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What is anemia?

Anemia is a condition that occurs when the number of red blood cells (RBCs) and/or the amount of hemoglobin found in the red blood cells drops below normal. Red blood cells and the hemoglobin contained within them are necessary to transport and deliver oxygen from the lungs to the rest of the body. Without a sufficient supply of oxygen, many tissues and organs throughout the body can be adversely affected.

Anemia is a fairly common condition, affecting both men and women of all ages, races, and ethnic groups. However, certain people have increased risk of developing anemia. These include people with diets poor in iron and vitamins, chronic diseases such as kidney disease, diabetes, cancer, inflammatory bowel disease, a family history of inherited anemia, chronic infections such as tuberculosis or HIV, and those who have had significant blood loss from injury or surgery. Anemia can be mild, moderate, or severe depending on how much the RBC count and/or hemoglobin levels are decreased.

In general, the main causes of anemia include:

  • Impaired or decreased production of RBCs by the bone marrow due to nutritional deficiency (e.g., iron deficiency, B vitamin deficiencies), bone marrow failure (e.g., aplastic anemia, myelodysplastic syndrome), or diseases that involve the bone marrow (e.g., infection, lymphoma, solid tumor)
  • Loss of RBCs due to bleeding or to increased destruction of RBCs as in hemolytic anemia

Anemia may be acute or chronic. Chronic anemia may develop slowly over a period of time with long-term illnesses such as diabetes, chronic kidney disease, or cancer. In these situations, the anemia may not be apparent because the underlying disease masks its symptoms. The presence of anemia in chronic conditions may often go undetected for a period of time and sometimes may only be discovered during tests or examinations for other conditions.

Anemia may also occur in acute episodes such as with substantial blood loss (extensive injury or invasive surgery) or with certain anemias in which a significant number of RBCs are destroyed known as hemolytic anemia. Signs and symptoms may become apparent very quickly, and the cause can be determined from a combination of physical examination, medical history, and testing.

Anemias can also be described based on the RBC size and concentration of hemoglobin in them. If cell size is much smaller than normal, it is known as microcytic anemia. If it is much bigger than normal, then it is macrocytic anemia. Likewise, if the concentration of hemoglobin is much lower than normal, it is hypochromic anemia; if the concentration is much higher than normal, the RBCs are called hyperchromic.

Within the two broad categories of general causes of anemia not associated with bleeding, there are several types with different specific causes. Some of the most common types are summarized in the table below. Click on the links to read more about each one.

Type of Anemia Description Examples of Causes
Iron Deficiency Deficiency of iron leads to decreased amounts of hemoglobin; low levels of hemoglobin in turn leads to decreased production of normal RBCs Blood loss; diet low in iron; poor absorption of iron
Pernicious Anemia and B Vitamin Deficiency RBCs do not develop as they normally would because of a lack of B vitamins (B12 and folate); leads to decreased production of RBCs Lack of intrinsic factor (needed for B12 absorption); diet low in B vitamins; decreased absorption of B vitamins
Aplastic Bone marrow is unable to produce enough blood cells; a life-threatening condition Cancer therapy, exposure to toxins, autoimmune disease, viral infections
Hemolytic RBCs are destroyed faster than the bone marrow can replace them Inherited causes such as sickle cell disease and thalassemia; other causes include transfusion of incompatible blood, autoimmune disease, certain drugs (penicillin)
Anemia of Chronic Diseases Various conditions over the long term can cause decreased production of RBCs Kidney disease, rheumatoid arthritis, diabetes, tuberculosis, HIV, Crohn disease, cancer, and others

This article only addresses some of the more common causes of anemia, especially those that are not typically associated with white blood cell and platelet abnormalities. There are many more conditions that can, for various reasons, result in some level of anemia, such as:

  • Bleeding—significant bleeding resulting from, for example, trauma or surgery (acute) or from gastrointestinal bleeding (ulcers) occurring over time (chronic)
  • Leukemia (acute or chronic)
  • Lymphoma
  • Myelodysplastic syndrome
  • Multiple myeloma
  • Myeloproliferative neoplasms
  • Infections (e.g., HIV)

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