Macrocytosis

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. ^[7]

Vitamin B12 and folate coenzymes are required for thymidylate and purine synthesis; thus, their deficiency results in retarded DNA synthesis. In vitamin B12 deficiency and folic acid deficiency, the defect in DNA synthesis affects other rapidly dividing cells as well, which may manifest 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 ¹³C-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. ^[8] 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, alcohol use disorder, and chronic obstructive pulmonary disease (COPD).

Patients with liver 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 alcohol use disorder. ^{[9, 10]} Although the macrocytosis in these cases may be secondary to poor nutrition with a resulting folate or vitamin B12 deficiency, it is more often due to direct toxicity of the alcohol on the marrow. The macrocytosis of alcohol use disorder usually reverses only after months of abstinence from alcohol.

Vitamin B12 deficiency is a cause of macrocytosis. Because DNA synthesis requires cyanocobalamin (vitamin B12) as a cofactor, a deficiency of the vitamin leads to decreased DNA synthesis in the erythrocyte, thus resulting in macrocytosis. A dietary deficiency of vitamin B12 is rare and usually only occurs in elderly persons on a "tea-and-toast diet" or in strict vegan vegetarians. However, deficiency can result from the following:

• Lack of intrinsic factor in patients who have undergone gastrectomy or who have pernicious anemia

• Malabsorption of vitamin B12 secondary to small bowel bacterial overgrowth, tapeworm, familial factors, drugs, ileal bypass, ileal enteritis, or sprue

Folate also is needed as a cofactor in the synthesis of DNA. Folate deficiency may be caused by any of the following:

• Dietary deficiency
• Increased requirements of pregnancy
• Congenital deficiency
• Sprue
• Alcohol use disorder
• Increased turnover due to conditions such as hemolysis or sickle cell disease, among others

Inherited disorders of DNA synthesis include the following:

• Lesch-Nyhan syndrome
• Deficient enzymes for folate metabolism
• Homocystinuria

Other genetic conditions associated with macrocytosis include the following:

• Benign familial macrocytosis is an inherited syndrome in which patients have mild asymptomatic macrocytosis. ^[11]
• Persons with Down syndrome are at higher risk for macrocytosis. The risk increases with age. ^[12]
• Large single mitochondrial DNA (mtDNA) deletion syndrome is a rare inborn error of metabolism that most commonly manifests as chronic progressive external ophthalmoplegia in adults, but may also be associated with macrocytosis without anemia. ^[13]  
• Mild macrocytosis has been reported in Williams-Beuren syndrome, a rare condition resulting from deletion of 26 to 28 genes on chromosome 7, including the elastin gene. ^[14]
• VEXAS (Vacuoles, E1 enzyme, X-linked, Autoinflammatory, Somatic) syndrome is a newly identified adult-onset autoinflammatory disease caused by somatic alterations in UBA1. Macrocytic anemia/macrocytosis features prominently in VEXAS syndrome, along with a wide spectrum of clinical manifestations, monocytopenia, thrombocytopenia, and bone marrow vacuolization of erythroid or myeloid precursors. ^[15]

Drug-induced macrocytosis is the most common cause in nonalcoholic patients. Usually, no associated anemia is present. The following categories of drugs are known to cause macrocytosis:

• Folate antagonists (eg, methotrexate ^[16] )
• Purine antagonists (eg, 6-mercaptopurine [6-MP])
• Pyrimidine antagonists (eg, cytosine arabinoside [ara-C])
• Alkylating agents (eg, cyclophosphamide)
• Tyrosine kinase inhibitors (eg, sunitinib, imatinib) ^[17]
• Zidovudine (AZT)
• Trimethoprim
• Oral contraceptive pills
• Phenytoin
• Metformin ^[18]
• Cyclin-dependent kinases 4,6 (CDK4/6) inhibitors (abemaciclib, palbociclib, and especially ribociclib) ^[19]

The tyrosine kinase inhibitors sunitinib and imatinib have been shown to induce macrocytosis when used in patients with a variety of cancers, including renal cell carcinomas (RCCs), gastrointestinal stromal tumors (GISTs), and breast cancer. ^[17] In patients with RCC, the development of macrocytosis following the institution of sunitinib treatment may potentially serve as a positive prognostic factor for overall survival. ^[20] Similarly, macrocytosis in patients with breast cancer treated with a CDK4/6 inhibitor has been linked with improved survival. ^[21]

Reticulocytosis may be due to posthemorrhagic blood loss or hemolysis. Reticulocytes are immature red cells released in response to decreased hematocrit levels.

Long-term alcohol abuse can result in macrocytosis through a direct effect on bone marrow. This effect is not related to the presence of liver disease or vitamin deficiency and resolves only after months of abstinence from alcohol. In addition, persons with alcohol use disorder may have secondary megaloblastosis from vitamin B12 and folate deficiency due to poor nutritional intake.

Heroin users have been found to have rates of macrocytosis 5 to 10 times higher than those in the general population. However, a study of 958 patients with a heroin use disorder entering methadone maintenance treatment found that high rates of comorbid alcohol use were more likely to be the cause of macrocytosis in this population. ^[22]  

Refractory anemias of the following types may cause macrocytosis:

• Myelodysplastic anemias
• Myelophthisic anemias (marrow replacement by neoplasm, granuloma, or fibrosis)
• Aplastic anemia
• Acquired sideroblastic anemia

Macrocytosis in patients with COPD is attributed to excess cell water secondary to carbon dioxide retention.

Macrocytosis of liver disease is secondary to increased cholesterol and phospholipids deposited on membranes of circulating erythrocytes. This deposition effectively increases the surface area of the erythrocyte.

Hypothyroidism can result in macrocytic anemia. More commonly, however, anemia associated with hypothyroidism is normocytic.

Artifactual elevation of the MCV must be considered in certain patients, although this occurs less frequently with newer cell-counting machines. Hyperglycemia and cold agglutinins may cause artifactually elevated MCVs. ^[23]