According to the Mayo Clinic, enlarged red blood cells, or macrocytosis, is a non-specific medical condition that has a number of different causes. Among the most common are vitamin B12 deficiency, folate deficiency, alcoholism, hypothyroidism (under-active thyroid) and liver disease. Other causes include increased red blood cell production secondary to acute blood loss or the side-effects of medications such as those used to treat cancer.
According to the American Society of Hematology, red blood cells are the largest component of human blood, making up 40 to 45 percent of its volume. They get their bright red color from the protein hemoglobin, which is responsible for transporting oxygen to the body’s cells. Under normal circumstances, red blood cells are a uniform shape and size, the latter of which is defined as the mean corpuscular volume or MCV. According to American Family Physician, when the MCV is greater than 100, macrocytosis exists.
The most common cause of macrocytosis is vitamin B12 deficiency or pernicious anemia, a disorder in which the cells of the intestine do not properly absorb Vitamin B12. In some cases, particularly in people with alcoholic liver disease, it also may be caused by poor nutrition. In those cases, it generally is accompanied by folate deficiency as well.
Medications may also cause macrocytosis. In recent years, a large increase in the number of patients diagnosed with this condition has been attributed to the reverse transcriptase inhibitors such as stavudine , lamivudine and zidovudine that are used in the treatment of HIV. In older persons, myeloproliferative disorders, also known as refractory anemia, may cause macrocytosis as well. Less severe elevations in the MCV are sometimes related to kidney, liver, thyroid, or chronic obstructive pulmonary disease.