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What is Anemia?

Anemia results from a decreased number of red blood cells (RBCs) or a decreased quantity of hemoglobin in the blood. It is the most common blood disorder.1-3

Classification of Anemia

Classification of anemia can be made by studying the morphology of RBCs, the underlying etiology, and the clinical symptoms. Based on the underlying etiology, anemia can be classified into two main types: anemia caused by the inadequate production of RBCs, and anemia caused by the excessive loss of RBCs (Table).1

Table. Etiological classification of anemia1

INADEQUATE production of red cells
  • Caused by inadequate "raw materials"
    • Iron deficiency
    • Vitamin B12 deficiency
    • Folic acid deficiency
  • Caused by impaired function of bone marrow factory
    • Anemia of chronic disease
    • Bone marrow damaged or destroyed (aplastic anemia)
    • Bone marrow replaced by foreign or abnormal cells (bone marrow replacement anemia)

EXCESSIVE loss of red cells
  • Caused by external blood loss (hemorrhage)
  • Caused by shortened survival of red cells in the circulation
    Defective red cells (hereditary hemolytic anemia)
    • Abnormal red cell shape
    • Abnormal hemoglobin within red cells
    • Defective hemoglobin synthesis within red cells
    • Deficient red cell enzymes
  • "Hostile environment"
    • Anti-red cell antibodies
    • Mechanical trauma to circulating red cells

Reproduced from Crowley LV. Essentials of human disease. Sudbury, MA, USA. Jones and Bartlett Publishers; 2011. pp. 226-7 © 2011 Jones and Bartlett Publishers.

Anemias Caused by Impaired Bone Marrow Function

One of the reasons for the inadequate production of RBCs is the impaired functioning of the bone marrow. Different types of anemia belong to this class and include:

Learn more about types of aplastic anemia >


  1. Crowley LV. Essentials of human disease. Sudbury, MA, USA: Jones and Bartlett Publishers; 2011.
  2. Center for Disease Control (CDC). Blood Disorders. Available at: Accessed April 2014.
  3. World Health Organization (WHO). Worldwide Prevalence of Anaemia 1993-2005. Available at: Accessed April 2014.

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