What Is a Hematologist?

A hematologist is a medical doctor who specializes in the diagnosis, treatment and prevention of diseases and disorders of the blood, including malignancies, hemophilia, leukemia, lymphoma, and sickle-cell anemia. Hematologists are involved in the physiology, pathology, etiology, diagnosis, treatment, prognosis and prevention of blood-related diseases and disorders, including immunologic, hemostatic (blood clotting) and vascular systems. They may work in hospitals, blood banks, pathology laboratories, or private clinics.

Hematology is the study of blood in health and disease. It includes problems with the red blood cells, white blood cells, platelets, blood vessels, bone marrow, lymph nodes, spleen, and the proteins involved in bleeding and clotting (hemostasis and thrombosis) (ASH, 2017). A hematologist is a medical doctor who applies this specialized knowledge to treat patients with blood conditions. Some hematologists may specialize in lymphatic organs and bone marrow, in order to diagnose blood and platelet abnormalities. Lymphatic organs include lymph nodes, the spleen, thymus, and lymphoid tissue.

Hematologists who work in blood banks maintain its safety and accessibility, in addition to supervising analysis of blood samples and assisting patients with genetic blood disorders. They work closely with surgeons, oncologists and other medical specialists to provide comprehensive medical care to their patients. A hematopathologist works in a laboratory setting, analyzing blood samples to diagnose hematological diseases.

Other hematologists may work directly with patients, performing and analyzing blood testing to identify complete blood count (CBC), bone marrow testing or biopsy, or other factors that may identify disorders of the blood. They may perform any of the following (ASH, 2017):

A hematologist may perform the following:

  • Examine and diagnose disorders and diseases of the blood, bone marrow, vascular and immune system.
  • Consult with patients to determine the appropriate course of treatment
  • Recommend and order tests to determine extent of illness or diagnose condition
  • Research diseases and treatments associated with red blood cells, white blood cells, platelets, blood vessels, bone marrow, lymph nodes, spleen, and proteins for involved with bleeding and clotting.
  • Hematologists may treat patients who have anemia, blood disorders such as hemophilia and blood clot, or blood cancers such as leukemia, lymphoma and myeloma.

Blood Diseases and Disorders

Blood cell disorders may involve the red blood cells, white blood cells, platelets, or any combination of the three. Each type of blood cell has its own unique function. For example, red blood cells transport oxygen to and from organs and tissue. White blood cells assist in resisting infections, and platelets have a huge role in clotting. Regardless of the type, all blood cells originate in bone marrow, which is the soft tissue inside the bone. A blood cell disorder may affect the formation or function of the blood cells.

Blood clotting, or coagulation, is an important process that prevents excessive bleeding when a blood vessel is injured, and platelets (a type of blood cell) and proteins in your plasma (the liquid part of blood) work together to stop the bleeding by forming a clot over the injury (ASH, 2017). Clots can form in the veins or arteries, causing damage to the entire circulatory system.

Symptoms of Blood Disorders

Although the symptoms will vary based on the type of disorder or disease, there are some common symptoms noted by hematologists. Typical symptoms of a red blood cell issue are fatigue, shortness of breath, difficulty concentrating, muscle weakness and fatigue, and an increased heart rate. Most symptoms of red blood cell disorders are due to the lack of oxygen in the blood and brain. Symptoms of white blood cell disorders include chronic infections, fatigue, unexplained weight loss, and a general un-well feeling. Platelet disorders present in symptoms such as cuts or wounds that do not heal properly, failure of blood clotting, skin that bruises easily, nosebleeds, or bleeding from the gums.


Red Blood Cell Disorders


Types of Anemia

Anemia is one of the most common red blood cell disorders seen by hematologists. It is typically caused by a lack of iron in the blood, which leads to a reduction of hemoglobin production. Hemoglobin plays a significant role in the transportation of oxygen throughout the body. There are many different types of anemia, and nearly all of them are caused by mineral deficiencies in the blood, although some are genetic or caused by other factors.

Iron or mineral-deficient

Iron-deficiency anemia is due to a lack of iron in the blood. Iron deficiency is usually due to blood loss but may occasionally be due to poor absorption of iron (ASH, 2017). Typical symptoms of iron-deficiency anemia include fatigue and shortness of breath due to the lack of oxygen in the lungs (because of the decreased oxygen in the blood reaching the lungs). In most cases, a mineral iron supplement will resolve the deficiency.

Pernicious anemia

Pernicious anemia is an auto-immune disorder that prevents the absorption of vitamin B-12 into the gastrointestinal tract. This inhibits the production of red blood cells, which can cause severe damage throughout the body’s tissue and organs. Prior to the introduction of B-12 injections, this disease was often fatal, because there was no treatment available. Vitamin-deficiency anemia is also the result of low levels of B-12 or folic acid.

Aplastic Anemia

Aplastic anemia is a rare bone marrow failure disorder in which the bone marrow stops making enough blood cells, and it occurs as a result of destruction or deficiency of blood-forming stem cells in your bone marrow, in particular when the body’s own immune system attacks the stem cells (ASH, 2017). Although production may not cease completely, the few blood cells the marrow does make are normal. Viral infections, ionizing radiation, and exposure to toxic chemicals or drugs can also result in aplastic anemia. Symptoms of this disease include prolonged infections that are difficult to overcome, uncontrolled bleeding, and fatigue.


Autoimmune Hemolytic Anemia

Autoimmune hemolytic anemia (AHA) is an immune system disorder in which the red blood cells are destroyed by the immune system faster than the blood cells can be replaced, which results in a low red blood cell count. Hemolytic anemia may be due to mechanical causes (leaky heart valves or aneurysms), infections, autoimmune disorders, or congenital abnormalities in the red blood cell (ASH, 2017). Some inherited abnormalities may affect the formation or function of hemoglobin or red blood cells.

Sickle Cell Anemia

Approximately 70,000 to 100,000 Americans have sickle cell anemia, the most common form of an inherited blood disorder that causes the production of abnormal hemoglobin, a protein that attaches to oxygen in the lungs and carries it to all parts of the body (ASH, 2017). Sickle cell anemia is caused by a genetic mutation, which causes the production of abnormal hemoglobin proteins that are shaped differently than normal hemoglobin proteins, as they have a sickle shape that is more curved and rigid. This abnormal formation causes a reduction in the amount of oxygen in the blood, and it causes obstruction of the blood vessels, which can prevent blood from reaching certain tissues and organs.


Thalassemia are blood disorders due to genetic mutations, similar to sickle-cell anemia. Thalassemia can cause bone deformities, enlarged spleen, heart problems, and growth and developmental disorders in children. Polycythemia is a blood cancer caused by a genetic mutation, and it involves over-production of red blood cells in the bone marrow, which causes thickening of the blood. This can cause a reduction in the rate of blood flow throughout the body, increasing the risk of clotting, heart attack and stroke.

White Blood Cell Disorders

Disorders or diseases of the white blood cells affect the response of the immune system and its ability to fight off infections. Lymphoma, leukemia, and myelodysplastic syndrome (MDS) are all disorders involving white blood cells.

Lymphoma is a blood cancer originating in the lymphatic system. With lymphoma, the white blood cells structure is altered, and they begin to continuously replicate, similar to cancer cells, growing out of control. Abnormal lymphocytes become lymphoma cells, which multiply and collect in your lymph nodes and other tissues. Over time, these cancerous cells impair your immune system (ASH, 2017). Hodgkin’s and non-Hodgkin’s lymphoma are two of the most common types of lymphoma.

Leukemia is another type of cancer of the blood, and it may be acute or chronic. Malignant white blood cells begin to uncontrollably replicate in the bone marrow. The high number of abnormal white blood cells are not able to fight infection, and they impair the ability of the bone marrow to produce red blood cells and platelets (ASH, 2017).

Myelodysplastic syndrome (MDS) is another condition involving white blood cell disorders. Blasts, or immature blood cells, are produced in the bone marrow, where they begin to multiply, thus overtaking the healthy, mature white blood cells. MDS can sometimes lead to leukemia, and its progression varies by patient – some progress slowly while in others, it progresses more quickly.

Platelet Disorders

Platelet disorders may result in several different abnormalities: reduced platelet production, over-production of platelets, or deformities in the platelet structure that prevent proper clotting. These abnormalities can cause severe loss of blood due to the lack of clotting, and over-production can lead to over-clotting, which can block arteries and vessels causing heart attack or stroke. Most platelet orders are genetic in nature.

Von Willebrand disease is one of the most common inherited platelet disorders. This disease is caused by a lack of protein, which reduces the platelet’s ability to properly clot the blood. Hemophilia only affects males, and it is a blood-clotting disorder that can result in excessive, prolonged bleeding both inside and outside of the body. Primary thrombocythemia involves the over-production of platelets, which can lead to excessive clotting.

Plasma Cell Disorders

Plasma is part of a white blood cell that produces the antibodies used to fight viral and bacterial infections. Plasma cell myeloma is a rare cancer of the plasma cells in the bone marrow. Plasmacytomas are malignant plasma cells that accumulate in the bone marrow and become tumors, and they are typically formed in the spine, hip, or rib bones. Plasma cell myeloma can result in thickened blood and severe kidney damage. Myeloma cells prevent the normal production of antibodies, leaving your body's immune system weakened and susceptible to infection (ASH, 2017).

History of Hematology

The beginnings of hematology occurred in Europe in the 19th century, and each European country seems to have its own "father of hematology." For example, Hewson in England, Hayem in France, and Ehrlich in Germany are all known as the father of hematology in their respective countries. Although he was born in Canada, Osler is known as the American father of hematology, due to his microscopic evaluation of blood cells during his time at the Johns Hopkins clinic. He is responsible for establishing the Division of Clinical Microscopy at Johns Hopkins as well, and he was one of the first to define platelets and describe their function (Coller, 2015).

Osler published his textbook, The Principles and Practice of Medicine, in 1892. In this written work, he describes diseases of the blood including anemia, hemorrhage, renal insufficiency, inanition and heavy metal toxins and their effect on the blood and kidney. He also describes iron-deficiency as a contributing factor to anemia, as well as descriptions of leukemia, Hodgkin disease, hemophilia, and white blood cell abnormalities.

The discovery of blood cell staining using dyes for microscopic analysis led to an explosive growth in hematologic investigation, resulting in the production of numerous books and atlases of blood cell morphology in health and disease (Coller, 2005). The staining method led to the differentiation of leukocyte counts and precursors to platelet production. The first sickle cell was identified in 1910, and over a decade later, the first case of thalassemia was identified.

The hemocytometer was developed in the early 1900s, and it allowed for the quantification of red blood cells in the blood. Although reproducibility was challenging, this development led to the discovery that clinical hemorrhage, low platelet counts, and prolonged bleeding were inter-related. In addition, whole blood transfusions led to an increase in the platelet blood count, which stopped the prolonged bleeding.

The most dramatic and far reaching event in hematology in the United States in the pre-Blood period was Minot and Murphy’s 1926 report that feeding liver to patients with pernicious anemia could cure this otherwise fatal disorder (Coller, 2015). This discovery led to experimentation with vitamin B-12 and the measurement of patient’s response of increased reticulocyte production.

Blood Coagulation Discoveries

Theories of blood coagulation go back at least to the Greeks, but Hewson in 1770 in London was the first to develop a method of separating plasma from the formed elements in blood using a trick known by those who made blood sausage that high concentrations of salt impeded coagulation (Gulliver, 1846, Coller, 2015). He went on to discover fibrinogen, which only needed plasma to form. European scientists discovered that enzymes such as serum and thrombin could clot the fibrinogen.

In 1916, a medical student working at Johns Hopkins, isolated a fraction from hepatic tissue that displayed a strong affinity to the inhibition of coagulation. Jay McLean named this marker heparin. The clinical use of heparin began just 30 years after this discovery.

Platelet Research

Several qualitative platelet disorders were described by the early 1900s, due to the discovery of their role in both hemostasis and thrombosis using intravital microscopy by an Italian scientist known as Bizzozero (Coller, 2015). The significant role platelets have in coagulation and clotting was discovered in 1936 and was further expanded in 1947. In 1926, Finland researcher von Wellebrand described the disorder that bears his name, in which inherited defects in platelet function resulted in a different disease than hemophilia, because it also affected females.

White Blood Cells (Leukocytes) and Their Function

In addition to his contributions to coagulation, Hewson also performed landmark studies of the lymphatic system, including delineating the anatomy and dual circulation of lymph nodes, noting the involution of the thymus with age, describing lymphocytes in lymph (chyle), and postulating that the lymphatics drained body cavities (Gulliver, 1846). In the 19th century, there were differing opinions in regard to the circulatory nature of white blood cells, although they were known to be present in pus.

A Russian scientist was the first to recognize the phagocytic ability of leukocytes, and the role they have in protecting the blood from infections. This was a significant discovery, because previous theories identified white blood cells as compounding infections by harboring organisms and allowing them to grow. In 1926, lymphocytes were identified as a factor in preventing the growth of tumors.

Hematology in World War II and Its Aftermath

During the second World War, blood collection, preservation, and transfusion became the top priority for hematologists, particularly for treating hemorrhagic shock. Albumin was used to treat hemorrhagic shock on the battlefield, which dramatically reduced the number of fatalities due to this trauma, and it also significantly advanced knowledge of the protein chemistry of the blood.

The traumas of war also led to an increase in the treatment of burns, including skin grafts. One of the greatest challenges with skin grafting at this time was the risk of infection, and the understanding of the role of lymphocytes led to advancements and successes in skin grafting. Radiation biology was also studied to identify the effects of possible radiation exposure. Platelet counting methods were used to identify radiation doses, which then led to studies of effects in the bone marrow and the benefits of bone marrow transplants.

How to Become a Hematologist

To become a hematologist, an undergraduate degree, medical school, and a residency program are required, although many students also complete a fellowship program, particularly those who wish to practice in a specialized hematology field. Specialized fields of hematology include coagulation, adult hematology, oncology, pediatrics, and pathology.

Undergraduate Degree

While there is not specific degree required for medical school acceptance, most aspiring hematologists major in a pre-medical field, chemistry, biology or physics, although some may pursue a social science degree, such as sociology. Students must take and pass the Medical College Admissions Test (MCAT) to be accepted into an accredited medical school. A high MCAT score, a high GPA, and participation in multiple extracurricular activities are all recommended to increase the chances of acceptance, as well as volunteering or working at a hospital or clinical center.

The MCAT will test a students critical thinking and problem-solving skills, in addition to their knowledge of physical sciences and their writing ability. Medical schools do have a minimum MCAT score requirement, so this is essential for acceptance. Most students take the MCAT during the third year of their undergraduate program.

Medical School

Medical consists of four years of education and training that will result in a Doctorate of Medicine (M.D.) or Doctor of Osteopathy (D.O.) degree. A D.O. degree typically includes a one-year internship before medical school is completed. The first two years of medical school consist of coursework in subjects such as biochemistry, anatomy, psychology, pharmacology, medical law and ethics. During the last two years, students will participate in supervised hospital rotations where they practice examining, diagnosing and treating patients. Rotations allow students to be exposed to a variety of medical specialty fields including internal medicine, pediatrics, cardiology, and many more.

In addition to a medical degree, the United States Medical Licensing Exam (USMLE) must be passed for a hematologist to practice medicine in any state, although most states require completion of a residency program as well.

Residency and Fellowship Programs

When medical school is completed, the physician will begin a residency program that may last anywhere between three to five years. A future hematologist will typically focus on pediatrics, internal medicine, or pathology, depending on their choice of future specialty field. During the residency, the doctor will work with other physicians and hematologists to examine, diagnose, and treat patients.

A fellowship allows for an additional two to three years of clinical hematology training, in addition to training in hematology specialty fields, such as pediatrics or oncology. The fellowship must be accredited by the Council for Graduate Medical Education (ACGME).

The American Board of Internal Medicine (ABIM) is the certifying board for Hematology. Hematologists must first become certified in internal medicine before they can apply for Hematology certification. Certification is not required to practice hematology, but it demonstrates proficiency in the field.



ASH – American Society of Hematology. Blood Disorders. 2017. Retrieved November 21, 2017 from: http://www.hematology.org/Patients/Blood-Disorders.aspx

Coller, Barry S. Blood at 70: its roots in the history of hematology and its birth. Blood 2015, 126:2548-2560.

Gulliver G. London, England: The Sydenham Society; 1846. ed. The Works of William Hewson, F.R.S

ABIM – American Board of Internal Medicine. Hematology Policies. 2017. Retrieved November 22, 2017 from: http://www.abim.org/certification/policies/internal-medicine-subspecialty-policies/hematology.aspx


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