Hemoglobin Grades Severity, MCV Names the Category | AI Learning Guide

Start with the distinction that matters most: hemoglobin tells you how anemic the patient is, but the MCV tells you what kind. Two patients with the same hemoglobin of eight can have completely different diseases. The MCV is the average red cell volume in femtoliters, and it sorts anemia into three categories. Below eighty is microcytic, and the differential is short: iron deficiency, thalassemia, anemia of chronic disease, sideroblastic anemia, and lead poisoning. Between eighty and one hundred is normocytic, the broadest category. Above one hundred is macrocytic, which splits into megaloblastic causes like B12 and folate deficiency, and non-megaloblastic causes like alcohol, liver disease, and hypothyroidism. One caution: the MCV is a mean, so if two cell populations coexist, the average can look normal and hide both problems.

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Hemoglobin and hematocrit measure severity. MCV measures type. A low hemoglobin tells you the patient is anemic; only the MCV tells you which differential list to open.

The three morphologic categories

Microcytic (MCV < 80 fL)

Iron deficiency
Thalassemia
Anemia of chronic disease (some cases)
Sideroblastic anemia
Lead poisoning

Normocytic (MCV 80-100 fL)

Anemia of chronic disease
Acute blood loss
Early nutritional deficiency
Hemolysis
Marrow failure

Macrocytic (MCV > 100 fL)

B12 deficiency (megaloblastic)
Folate deficiency (megaloblastic)
Alcohol, liver disease (non-megaloblastic)
Hypothyroidism (non-megaloblastic)
Myelodysplasia (non-megaloblastic)

MCV is an average. When two red cell populations coexist, such as iron deficiency plus B12 deficiency, the average can land in the normal range and hide both problems. A normal MCV does not exclude a mixed deficiency.