Differentiation Between Macrocytic and Microcytic Anemia

Initial Classification Based on MCV

The primary method to differentiate between macrocytic and microcytic anemia is through the mean corpuscular volume (MCV), with microcytic anemia defined as MCV <80 fL and macrocytic anemia as MCV >100 fL 1, 2
Microcytic anemia (MCV <80 fL) is most commonly caused by iron deficiency; other causes include thalassemia, anemia of chronic disease, and sideroblastic anemia 1, 2
Macrocytic anemia (MCV >100 fL) is most commonly megaloblastic, indicating vitamin B12 or folate deficiency; non-megaloblastic causes include alcoholism, certain medications, and myelodysplastic syndrome 1, 2

A low reticulocyte index with microcytic anemia suggests decreased RBC production due to iron deficiency, thalassemia, or anemia of chronic disease 1, 3
A high reticulocyte index indicates increased RBC production, suggesting blood loss or hemolysis regardless of MCV 1, 3

Serum ferritin <30 μg/L indicates iron deficiency in the absence of inflammation 4, 5
Transferrin saturation <16% suggests iron deficiency 5
Red cell distribution width (RDW) >14% with low MCV suggests iron deficiency anemia 6
RDW ≤14% with low MCV suggests thalassemia minor 6

Failing to consider mixed anemias can lead to incorrect diagnosis, as microcytosis and macrocytosis can neutralize each other, resulting in a normal MCV 7

Iron studies showing low iron, high TIBC, and low ferritin suggest iron deficiency 1
Normal iron studies with persistent microcytosis suggest thalassemia trait; hemoglobin electrophoresis is recommended 5
Elevated ferritin with low transferrin saturation suggests anemia of chronic disease 4
Elevated erythrocyte protoporphyrin (>30 μg/dL) suggests iron deficiency, but can also be elevated in lead poisoning and inflammation 6

Determine MCV from complete blood count 7
Calculate reticulocyte index to assess bone marrow response 7
For microcytic anemia, check iron studies (ferritin, iron, TIBC, transferrin saturation) 5
For macrocytic anemia, check vitamin B12 and folate levels 7
Review medications and alcohol intake 2
Check liver and thyroid function tests 2