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Introduction to Hematology

An introduction to hematology, blood, blood-forming tissues, major blood-cell types, and hematology testing.

5 min read Hematology #nrbc #rbc-indices #reticulocyte #hematology #differential
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Learning Objectives

By the end of this resource you should be able to:

- Define hematology and describe its scope.
- Identify the major components of blood.
- Describe the basic roles of red blood cells, white blood cells, platelets, and plasma.
- Explain the role of bone marrow in blood-cell production.
- Describe the role of laboratory hematology testing.

Introduction to Hematology

Overview

Hematology is the branch of laboratory and clinical medicine concerned with blood, blood-forming tissues, and disorders affecting blood cells and related processes.

The hematology laboratory provides information about cellular components of blood and, depending on the laboratory, may also perform investigations related to hemostasis, coagulation, morphology, transfusion support, and specialized hematologic disorders.

Blood and its major components

Blood consists broadly of a liquid component, plasma, and formed elements.

Red blood cells

Red blood cells (erythrocytes) contain hemoglobin and are specialized for gas transport. Their number, size, hemoglobin content, and distribution can be assessed using hematology testing.

Common red-cell measurements include:

  • Red blood cell count (RBC)
  • Hemoglobin concentration (Hb)
  • Hematocrit or packed cell volume (Hct/PCV)
  • Mean corpuscular volume (MCV)
  • Mean corpuscular hemoglobin (MCH)
  • Mean corpuscular hemoglobin concentration (MCHC)
  • Red-cell distribution width (RDW)

These parameters are commonly considered together rather than interpreted as isolated numbers.

White blood cells

White blood cells (leukocytes) participate in immune and inflammatory responses. Major circulating populations include:

  • Neutrophils
  • Lymphocytes
  • Monocytes
  • Eosinophils
  • Basophils

A complete blood count with differential can provide quantitative information about these populations. A blood film may provide additional information about cell morphology and unusual populations.

Platelets

Platelets are cellular fragments derived from megakaryocytes and have an important role in primary hemostasis.

Laboratory platelet assessment may include platelet count and selected platelet indices. The significance of a result depends on the patient's clinical context, specimen quality, analytical method, and laboratory reference information.

Plasma

Plasma is the liquid portion of blood. It contains water, proteins, electrolytes, nutrients, hormones, gases, and other dissolved substances.

Hematopoiesis

Hematopoiesis is the process through which blood cells are produced from hematopoietic stem and progenitor cells. Bone marrow is a major site of blood-cell production in healthy adults.

Production is regulated by cellular differentiation, growth factors, cytokines, tissue signals, and the body's physiological requirements.

A simplified view includes production of:

  • Erythroid cells leading to mature red blood cells
  • Myeloid cells including granulocytes and monocytes
  • Megakaryocytes leading to platelets
  • Lymphoid cells including B- and T-cell lineages

Common hematology investigations

Complete blood count

The complete blood count (CBC) provides quantitative information about major blood-cell populations and commonly includes hemoglobin, red-cell measurements, white-cell count, differential information, and platelet count.

Blood film examination

Peripheral blood-film examination allows laboratory personnel to assess cell morphology and can provide information that may not be captured adequately by numerical parameters alone.

Reticulocyte assessment

Reticulocytes are immature red blood cells. Reticulocyte measurements can provide information about erythropoietic activity and are often considered in the investigation or monitoring of anemia.

Specialized investigations

Depending on laboratory scope, hematology services may include investigations of coagulation, hemoglobin variants, bone-marrow disorders, and other specialized conditions.

Specimen considerations

Many routine hematology examinations use anticoagulated whole blood, commonly collected into a tube containing an appropriate anticoagulant according to the laboratory's validated requirements.

Important pre-analytical considerations include:

  • Correct patient identification
  • Correct specimen container
  • Adequate volume
  • Appropriate blood-to-anticoagulant relationship
  • Proper mixing where required
  • Appropriate transport and storage
  • Examination within the validated stability period

Specific requirements are test- and method-dependent and should be taken from the laboratory's current procedure and applicable manufacturer instructions.

Factors that can affect hematology results

Potential sources of error include:

  • Specimen clotting
  • Hemolysis
  • Inadequate mixing
  • Incorrect blood-to-anticoagulant ratio
  • Delayed analysis
  • Inappropriate storage
  • Instrument flags requiring review
  • Platelet clumping
  • Abnormal cell populations
  • Specimen contamination or identification errors

Laboratory quality considerations

Hematology quality management includes appropriate internal quality control, instrument maintenance, calibration or verification where applicable, reagent management, review of instrument flags, staff competency, and documented procedures.

Quality-control results should be evaluated according to the laboratory's established criteria rather than relying on an improvised rule.

Key points

  • Hematology focuses on blood, blood-forming tissues, and related disorders.
  • RBCs, WBCs, platelets, and plasma have different biological and laboratory roles.
  • The CBC provides multiple related measurements that should be considered together.
  • Blood-film examination can add morphological information.
  • Specimen quality is essential to reliable hematology results.
  • Detailed reference intervals, result interpretation, and calculators should be consulted in their dedicated LabGoTo sections.

References

  • NCBI. MeSH: Hematology.
  • MedlinePlus. Blood Count Tests.
  • ISO 15189:2022. Medical laboratories — Requirements for quality and competence.