Diagnostic Approach to Macrocytic Anemia

Initial Evaluation

The American Gastroenterological Association recommends beginning with a complete blood count (CBC) with mean corpuscular volume (MCV), reticulocyte count, and peripheral blood smear to differentiate megaloblastic from nonmegaloblastic causes and assess bone marrow response 1, 2
The National Comprehensive Cancer Network suggests that the minimum workup for macrocytic anemia (MCV >100 fL) should include CBC with differential and MCV to confirm macrocytosis, and reticulocyte count to distinguish between decreased RBC production versus increased destruction/loss 1, 3

The American Gastroenterological Association recommends ordering vitamin B12 and red blood cell folate levels immediately if megaloblastic features are present on peripheral smear, as these are the most common causes of megaloblastic macrocytic anemia 1, 3
The Journal of Crohn's and Colitis suggests checking reticulocyte index to guide further testing if nonmegaloblastic features are present, and evaluating for hemolysis or hemorrhage if the reticulocyte index is elevated 2, 3
The National Comprehensive Cancer Network recommends evaluating for decreased RBC production if the reticulocyte index is low or normal, and considering thyroid function tests, liver function tests, medication review, and alcohol history 1, 3

The Journal of Crohn's and Colitis suggests obtaining soluble transferrin receptor and percentage of hypochromic red cells to evaluate for combined iron deficiency if the cause remains unclear after initial testing 2, 3
The American Gastroenterological Association recommends considering bone marrow aspirate and biopsy with hematology consultation if myelodysplastic syndrome is suspected in elderly patients with cytopenias 2, 3

The National Comprehensive Cancer Network recommends evaluating for myelosuppression as a cause of macrocytosis in cancer patients receiving chemotherapy, and considering bone marrow biopsy if MDS is suspected 1
The Journal of Crohn's and Colitis suggests using transferrin saturation <15% as the primary indicator of iron deficiency when inflammation is present, as inflammatory conditions can falsely elevate ferritin 2, 3