Comprehensive Introduction to Hematology: Blood Composition, Testing, and Cellular Analysis

Tests Conducted in the Hematology Laboratory

• The hematology section is responsible for a variety of diagnostic procedures used to analyze blood components and coagulation factors. These include:

  • Complete blood count (CBC): A comprehensive assessment of all cellular components in the blood.

  • Peripheral blood smear (PBS): Microscopic examination of blood cells to evaluate morphology.

  • Reticulocyte count: Assessment of immature red blood cells to determine bone marrow activity.

  • Malarial parasite detection: Identification of plasmodium species in blood samples.

  • Erythrocyte sedimentation rate (ESR): A measure of the rate at which red blood cells sink in a tube, used as a marker for inflammation.

  • Clotting time/Bleeding time (CT/BT): Basic screening tests for primary and secondary hemostasis.

  • Prothrombin time (PT): Evaluation of the extrinsic and common pathways of coagulation.

  • Partial thromboplastin time (PTT): Evaluation of the intrinsic and common pathways of coagulation.

  • Fibrinogen assay: Measurement of the concentration of fibrinogen (Factor I).

  • D-dimer assay: Assessment of fibrin degradation products, useful in diagnosing DIC or deep vein thrombosis.

  • Bone marrow examination: Direct analysis of the tissue where blood cells are produced.

  • Flow cytometry immunophenotyping: Cell analysis based on surface markers.

  • Cytogenetics and molecular diagnostics: Investigation of genetic abnormalities and molecular mutations.

• Instrumentation noted: mindray BC-6800 analyzer.

Fundamental Properties of Blood

• Definition: Blood is a complex mixture of different cells that circulate within a fluid medium known as plasma.

• Primary Function: It serves as the main transport medium of the body, moving nutrients, gases, and waste products.

• Physical Characteristics:

  • Volume: The average human body contains approximately $5\,L$ of blood.

  • Weight Contribution: Blood accounts for approximately $8\%$ of total body weight.

  • Density and Viscosity: Blood is denser and approximately $5 \times$ more viscous than water.

  • Temperature: The normal physiological temperature of blood is $37.4\,^{\circ}\text{C}$.

  • pH Range: The normal pH of blood is slightly basic, ranging from $7.35\text{--}7.45$.

Major Components of Blood and Hematocrit

• Blood is categorized into three primary layers after centrifugation:

  • Plasma: Comprised of water, proteins, and other solutes.

  • Buffy coat: A thin layer containing white blood cells (WBCs) and platelets.

  • Packed RBCs: Also known as the hematocrit.

• Hematocrit (Hct) Reference Ranges:

  • Normal Females: $37\%\text{--}47\%$

  • Normal Males: $42\%\text{--}52\%$

• Clinical Indications Based on Hematocrit:

  • Anemia: Characterized by a depressed or low hematocrit percentage.

  • Polycythemia: Characterized by an elevated or high hematocrit percentage.

Red Blood Cells (Erythrocytes)

• Morphology: RBCs are flat biconcave disks, allowing for increased surface area for gas exchange.

• Nucleation: These cells are anucleate (contain no nucleus) in their mature state.

• Color: They appear pale red or salmon pink under microscopy.

• Size: They have a diameter of $7\text{--}8\,\mu\text{m}$.

• Lifespan: The typical lifespan of an erythrocyte is $120\,\text{days}$.

• Optical Characteristic: They possess a "zone of pallor" that occupies the central $1/3$ of the cell, which is a direct reflection of its biconcave shape.

• Functional Component: Hemoglobin is the primary protein that facilitates the transport of $O_2$ and $CO_2$ between the lungs and bodily tissues.

• Normal Values (NV) for Red Blood Cell Count:

  • Males (M): $4.2\text{--}6.0 \times 10^{12}\,L^{-1}$

  • Females (F): $3.8\text{--}5.2 \times 10^{12}\,L^{-1}$

Plasma Composition

• Plasma is the liquid portion of blood and is separated from whole blood for therapeutic use in patients with coagulation disorders.

• Water Content: Predominantly composed of water ($90\%$).

• Plasma Proteins ($5\%\text{--}7\%$):

  • Albumin: Crucial for maintaining osmotic concentration/pressure.

  • Globulin: Includes Acute Phase Reactants (APRs) and proteins involved in adaptive immunity.

  • Fibrinogen: Essential for the process of hemostasis (blood clotting).

• Regulatory Proteins (< $1\%$): Includes hormones and various enzymes.

• Other Solutes ($1\%$): Includes nutrients, dissolved gases, electrolytes, and metabolic waste products.

Platelets (Thrombocytes)

• Morphology: Round, anucleate, and slightly granular.

• Size: They are small, measuring $2\text{--}4\,\mu\text{m}$ in diameter.

• Origin: They are fragments derived from large megakaryocytes located in the bone marrow.

• Function: Primary role is the control of hemostasis.

• Normal Value (NV): $150\text{--}450 \times 10^9\,L^{-1}$

• Clinical Significance:

  • Thrombocytosis ($\uparrow\text{PLT}$): Secondary to other conditions, essential thrombocythemia, polycythemia vera (PV), primary myelofibrosis (PMF), chronic myeloid leukemia (CML), or megakaryocytic leukemias.

  • Thrombocytopenia ($\downarrow\text{PLT}$): Caused by bone marrow (BM) suppression, immune-mediated destruction, heparin-induced thrombocytopenia, thrombosis, hypersplenism, or massive blood transfusion.

White Blood Cells (Leukocytes)

• Total WBC Reference Range: $4.5\text{--}11.0 \times 10^9\,L^{-1}$

Neutrophils

• Abundance: The most common leukocyte.

• Morphology: Features a lobulated or segmented nucleus with $3\text{--}5$ lobes and pink- or lavender-staining granules containing bactericidal substances.

• Function: Phagocytic; they engulf and destroy microorganisms and foreign debris.

• Reference Range: $50\%\text{--}70\%$

• Clinical Significance:

  • Neutrophilia ($\uparrow\text{NEUT}$): Bacterial/fungal infection (sepsis) or myelogenous leukemia.

  • Neutropenia ($\downarrow\text{NEUT}$): Bone marrow (BM) suppression or viral infection.

Lymphocytes

• Morphology: Round and slightly larger than RBCs with a large purple-staining nucleus and scant, non-granular cytoplasm.

• Types: T cells, B cells, and Natural Killer (NK) cells.

• Memory Cells: Form after pathogen exposure to provide rapid responses upon subsequent exposure.

• Reference Range: $18\%\text{--}42\%$

• Clinical Significance:

  • Lymphocytosis ($\uparrow\text{LYMPH}$): Viral infections, Infectious Mononucleosis (IM), pertussis, toxoplasmosis, Acute Lymphoblastic Leukemia (ALL), or Chronic Lymphocytic Leukemia (CLL).

  • Lymphocytopenia ($\downarrow\text{LYMPH}$): HIV (AIDS), BM failure, steroids, or chemotherapy treatment.

Monocytes

• Morphology: Indented or horseshoe-shaped nucleus; blue-gray cytoplasm with fine azure granules, giving it a "ground glass" appearance.

• Nature: Referred to as macrophages once they leave the circulation; they are the most abundant cell type in the body.

• Function: Highly effective phagocytic cells and serve as Antigen-Presenting Cells (APCs) for other immune cells.

• Reference Range: $2\%\text{--}11\%$

• Clinical Significance:

  • Monocytosis ($\uparrow\text{MONO}$): Chronic infection, autoimmune disorders, monocytic leukemias, or myelodysplastic syndromes (MDS).

Eosinophils

• Morphology: Bilobed nucleus with coarse orange-red granules filled with immune-regulating proteins.

• Function: Phagocytic; particularly effective against antigen-antibody complexes; they also release antihistamines.

• Reference Range: $1\%\text{--}3\%$

• Clinical Significance:

  • Eosinophilia ($\uparrow\text{EO}$): Allergy or parasitic infection.

Basophils

• Abundance: The least common leukocyte.

• Morphology: Bilobed nucleus; dark purple irregular granules that often obscure the nucleus.

• Function: Granules contain histamines; they release heparin and immune cytokines; they express IgE receptors on their surfaces.

• Reference Range: $0\%\text{--}2\%$

• Clinical Significance:

  • Basophilia ($\uparrow\text{BASO}$): Type I hypersensitivity reactions, Chronic Myeloid Leukemia (CML), or Polycythemia Vera (PV).

Tests Conducted in the Hematology Laboratory

• Complete blood count (CBC): Assesses blood cellular components.

• Peripheral blood smear (PBS): Examines blood cell morphology.

• Reticulocyte count: Evaluates bone marrow activity.

• Malarial parasite detection: Identifies plasmodium in blood samples.

• Erythrocyte sedimentation rate (ESR): Measures red blood cell settling rate, indicating inflammation.

• Clotting time/Bleeding time (CT/BT): Screens for hemostasis issues.

• Prothrombin time (PT): Assesses extrinsic and common coagulation pathways.

• Partial thromboplastin time (PTT): Assesses intrinsic and common coagulation pathways.

• Fibrinogen assay: Measures fibrinogen levels.

• D-dimer assay: Checks for fibrin degradation products, useful in diagnosing conditions like DIC or deep vein thrombosis.

• Bone marrow examination: Analyzes blood cell production tissue.

• Flow cytometry: Analyzes cell surface markers.

• Cytogenetics and molecular diagnostics: Investigates genetic abnormalities.

Instrumentation noted: mindray BC-6800 analyzer.

Fundamental Properties of Blood

• Definition: Blood is a mixture of cells in plasma.

• Function: Main transport medium in the body.

• Physical Characteristics:

  • Volume: Average of $5\,L$.

  • Weight: Constitutes $8\%$ of body weight.

  • Density: Denser and $5\times$ more viscous than water.

  • Temperature: Normal is $37.4\,^{\circ}\text{C}$.

  • pH: Slightly basic, $7.35\text{--}7.45$.

Major Components of Blood and Hematocrit

• Blood separates into three layers after centrifugation:

  • Plasma: Composed of water and proteins.

  • Buffy coat: Contains white blood cells and platelets.

  • Packed RBCs: Known as hematocrit.

• Hematocrit (Hct) Reference Ranges:

  • Normal Females: $37\%\text{--}47\%$

  • Normal Males: $42\%\text{--}52\%$

• Clinical Indications:

  • Anemia: Low hematocrit.

  • Polycythemia: High hematocrit.

Red Blood Cells (Erythrocytes)

• Morphology: Biconcave disks.

• Nucleation: Anucleate.

• Color: Pale red.

• Size: Diameter $7\text{--}8\,\mu\text{m}$.

• Lifespan: $120\,days$.

• Main Function: Hemoglobin transports $O<em>2$ and $CO</em>2$.

• Normal Values:

  • Males: $4.2\text{--}6.0 \times 10^{12}\,L^{-1}$

  • Females: $3.8\text{--}5.2 \times 10^{12}\,L^{-1}$

Plasma Composition

• Plasma is the liquid portion of blood, used in coagulation therapy.

• Main Components:

  • Water: $90\%$.

  • Proteins: $5\text{--}7\%$ (includes albumin, globulin, fibrinogen).

  • Other solutes: $1\%$ (nutrients, gases, electrolytes).

Platelets (Thrombocytes)

• Morphology: Round, anucleate, granular.

• Size: $2\text{--}4\,\mu\text{m}$.

• Origin: From megakaryocytes in the bone marrow.

• Function: Control of hemostasis.

• Normal Value: $150\text{--}450 \times 10^9\,L^{-1}$.

• Clinical Significance:

  • Thrombocytosis: High platelet count.

  • Thrombocytopenia: Low platelet count.

White Blood Cells (Leukocytes)

• Total WBC Reference Range: $4.5\text{--}11.0 \times 10^9\,L^{-1}$.

• Neutrophils: Most common, phagocytic, $50\%\text{--}70\%$.

• Lymphocytes: Include T, B, and NK cells, $18\%\text{--}42\%$.

• Monocytes: Effective phagocytes, $2\%\text{--}11\%$.

• Eosinophils: Effective against parasites, $1\%\text{--}3\%$.

• Basophils: Least common, $0\%\text{--}2\%$.