🧬 Educational content only. This article is an educational overview of haematology as a scientific and medical discipline. It is not medical advice and does not relate to individual health conditions or test results.
Haematology (spelled “hematology” in American English) is the branch of medicine and biomedical science concerned with blood, the blood-forming organs (primarily the bone marrow), and the diseases that affect them. Blood is one of the most studied and clinically important tissues in the human body — analysing its components reveals a remarkable amount of information about a person’s health. The laboratory science of haematology is one of the core disciplines in any clinical laboratory, and haematology tests are among the most commonly performed investigations in medicine.
Key Takeaways
- Haematology studies blood cells (red cells, white cells, and platelets), plasma proteins, coagulation, and the bone marrow that produces blood cells.
- The full blood count (FBC/CBC) is the most common haematology test, measuring the numbers and characteristics of blood cells.
- Clinical haematologists diagnose and treat blood disorders including anaemia, leukaemia, lymphoma, clotting disorders, and haemoglobinopathies.
- Laboratory haematologists and biomedical scientists perform and interpret haematology tests, operate automated analysers, and examine blood films under the microscope.
- Modern haematology encompasses molecular diagnostics, flow cytometry, bone marrow examination, and haemostasis testing.
The Components of Blood
Blood is a connective tissue composed of cells suspended in a liquid medium called plasma. It makes up approximately 7–8% of body weight in adults — around 5 litres in an average adult. The cellular components of blood are: red blood cells (erythrocytes), which carry oxygen from the lungs to tissues using haemoglobin; white blood cells (leucocytes), which are the immune system’s primary defenders; and platelets (thrombocytes), which are small cell fragments essential for blood clotting. Plasma — the liquid component — is approximately 90% water and contains proteins (including albumin, clotting factors, immunoglobulins, and transport proteins), nutrients, hormones, and waste products. When blood clots and the clotting proteins are removed, the remaining fluid is called serum.
Haematopoiesis: Where Blood Cells Come From
All blood cells originate from a common precursor — the haematopoietic stem cell — located in the bone marrow (primarily in flat bones such as the sternum, pelvis, and vertebrae in adults). This process, called haematopoiesis, produces billions of new blood cells every day to replace those that age and die. The differentiation pathway branches into two main lineages: the myeloid lineage, which produces red blood cells, platelets, granulocytes (neutrophils, eosinophils, basophils), and monocytes; and the lymphoid lineage, which produces lymphocytes (T cells, B cells, and natural killer cells). Understanding haematopoiesis is essential for understanding blood cancers such as leukaemia and lymphoma, which arise from abnormalities in this process.
The Full Blood Count: Haematology’s Frontline Test
The full blood count (FBC) — called a complete blood count (CBC) in North America — is the cornerstone haematology investigation. It measures the haemoglobin concentration, red blood cell count, haematocrit (packed cell volume), red cell indices (MCV, MCH, MCHC, RDW), total white blood cell count and differential (breaking down each type of white cell), and platelet count. The FBC is used for initial investigation of anaemia, infection, bleeding disorders, and blood cancers, as well as routine monitoring across many medical conditions. A peripheral blood film — where blood is spread on a glass slide and examined under a microscope by a trained scientist — adds critical visual information about cell morphology that automated analysers cannot fully capture.
Coagulation (Blood Clotting)
The study of haemostasis and coagulation — the processes by which the body stops bleeding and maintains blood flow — is a major subdiscipline of haematology. When a blood vessel is damaged, a cascade of reactions involving platelets and clotting proteins (coagulation factors) forms a clot to seal the vessel. Laboratory tests of coagulation include PT/INR (prothrombin time), APTT (activated partial thromboplastin time), fibrinogen, D-dimer, and specific clotting factor assays. These are used to diagnose inherited bleeding disorders (such as haemophilia A and B, and von Willebrand disease), acquired bleeding problems (such as in liver disease or disseminated intravascular coagulation, DIC), and to monitor anticoagulant therapy (e.g., warfarin monitoring via INR).
Common Haematological Conditions
Anaemia (reduced haemoglobin) is the most common haematological finding globally and has many causes including iron deficiency, vitamin B12 or folate deficiency, haemolysis, and chronic disease. Leukaemia is a cancer of the white blood cells arising in the bone marrow; it is classified as acute or chronic, and as myeloid or lymphoid (giving four main types: ALL, AML, CLL, CML). Lymphoma is a cancer of the lymphatic system, including Hodgkin and non-Hodgkin lymphoma. Myeloma (multiple myeloma) is a cancer of plasma cells. Haemoglobinopathies are inherited disorders of haemoglobin structure or production, including sickle cell disease and thalassaemia. Thrombocytopenia (low platelet count) increases bleeding risk; thrombocytosis (high platelet count) can increase clotting risk.
Haematology in the Laboratory
In the clinical laboratory, haematology is performed by biomedical scientists (UK) or medical laboratory scientists (US), using highly automated full blood count analysers, blood film microscopy, flow cytometers (for immunophenotyping of blood cells and diagnosing leukaemia/lymphoma), coagulation analysers, and molecular diagnostic platforms (for detecting gene mutations associated with haematological malignancies). Clinical haematologists — medical doctors specialising in blood diseases — oversee complex diagnostics, manage patients with blood disorders, and run services such as bone marrow transplantation and anticoagulation clinics.
References
- National Library of Medicine. Blood Disorders. MedlinePlus. https://medlineplus.gov/blooddisorders.html
- NHS. Blood tests. https://www.nhs.uk/conditions/blood-tests/
- Hoffbrand AV, Moss PAH. Hoffbrand’s Essential Haematology. 8th ed. Wiley-Blackwell; 2024.
- Institute of Biomedical Science. Haematology. https://www.ibms.org/go/nm:haematology
- World Health Organization. Classification of Tumours of Haematopoietic and Lymphoid Tissues. 5th ed. IARC; 2022.