Mean Corpuscular VolumeMCV
Average red cell volume; the first axis of anemia classification.
What it measures
Calculated by automated counters (Hct/RBC × 10) or measured directly by impedance. Reflects average erythrocyte size at the moment of sampling, not the dynamics of erythropoiesis.
Why we order it
Classify anemia as microcytic (<80), normocytic, or macrocytic (>100); narrows differential before more expensive testing.
Interpretation by lens
Classify, then test by mechanism. Microcytic → iron studies first; macrocytic → B12 + folate; normocytic → reticulocyte index.
An MCV at the upper bound of 'normal' (96–100) is an early marker of B12 status decline; consider MMA before the value crosses 100.
MCV is sensitive to in-vitro storage swelling; use freshly drawn EDTA samples within 6 h for research-grade values.
Causes of abnormal values
- ↑B12 / folate deficiency
- ↑Liver disease (round macrocytes)
- ↑Hypothyroidism
- ↑Reticulocytosis (large young cells)
- ↑Drugs: hydroxyurea, methotrexate, zidovudine, alcohol
- ↑Myelodysplastic syndrome
- ↓Iron deficiency
- ↓Thalassemia (very low MCV with normal/high RBC count — Mentzer index <13)
- ↓Anemia of chronic disease (often normocytic but can be mildly microcytic)
- ↓Lead poisoning (with basophilic stippling)
- ↓Sideroblastic anemia
Pattern recognition
Think thalassemia trait; order Hb electrophoresis.
MDS or severe B12 deficiency; do not delay marrow workup.
Pre-analytic & interpretation traps
- !Cold agglutinins falsely raise MCV (RBCs clump and are counted as one cell). Warm sample to 37 °C and re-run.
- !Mixed deficiency (B12 + iron) can produce a normal MCV with high RDW — the RDW is the clue.
- !Severe hyperglycemia osmotically swells RBCs in vitro, falsely raising MCV.