Bit · Heme/Onc
Anemia - Microcytic vs Normocytic vs Macrocytic
The first split on every anemia is MCV. It collapses the differential by half in one number.
Mechanism#
Mean corpuscular volume (MCV) classifies anemia by red-cell size - and the size points at the mechanism:
- Microcytic (MCV < 80) - defective hemoglobin synthesis. Either iron, globin, or heme is the problem.
- Normocytic (MCV 80-100) - either acute blood loss, hemolysis, marrow failure, or chronic disease (early). Split further by reticulocyte count.
- Macrocytic (MCV > 100) - DNA synthesis problem (megaloblastic) or non-megaloblastic (alcohol, liver disease, hypothyroidism).
Differentiator Table#
| MCV | Category | Causes |
|---|---|---|
| < 80 | Microcytic | Iron deficiency, anemia of chronic disease (later), thalassemia, sideroblastic anemia, lead poisoning |
| 80-100 | Normocytic - reticulocyte count needed | High retic: hemolysis, acute blood loss. Low retic: aplastic anemia, marrow infiltration, anemia of chronic disease early, CKD (↓ EPO) |
| > 100, megaloblastic (hypersegmented neutrophils) | Macrocytic | B12 deficiency (+ neuro), folate deficiency (no neuro), methotrexate, hydroxyurea, AZT |
| > 100, non-megaloblastic | Macrocytic | Alcohol use, liver disease, hypothyroidism, reticulocytosis (large young RBCs), MDS |
The Pivot#
Two steps:
- Check MCV.
- If normocytic, check reticulocyte index. High (> 2%) → losing or destroying RBCs. Low → can't make them.
From there, the differential collapses to 3-4 entities and standard labs (iron studies, B12/folate, smear, retic) close it.
NBME-Style Stem#
A 24-year-old woman with menorrhagia presents with fatigue. Hemoglobin is 9.6 g/dL, MCV 68 fL, RDW 18%. Ferritin is 6 ng/mL, transferrin saturation 8%. Which of the following is the most likely diagnosis?
Concept Anchor
MCV cuts the anemia differential into three categories before any other lab; reticulocyte count subdivides the middle group. Two numbers and you're halfway to the diagnosis.