The size of the red blood cells helps classify the type of anemia.
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The term anemia, meaning вЂњno blood,вЂќ is used to describe a number of disorders that result from having too few healthy red blood cells in your circulation. Over the years, scientists have learned the average size of your red blood cells offers a clue to the underlying cause of your anemia. So, anemias can be classified as microcytic, normocytic or macrocytic, depending on whether your red blood cells are smaller than normal, of normal size or larger than normal, respectively. While these categories overlap somewhat, this classification system allows quick diagnosis of most people's anemia.
Small Red Blood Cells
The most common cause of microcytic anemia in the U.S. is iron deficiency, according to a 2010 review in вЂњAmerican Family Physician.вЂќ Iron deficiency anemia occurs in people who do not get enough iron in their diet, who do not absorb iron well or whose iron stores are depleted due to blood loss.
Anemia from chronic disease, such as that occurring in people with rheumatoid arthritis, and sideroblastic anemia, which can be inherited or develop as the result of bone marrow disease, are relatively common forms of microcytic anemia. Both are thought to result from inefficient use of available iron stores. Lead poisoning and thalassemias -- genetic disorders that interfere with red blood cell production -- are also associated with microcytic anemia.
Normal-Sized Red Blood Cells
Many disorders are associated with normocytic anemia, which is characterized by a decreased number of normal-sized red blood cells or red blood cells that contain less hemoglobin than usual. Hemoglobin is the oxygen-carrying protein that is packed into your red cells when they are manufactured in your bone marrow. If your red blood cells' hemoglobin content is decreased, oxygen delivery to your tissues may be impaired. Conditions that cause sudden blood loss or increased destruction of red blood cells are frequent causes of normocytic anemia. Pregnancy, which causes your blood volume to expand without a proportional increase in red blood cell numbers, often causes normocytic anemia.
Sickle cell disease, spherocytosis, elliptocytosis, glucose-6-phosphate dehydrogenase deficiency and pyruvate kinase deficiency are inherited disorders that accelerate red blood cell destruction. Liver disease, bone marrow failure, multiple myeloma, leukemia, thyroid disease and kidney failure are acquired causes of normocytic anemia. Some of these conditions may also cause microcytic or macrocytic anemia. In addition to causing anemia, many of these disorders, such as sickle cell disease, spherocytosis and leukemia, generate oddly shaped red blood cells or are accompanied by white blood cell abnormalities. These anomalies can be detected during examination of your blood or bone marrow under a microscope.
Large Red Blood Cells
The most common causes of macrocytic anemia are vitamin B12 and folate deficiencies, alcoholism and the use of certain medications, such as methotrexate, HIV drugs, phenytoin (Dilantin) or metformin (Glucophage). Elderly people frequently develop macrocytic anemia due to atrophic gastritis, a stomach condition that impairs absorption of vitamin B12. Hypothyroidism -- low thyroid function -- liver disease, kidney disease and chronic obstructive pulmonary disease are also causes of macrocytic anemia. Although unusual, copper or vitamin C deficiencies can cause macrocytic anemia, as both nutrients are involved in manufacturing hemoglobin.
While classifying anemias on the basis of red blood cell size has certain advantages, this system has its drawbacks. Many conditions, such as lead poisoning, iron deficiency, thyroid disease or spherocytosis, may cross from 1 classification to another, depending on how long the anemia has been established. So, many doctors prefer to classify anemias based on their cause. Such a system might separate anemias into those caused by acute or chronic blood loss, deficient red blood cell production or excessive red blood cell destruction. Still other systems combine the cause and red blood cell size to classify anemias. Regardless of the system used, treatment for anemia is directed at the underlying cause, once it is identified.