Red blood cells, also called erythrocytes, carry oxygen from the lungs throughout the body using the circulatory system. Various medical conditions can affect these cells, causing small red blood cells, enlarged red blood cells, and also affecting their shape and color. The most common cause of small cells is iron deficiency anemia, which is usually the result of insufficient iron intake, excessive bleeding, or problems with iron absorption. The medical term used when it is caused by anemia is microcytic anemia, and treatment varies depending on the underlying cause. There are also some rare blood disorders that can cause small red blood cells such as spherocytosis, various forms of thalassemia, and sideroblastic anemia.
Anemia and other blood disorders are commonly diagnosed using a complete blood count, or CBC. This blood test examines red blood cells in detail, using various red blood cell indices. For example, the number of blood cells and the variation in their size are observed. To ensure proper diagnosis and treatment, it is important to see a medical professional if a blood test shows small red blood cells.
Iron deficiency anemia is usually caused by a lack of iron in the diet; from blood loss, for example from a stomach ulcer or childbirth; or by iron malabsorption, sometimes caused by stomach disorders such as Crohn's disease or celiac disease. The lack of iron causes fewer red blood cells to be produced, and the cells that do form are small and pale due to the lack of hemoglobin. Hemoglobin is what gives red blood cells their color, and it's also what makes them capable of carrying oxygen. Iron is essential for the formation of hemoglobin. The most common treatment for iron deficiency anemia is iron supplements, but other medications are sometimes needed to treat the cause.
Other blood disorders can also cause small red blood cells, although these conditions are rare. One example is sideroblastic anemia, which is not caused by a lack of iron, but by problems with the body's production of hemoglobin, resulting in fewer, small, pale-colored red blood cells. Spherocytosis is another rare genetic blood disorder that causes red blood cells to be small and abnormally shaped. Various forms of thalassemia, sometimes called Cooley's anemia, can also cause small, pale, and fragile red blood cells. Thalassemias are generally inherited and vary in severity.