Causes of Macrocytic Anemia
Macrocytic anemia (MCV >100 fL) is most commonly caused by vitamin B12 or folate deficiency (megaloblastic), followed by myelodysplastic syndrome, alcoholism, liver disease, and certain medications like hydroxyurea or azathioprine. 1
Classification Framework
Macrocytic anemia divides into two major categories based on reticulocyte count and peripheral smear findings 1:
Megaloblastic Causes (with normal or low reticulocytes)
These result from impaired DNA synthesis, producing macro-ovalocytes and hypersegmented neutrophils on peripheral smear 1, 2:
- Vitamin B12 deficiency - Most common megaloblastic cause, from insufficient uptake or inadequate absorption through lack of intrinsic factor (pernicious anemia, H. pylori gastritis, antacids, veganism) 1
- Folate deficiency - Increased requirement in pregnancy, hemolysis, chronic myeloid leukemia, or malabsorption 1
- Combined B12 and folate deficiency - Very rare, usually in malabsorption syndromes 1
Non-Megaloblastic Causes (with normal or low reticulocytes)
These show normal DNA synthesis without megaloblastic changes 1, 2:
- Myelodysplastic syndrome (MDS) - Leading cause in many series (19.3% of cases), particularly common in elderly patients 1, 3
- Alcoholism - Can cause isolated macrocytosis without anemia initially 1
- Liver disease/dysfunction - Chronic hepatic disease 1, 4
- Medications - Hydroxyurea, diphenytoin, methotrexate, azathioprine 1
- Hypothyroidism 1, 4
- Bone marrow failure syndromes - Aplastic anemia, pure red cell aplasia (11.8% of cases) 1, 3
Causes with Elevated Reticulocytes
These represent appropriate marrow response 1:
- Hemolytic anemia - Creates "false macrocytosis" from reticulocyte release 1
- Acute hemorrhage - May initially show elevated reticulocytes 1
- MDS with hemolysis 1
Critical MCV Thresholds for Differential Diagnosis
MCV >130 fL strongly suggests megaloblastic anemia (vitamin B12/folate deficiency) or medication effect 3
MCV 114-130 fL suggests alcoholism/liver disease, bone marrow failure, myeloid malignancy, or hemolytic anemia 3
MCV 100-114 fL suggests lymphoid malignancy, chronic renal failure, hypothyroidism, or solid tumors 3
MCV >110 fL makes megaloblastic anemia significantly more likely (p=0.0007) 5
Common Pitfalls
Three patients with megaloblastic marrow who failed vitamin replacement were ultimately diagnosed with MDS - always consider MDS in elderly patients with macrocytic anemia unresponsive to vitamin therapy 5. The combination of macrocytic anemia with leukopenia and/or thrombocytopenia strongly suggests MDS and warrants hematology consultation 4.
Pernicious anemia represents late-stage autoimmune gastritis - patients with new pernicious anemia diagnosis require endoscopy with topographical biopsies to confirm corpus-predominant atrophic gastritis and rule out gastric neoplasia including neuroendocrine tumors 1.
In inflammatory bowel disease, macrocytosis may indicate vitamin B12 or folic acid deficiency, particularly with extensive small bowel resection or extensive ileal Crohn's disease 1.
Plasma cell dyscrasia accounts for 7.2% of macrocytic anemia cases and should be considered in the differential 3.