrombinase Assembly: Factor Xa combines with activated Factor V (Va), ionic $Ca^{2+}$, and platelet membrane phospholipids (PF3) to form the multi-protein enzymatic complex known as Prothrombinase (Prothrombin Activator).
3. Thrombin Generation: Prothrombinase catalyzes the rapid proteolytic cleavage of the inactive plasma proenzyme Prothrombin (Factor II) into the active, powerful serine protease enzyme Thrombin.
4. Fibrin Polymerization: Active Thrombin executes two critical functions:
   • It cleaves soluble plasma Fibrinogen (Factor I), releasing small fibrinopeptides and converting fibrinogen into insoluble Fibrin monomers. These monomers spontaneously polymerize end-to-end and side-to-side into long, insoluble fibrin strands that drape across the platelet plug.
   • Thrombin activates Factor XIII (Fibrin-Stabilizing Factor) in the presence of $Ca^{2+}$. Factor XIIIa forms covalent cross-linking peptide bonds between adjacent fibrin polymer chains, turning a loose fibrin tangle into a stable, structural, three-dimensional mesh.
5. Clot Formation: The cross-linked fibrin meshwork traps passing red blood cells, leukocytes, and platelets, forming the definitive hemostatic clot that completely seals the vascular defect.

Hemostasis 3 Phases Step-by-Step Table

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Clot Retraction, Tissue Repair & Fibrinolysis

Once a definitive hemostatic clot has stabilized the vascular rupture, subsequent physiological mechanisms consolidate the seal, rebuild the vessel wall, and safely dissolve the clot once tissue architecture is restored.

1. Clot Retraction (Consolidation)

Within 30 to 60 minutes following clot formation, the clot undergoes retraction:

• Platelets trapped within the fibrin mesh extend pseudopods that adhere tightly to fibrin strands.
• The actin and myosin contractile microfilaments within the platelets contract, pulling the attached fibrin strands inward.
• As the mesh compacts, it squeezes out a clear, straw-colored fluid termed serum (plasma devoid of fibrinogen and consumed clotting factors).
• This contraction pulls the severed edges of the ruptured blood vessel wall into close physical alignment, reducing wound surface area and facilitating tissue repair.

2. Vascular Wall Repair

Platelets trapped within the retracted clot degranulate, releasing growth factors that orchestrate permanent tissue reconstruction:

• Platelet-Derived Growth Factor (PDGF): Stimulates division and migration of smooth muscle cells and fibroblasts in the vessel tunica media and adventitia to rebuild the connective tissue wall.
• Vascular Endothelial Growth Factor (VEGF): Stimulates adjacent endothelial cells to proliferate and reconstruct the smooth, non-thrombogenic endothelial lining.

3. Fibrinolysis (Clot Dissolution)

If a blood clot remained permanently within a vessel, it would organize into a fibrous scar that permanently occluded the vascular lumen, or pieces could break loose to form fatal thromboemboli (such as a pulmonary embolism). The removal of unneeded clots following tissue healing is accomplished via fibrinolysis:

• Plasminogen Incorporation: During initial clot formation, an inactive plasma proenzyme synthesized by the liver, plasminogen, is incorporated and trapped directly within the polymerizing fibrin mesh.
• Activation by tPA: Over several days, repairing endothelial cells synthesize and slowly secrete Tissue Plasminogen Activator (tPA) into the clot.
• Plasmin Generation: In the presence of fibrin, tPA cleaves inactive plasminogen into the active, powerful fibrin-digesting enzyme Plasmin.
• Enzymatic Degradation: Plasmin digests the cross-linked fibrin meshwork into soluble fibrin degradation products (FDPs), dissolving the thrombus and restoring unhindered blood flow through the recanalized vessel.
• Clinical D-Dimer Test: A specific fibrin degradation product—termed D-dimer—is produced exclusively when cross-linked fibrin is degraded by plasmin. A normal D-dimer level helps clinicians rule out acute deep vein thrombosis (DVT) and pulmonary embolism (PE) in lower-risk patients; an elevated level is sensitive but nonspecific and usually prompts imaging.

Clinical Anticoagulants and Pharmacology

Physiological and pharmacological anticoagulants prevent catastrophic intravascular thrombosis:

• Heparin: A natural glycosaminoglycan produced by basophils and tissue mast cells. Heparin binds to and dramatically accelerates the activity of antithrombin III, an endogenous plasma protein that inactivates thrombin and Factor Xa. Clinically, unfractionated or low-molecular-weight heparin is administered intravenously or subcutaneously for immediate inpatient anticoagulation.
• Warfarin (Coumadin): An oral anticoagulant that acts as a competitive antagonist of vitamin K epoxide reductase. By blocking the recycling of oxidized vitamin K, warfarin depletes functional vitamin K, preventing hepatic synthesis of Factors II, VII, IX, and X. Because pre-existing circulating factors must turn over, warfarin's therapeutic anticoagulant effect takes 36 to 72 hours to develop. Its clinical efficacy is routinely monitored using the Prothrombin Time (PT) and International Normalized Ratio (INR).