|Classification and external resources|
Macrocytosis is the enlargement of red blood cells with near-constant hemoglobin concentration, and is defined by a mean corpuscular volume (MCV) of greater than 100 femtolitres (the precise criterion varies between laboratories). The enlarged erythrocytes are called macrocytes or megalocytes (both words have roots meaning "big cell").
Poor absorption of vitamin B12 in the digestive tract can also cause macrocytosis.
Gastrointestinal diseases that may cause macrocytosis include celiac disease (severe sensitivity to gluten from wheat and other grains that causes intestinal damage) and Crohn’s disease (inflammatory bowel disease that can affect any part of the gastrointestinal tract). (Source healthgrades.com)
Other causes may include:
- megaloblastosis (vitamin B12 or folate deficiency; or DNA synthesis-inhibiting drugs)
- chronic obstructive airway disease
- aplastic anemia
- reticulocytosis (commonly from hemolysis or a recent history of blood loss).
- liver disease
- myeloproliferative disease
- myelodysplastic syndrome which most commonly presents with macrocytic anemia 
- chronic exposure to benzene
- pregnancy (most common, and requires no treatment as the person affected will return to normal post-partum)
- Greenberg, P. L., et al. "Myelodysplastic syndromes: clinical practice guidelines in oncology." Journal of the National Comprehensive Cancer Network: JNCCN 11.7 (2013): 838-874.
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