Macrocytosis is a term used to describe erythrocytes that are larger than normal, typically reported as mean cell volume (MCV) greater than 100 fL. The amount of hemoglobin increases proportionately with the increase in cell size. Therefore, if the increase in MCV is not related to macrocytic anemia, the mean cell hemoglobin concentration (MCHC) also increases in proportion. Increased MCV is an indication of macrocytic anemia when any of the following is present  :
The most common cause of macrocytic anemia is megaloblastic anemia, which is the result of impaired DNA synthesis. Although DNA synthesis is impaired, RNA synthesis is unaffected, leading to a buildup of cytoplasmic components in a slowly dividing cell. This results in a larger-than-normal cell. The nuclear chromatin of these cells also has an altered appearance. 
Vitamin B-12 and folate coenzymes are required for thymidylate and purine synthesis; thus, their deficiency results in retarded DNA synthesis. In vitamin B-12 deficiency and folic acid deficiency, the defect in DNA synthesis affects other rapidly dividing cells as well, which may be manifested as glossitis, skin changes, and flattening of intestinal villi.
DNA synthesis may also be delayed when certain chemotherapeutic agents are used, including folate antagonists, purine antagonists, pyrimidine antagonists, and even folate antagonist antimicrobials.
Hydroxyurea, an agent now commonly used to decrease the number of vaso-occlusive pain crises in patients with sickle cell disease, interferes with DNA synthesis, causing macrocytosis by which compliance with therapy may be monitored. Patient compliance with zidovudine, an agent used in the treatment of patients with HIV infection, may be monitored in the same way.
Sternfeld et al, in a study using the 13 C-methionine breath test to analyze hepatic mitochondrial function in vivo in antiretroviral-treated HIV-infected patients with macrocytosis, found a significantly negative correlation between mean corpuscular erythrocyte volume and the breath test results.  They concluded that there is an association between an increase in mean corpuscular erythrocyte volume from treatment with nucleoside reverse transcriptase inhibitors and the hepatic mitochondrial function in vivo.
Nonmegaloblastic macrocytic anemias are those in which no impairment of DNA synthesis occurs. Included in this category are disorders associated with increased membrane surface area, accelerated erythropoiesis, alcoholism, and chronic obstructive pulmonary disease (COPD).
Patients with hepatic disease and obstructive jaundice have macrocytosis that is secondary to increased deposition of cholesterol or phospholipids on the membranes of circulating red blood cells (RBCs). Similarly, in splenectomized patients, RBC membrane lipids that usually are removed during maturation in the spleen are not effectively removed, and the result is a larger-than-normal cell.
In patients with hemolytic anemia or posthemorrhagic anemia, the reticulocyte count increases. The reticulocyte, an immature RBC, is approximately 20% larger than the more mature RBC. When the reticulocyte is released prematurely from the marrow, its volume is averaged with the volume of the more mature RBC, and the resultant MCV is increased.
Macrocytosis, sometimes without associated anemia, is often evident in persons with chronic alcoholism. [8, 9] Although the macrocytosis of alcoholism may be secondary to poor nutrition with a resulting folate or vitamin B-12 deficiency, it is more often due to direct toxicity of the alcohol on the marrow. The macrocytosis of alcoholism usually reverses only after months of abstinence from alcohol.
The macrocytosis associated with COPD is attributed to excess cell water that is secondary to carbon dioxide retention.
A murine study found that disruption of the Gardos channel (the erythrocyte Ca 2+ -activated K + channel [KCa3.1]) caused subtle erythrocyte macrocytosis and led to mild but progressive splenomegaly.