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.