Macrocytosis Causes and Diagnostic Approach

Megaloblastic Causes

Vitamin B12 deficiency is a primary cause of macrocytosis, resulting in megaloblastic changes with macro-ovalocytes and hypersegmented neutrophils on peripheral smear, according to the National Comprehensive Cancer Network 1
Ileal resection >30 cm leads to B12 malabsorption since the distal ileum is the absorption site 2
Active inflammatory bowel disease with ileal involvement impairs B12 absorption 3
Folate deficiency, particularly in alcoholics and malnourished patients, produces identical megaloblastic morphology to B12 deficiency 1
Malabsorption from jejunal inflammatory bowel disease and sulfasalazine blocking folate absorption contribute to folate deficiency 2, 3

Methotrexate inhibits dihydrofolate reductase, blocking conversion to tetrahydrofolic acid and causing macrocytosis through impaired DNA synthesis 2, 3
Thiopurines (azathioprine, 6-mercaptopurine) cause direct myelosuppression rather than vitamin deficiency 2, 3
Hydroxyurea is a well-established cause of drug-induced macrocytosis 1
Alcoholism is one of the most common causes, producing non-megaloblastic macrocytosis 1
Chronic liver disease of any etiology causes macrocytosis 1
Hypothyroidism produces macrocytosis through decreased erythropoiesis 1
Reticulocytosis from hemolysis or hemorrhage causes macrocytosis because immature reticulocytes are larger cells 2, 3
Myelodysplastic syndrome and primary bone marrow disorders contribute to macrocytosis 1
Erythropoietin therapy shifts immature reticulocytes into circulation 2

Concurrent iron deficiency with B12 or folate deficiency produces falsely normal MCV as microcytosis and macrocytosis cancel out 2
Elevated red cell distribution width (RDW) suggests a mixed picture when MCV appears normal 2
Mean corpuscular hemoglobin (MCH) is more sensitive than MCV for detecting iron deficiency in this scenario 2
In inflammatory conditions like IBD, ferritin <50 μg/L may still indicate iron deficiency despite inflammation 2
Never give folic acid before excluding B12 deficiency, as folate supplementation masks B12 depletion and can precipitate subacute combined degeneration of the spinal cord 2, 3, 4

Peripheral blood smear to distinguish megaloblastic (macro-ovalocytes, hypersegmented neutrophils) from non-megaloblastic morphology is essential 1
Reticulocyte count differentiates production defects (low/normal) from hemolysis/hemorrhage (elevated) 1, 3
Vitamin B12 and folate levels are essential first-line tests 1

Evaluate for hemolysis by checking haptoglobin, LDH, indirect bilirubin, and direct antibody test (Coombs) in patients with non-megaloblastic morphology and elevated reticulocyte count, as recommended by the American Society of Clinical Oncology 5
Assess for recent hemorrhage and consider peripheral blood smear for schistocytes in patients with non-megaloblastic morphology and elevated reticulocyte count, as recommended by the American Society of Clinical Oncology 5

In inflammatory conditions, ferritin <100 μg/L may indicate iron deficiency, and transferrin saturation <16% with ferritin 30-100 μg/L suggests hypoferritinemia, according to the European Crohn’s and Colitis Organisation 6
Check iron studies if MCH is reduced despite macrocytosis, as this mixed picture requires different treatment approaches, and consider the guidelines from the European Crohn’s and Colitis Organisation 6