Elevated MCV and MCH: Primary Causes
Elevated MCV and MCH most commonly indicate vitamin B12 or folate deficiency, but also arise from thiopurine medications (azathioprine, 6-mercaptopurine), alcohol abuse, hypothyroidism, reticulocytosis, or myelodysplastic syndrome 1.
Primary Diagnostic Framework
When you encounter elevated MCV and MCH, work through this algorithmic approach:
Step 1: Check Reticulocyte Count
The reticulocyte count immediately divides macrocytic anemias into two pathways 1:
Low or normal reticulocytes → Deficiency states or bone marrow dysfunction
- Vitamin B12 deficiency
- Folate deficiency
- Myelodysplastic syndrome
- Primary bone marrow disease
- Medication effects (thiopurines, hydroxyurea, diphenytoin)
- Hypothyroidism
- Alcohol abuse
Elevated reticulocytes → Hemolysis (excludes deficiency states) 1
- Requires workup with haptoglobin, LDH, bilirubin
Step 2: Medication and Exposure History
Thiopurine treatment (azathioprine or 6-mercaptopurine) is a particularly common cause of macrocytosis in inflammatory bowel disease patients 1. Other medications causing macrocytosis include hydroxyurea and diphenytoin 2.
In pediatric populations, anticonvulsants, zidovudine, and immunosuppressive agents account for 35% of macrocytosis cases 3.
Alcohol abuse represents a major cause in adults—accounting for 18 of 70 cases (26%) in one series 4.
Step 3: Vitamin Deficiency Assessment
Order serum vitamin B12 and folate levels 1. Macrocytosis indicates vitamin deficiency in the context of low/normal reticulocytes 1.
In patients with extensive small bowel resection, extensive ileal Crohn's disease, or ileal-anal pouch, assess B12 and folate more frequently than annually 1.
Step 4: Consider Other Causes
- Hypothyroidism: Check TSH if clinical suspicion exists 1
- Liver disease: Chronic persistent hepatitis can cause macrocytosis 4
- Myelodysplastic syndrome: Particularly if pancytopenia present 2
- Hemochromatosis: Can cause elevated MCV, MCH, and MCHC due to increased iron uptake and hemoglobin synthesis by immature erythroid cells 5, 6
Critical Pitfalls to Avoid
Co-existing microcytosis and macrocytosis can neutralize each other, resulting in normal MCV despite underlying pathology 1. In this situation, an elevated red cell distribution width (RDW) serves as a critical clue—high RDW indicates iron deficiency even when MCV appears normal 1.
In inflammatory bowel disease specifically, macrocytosis may indicate B12 or folate deficiency, but normocytosis with normal or elevated ferritin can represent anemia of chronic disease 1. The clinical context matters significantly.
Minimum Workup Required
For macrocytosis with low/normal reticulocytes 1:
- Complete blood count with differential
- Reticulocyte count
- Vitamin B12 level
- Folate level
- Serum ferritin and transferrin saturation
- Thyroid function tests
- Liver function tests
If the cause remains unclear after this workup, hematology consultation is appropriate 1.
Special Populations
In children with macrocytosis, drug ingestion represents the most common cause (35%), followed by congenital heart disease and Down syndrome 3. Vitamin B12 or folate deficiency is notably rare in pediatric populations.
In hemochromatosis patients (particularly C282Y homozygotes), elevated MCV, MCH, and MCHC reflect increased iron uptake and hemoglobin synthesis rather than vitamin deficiency 5, 6.