Hematology 2 Reviewer

Introduction

This reviewer covers essential topics in Hematology 2, focusing on the critical components of the
hemostatic process and its clinical evaluation. Hemostasis is a complex and finely regulated system
responsible for maintaining blood fluidity while preventing excessive blood loss in case of vascular
injury. It involves three main stages: primary hemostasis (platelet plug formation), secondary
hemostasis (coagulation cascade and fibrin formation), and tertiary hemostasis (clot stabilization and
fibrinolysis).

The following sections will detail the mechanisms involved in each stage of hemostasis, along with
the laboratory tests used to evaluate platelet function, clot formation, and clot breakdown. Disorders
affecting the vascular system, platelets, and coagulation pathways will also be discussed, emphasizing
their clinical implications. The review concludes with an exploration of instrumentation in
coagulation and the role of automation in hematology.

This reviewer aims to provide a comprehensive understanding of the hemostatic system, enabling
students to apply theoretical knowledge to clinical practice. It includes reference ranges and
laboratory evaluation techniques essential for diagnosing and monitoring hematological conditions.
This material is particularly valuable for those preparing for examinations and clinical practice in
medical laboratory science.

Basic Terms on Hemostatic Process

Hemostasis is the process that maintains blood in a fluid state within the vascular system while
preventing excessive blood loss when injury occurs. It involves three stages:

1. Vasoconstriction: The immediate constriction of damaged blood vessels, reducing blood

flow to the injury site.
2. Platelet Plug Formation: Platelets adhere to the damaged endothelium, becoming
activated, and forming a temporary plug.
3. Coagulation Cascade: A series of enzymatic reactions that activate clotting factors, leading
to the formation of a fibrin mesh that stabilizes the platelet plug.

Key terms:

• Thrombus: A stationary blood clot within a vessel.

• Embolus: A blood clot that has moved to another location within the bloodstream.
• Fibrinolysis: The breakdown of fibrin in blood clots.
• Clotting Factors: Proteins involved in blood clotting (e.g., Factor I: Fibrinogen, Factor II:
Prothrombin).

Platelet Role and Vascular System in Primary Hemostasis

Platelets play a critical role in the first phase of hemostasis by forming a platelet plug at the injury
site. This process involves adhesion, activation, and aggregation:

• Adhesion: Platelets bind to von Willebrand Factor (vWF) at the site of endothelial injury.
• Activation: Activated platelets change shape, release granules (ADP, serotonin,
thromboxane A2), and recruit additional platelets.
• Aggregation: Platelets clump together to form a stable plug, sealing the wound temporarily.

Reference ranges:

• Platelet Count: 150,000 - 450,000/µL (convert to SI)

• Bleeding Time: 2-9 minutes
• Mean Platelet Volume (MPV): 7.5 - 12.0 fL (femtoliters)

Lab Evaluation of Primary Hemostasis

Laboratory tests assess platelet function and the vascular response:

1. Platelet Count: Measures the number of platelets in the blood.

2. Bleeding Time: Assesses the time it takes for bleeding to stop after a small cut.
3. Platelet Function Analyzer (PFA-100): Evaluates platelet adhesion and aggregation under
high shear conditions.
4. Platelet Aggregation Test: Measures the ability of platelets to aggregate in response to
various agonists (e.g., ADP, collagen, epinephrine).

Reference ranges:

• Bleeding Time: 2-9 minutes (depending on the method)

• Platelet Aggregation: Results vary based on agonist; full aggregation within 5-10
minutes is expected for most agonists.

Disorders Affecting Vascular System and Platelets

1. von Willebrand Disease (vWD): A deficiency or dysfunction of von Willebrand Factor,

impairing platelet adhesion.
2. Thrombocytopenia: A low platelet count (<150,000/µL), leading to increased bleeding
risk.
3. Thrombocytopathies: Platelet function disorders such as Glanzmann’s thrombasthenia or
Bernard-Soulier syndrome.
4. Vascular Disorders: Conditions like Ehlers-Danlos syndrome affect vessel integrity, making
vessels prone to rupture.

Secondary Hemostasis: Coagulation Proteins and Coagulation Pathway

Secondary hemostasis stabilizes the platelet plug through the activation of the coagulation cascade:
 • Extrinsic Pathway: Initiated by tissue factor (Factor III) exposed at the injury site, leading
to activation of Factor VII.
• Intrinsic Pathway: Activated by contact with damaged endothelium, involving Factors XII,
XI, IX, and VIII.
• Common Pathway: Convergence of the intrinsic and extrinsic pathways, involving Factors
I (fibrinogen), II (prothrombin), V, and X, culminating in fibrin formation.

Reference ranges:

• Prothrombin Time (PT): 11-15 seconds (for extrinsic pathway)

• Activated Partial Thromboplastin Time (APTT): 30-40 seconds (for intrinsic pathway)

Tertiary Hemostasis

Tertiary hemostasis refers to fibrinolysis, where the clot is broken down after tissue repair:

• Plasminogen: Converted to plasmin by tissue plasminogen activator (tPA), which degrades

fibrin.
• Fibrinolysis: The breakdown of fibrin into fibrin degradation products (FDPs) or D-
dimers.

Reference ranges:

• D-dimer: <500 ng/mL (used to rule out thrombotic events).

Lab Evaluation of Hemostasis

Laboratory tests evaluate the different components of the coagulation system:

1. Prothrombin Time (PT): Assesses the extrinsic pathway and is used to monitor warfarin
therapy.
2. Activated Partial Thromboplastin Time (APTT): Measures the intrinsic pathway and is
used to monitor heparin therapy.
3. Thrombin Time (TT): Assesses the conversion of fibrinogen to fibrin.
4. Fibrinogen Levels: Low levels can indicate consumption (as in DIC) or liver disease.

Reference ranges:

• PT: 11-15 seconds

• APTT: 30-40 seconds
• Fibrinogen: 200-400 mg/dL

Lab Evaluation of Fibrinolysis

• D-dimer Test: Measures the presence of fibrin degradation products, indicating recent clot
formation and breakdown.
 • Fibrin Degradation Products (FDPs): Elevated levels indicate enhanced fibrinolytic
activity.

Reference ranges:

• D-dimer: <500 ng/mL

• FDPs: 0-5 µg/mL

Disorders Leading to Hypocoagulation

Hypocoagulation refers to an impaired ability to form clots, resulting in excessive bleeding:

1. Hemophilia: Genetic deficiency of Factor VIII (Hemophilia A) or Factor IX (Hemophilia

B), leading to prolonged APTT.
2. Liver Disease: Reduces the production of clotting factors, leading to increased PT and
APTT.
3. Vitamin K Deficiency: Impairs the synthesis of Factors II, VII, IX, and X, prolonging PT.
4. Disseminated Intravascular Coagulation (DIC): Pathological activation of coagulation
and fibrinolysis, leading to consumption of clotting factors.

Lab Monitoring of Anticoagulation Therapy

1. Warfarin Therapy: Monitored using PT and International Normalized Ratio (INR). The
goal is typically an INR of 2.0-3.0 for most indications.
2. Heparin Therapy: Monitored using APTT. The therapeutic range is typically 1.5-2.5 times
the normal APTT value.
3. Direct Oral Anticoagulants (DOACs): May require specific tests, such as anti-Xa assays
for monitoring factor Xa inhibitors.

Reference ranges:

• INR: 2.0-3.0 (therapeutic range for warfarin)

• APTT: 1.5-2.5 times the control (for heparin therapy)

Instrumentation in Coagulation

Modern coagulation analyzers automate the measurement of clotting times using various methods:

1. Optical Density: Measures changes in light transmission as a clot forms.

2. Electromechanical Detection: Monitors changes in electrical resistance or other physical
properties during clot formation.
3. Thromboelastography (TEG): Provides a detailed analysis of clot formation, strength, and
lysis.

Automation in Hematology
Automation in hematology has revolutionized coagulation testing and complete blood count (CBC)
analysis:

1. Flow Cytometry: Used for counting and analyzing blood cells and platelets based on their
size, shape, and granularity.
2. Laser-Based Methods: Measure light scatter to analyze cell properties.
3. Automated Coagulation Analyzers: Provide rapid and consistent results for PT, APTT,
and other coagulation tests.