Evaluation and Management of High MCV with Elevated B12

Introduction to Macrocytosis with Elevated Vitamin B12

The National Comprehensive Cancer Network recommends immediate hematology evaluation for patients with macrocytosis and elevated vitamin B12 levels, as this combination suggests a myeloproliferative disorder 1
Elevated serum B12 with macrocytosis is commonly observed in myeloproliferative variants of hypereosinophilia, particularly in myeloid neoplasms with PDGFRA fusion genes 1

The American College of Physicians suggests checking serum tryptase levels immediately, as elevation alongside high B12 strongly suggests myeloproliferative disease 1
Obtain reticulocyte count to distinguish ineffective erythropoiesis from hemolysis/hemorrhage, with a strength of evidence rated as moderate 2, 3
Review peripheral blood smear for dysplastic features, hypersegmented neutrophils, schistocytes, eosinophilia, monocytosis, or circulating blasts, as recommended by the National Comprehensive Cancer Network 1, 2

Refer immediately to hematology if elevated serum tryptase alongside elevated B12 and macrocytosis is present, as recommended by the National Comprehensive Cancer Network 1
Any other cytopenias, such as anemia, thrombocytopenia, or leukopenia, should prompt urgent hematology referral, with a strength of evidence rated as high 2

Bone marrow aspirate and biopsy with specialized testing is indicated when confirming suspected myeloid/lymphoid neoplasm with eosinophilia, as recommended by the National Comprehensive Cancer Network 1
The workup should include immunohistochemistry for CD117, CD25, and tryptase, with conventional cytogenetics and FISH testing to detect TK fusion gene rearrangements 1

In patients with inflammatory bowel disease, ferritin up to 100 μg/L may still indicate iron deficiency when inflammation is present, with a strength of evidence rated as moderate 2, 3
Thiopurines cause macrocytosis through myelosuppressive activity rather than vitamin deficiency, and regular CBC monitoring is necessary to track MCV stability, as recommended by the American Gastroenterological Association 2