Evaluation of Macrocytic Anemia

Initial Assessment

The American Gastroenterological Association recommends obtaining a reticulocyte count to differentiate between megaloblastic and non-megaloblastic causes and to determine if the bone marrow is responding appropriately, with a strength of evidence level of moderate 2, 1
The American College of Gastroenterology suggests ordering thyroid function tests (TSH, free T4) as hypothyroidism can cause macrocytic anemia, with a strength of evidence level of high 1
Assess for hemolysis with haptoglobin, LDH, and bilirubin levels, as recommended by the American Association for the Study of Liver Diseases, with a strength of evidence level of moderate 2, 3

The American College of Gastroenterology recommends reviewing all medications, particularly those known to cause macrocytosis, such as azathioprine and other immunosuppressants, with a strength of evidence level of moderate 2, 3

If reticulocyte count is elevated, consider hemolysis or recent hemorrhage as potential causes, as suggested by the American Society of Hematology, with a strength of evidence level of low 3
If initial workup is inconclusive, consider bone marrow examination to evaluate for myelodysplastic syndromes or other primary bone marrow disorders, as recommended by the American College of Physicians, with a strength of evidence level of moderate 1
Red cell distribution width (RDW) can help identify mixed deficiency states where both microcytosis and macrocytosis may be present, with a strength of evidence level of moderate 2, 4

Despite normal B12 levels, consider testing methylmalonic acid and homocysteine levels if clinical suspicion for B12 deficiency remains high, as serum B12 may be normal in some cases of functional B12 deficiency, as recommended by the American College of Physicians, with a strength of evidence level of moderate 1
Remember that multiple causes of macrocytosis can coexist, particularly in patients with chronic conditions, as suggested by the American Gastroenterological Association, with a strength of evidence level of moderate 2, 3