Evaluation of Macrocytosis with Normal B12 and Folate

Immediate Laboratory Assessment

The American College of Physicians recommends checking reticulocyte count to distinguish between ineffective erythropoiesis and increased red cell production, with low/normal reticulocytes suggesting bone marrow disorder or deficiency, and elevated reticulocytes suggesting hemolysis or hemorrhage 2
Complete iron studies, including MCH, should be checked to detect concurrent iron deficiency, as MCH is more sensitive than MCV for detecting iron deficiency 1, 2
A peripheral blood smear should be examined for hypersegmented neutrophils, schistocytes, or dysplastic features, which can indicate megaloblastic process, hemolysis, or myelodysplasia 2
TSH levels should be checked, as hypothyroidism commonly causes macrocytosis without anemia 3
Liver function tests and GGT should be checked, as chronic alcohol use can cause macrocytosis independent of nutritional deficiencies 3

Up to 70-83% of B12-deficient patients have normal MCV, and tissue-level deficiency can occur even with "normal" laboratory B12 values, highlighting the importance of measuring homocysteine and methylmalonic acid in patients with high clinical suspicion 1
Methylmalonic acid is more specific for B12 deficiency than homocysteine, which can be elevated in folate deficiency, pyridoxine deficiency, renal insufficiency, and hypothyroidism 4, 1
A low-normal folate level warrants closer attention, and measuring erythrocyte folate can provide a more accurate assessment of tissue folate stores 3

A systematic review of medication history should be conducted to identify macrocytosis-inducing medications, such as thiopurines, anticonvulsants, methotrexate, hydroxyurea, and antiretrovirals 2, 3
Alcohol consumption should be quantified, as chronic alcohol use can cause macrocytosis independent of folate deficiency 3

Patients with other cytopenias, progressive or severe macrocytosis, dysplastic features on peripheral smear, elevated RDW, or unexplained macrocytosis after initial workup should be referred to hematology 2

Normal B12 levels do not rule out B12-related pathology, and concurrent iron deficiency is common in B12-deficient patients, making MCH assessment valuable 1, 2
Follow-up is essential, even in cases of unexplained macrocytosis, as patients may develop primary bone marrow disorders or worsening cytopenias over time 2