🩸 Lab Values & Diagnostics Lesson

MCV tells the story: small, normal, or large

One number on the CBC — mean corpuscular volume — narrows dozens of possible anemia causes down to a short, manageable list before you order a single follow-up test.

<80

Microcytic

80-100

Normocytic

>100

Macrocytic

📖 Full Breakdown

The three MCV categories and their leading causes

MCV (mean corpuscular volume) measures the average size of a single red blood cell. Every anemia falls into one of three size categories, and each category has its own short list of usual suspects.

<80

Microcytic anemia — small cells

The cell doesn't have enough hemoglobin to fill out to normal size. Iron deficiency is by far the most common cause — worldwide and in exam questions. Other causes: thalassemia (an inherited hemoglobin-production disorder), sideroblastic anemia, lead poisoning, and long-standing anemia of chronic disease (which usually starts normocytic and can drift microcytic over time).

80-100

Normocytic anemia — normal-sized cells

The red cells themselves are the right size, but there aren't enough of them, or they're being destroyed too fast. Causes: acute blood loss (trauma, surgery, GI bleed), hemolysis (red cells being destroyed prematurely), early-stage anemia of chronic disease, and aplastic anemia (bone marrow failing to produce enough of all cell lines).

>100

Macrocytic anemia — large cells

The cell divides fewer times than normal before release, so it ends up larger. Causes: vitamin B12 deficiency and folate deficiency are the two classic causes (both are needed for DNA synthesis during cell division), plus liver disease, hypothyroidism, chronic alcohol use, and certain medications and myelodysplastic syndromes.

🩺 Clinical / Exam Application

A CBC shows hemoglobin 9 g/dL with MCV 72 fL. The low MCV immediately narrows this to the microcytic category — the next step is checking iron studies (ferritin, TIBC) to confirm iron deficiency before reaching for thalassemia or sideroblastic causes, since iron deficiency is dramatically more common and the treatment is completely different.

⚠️ Exam Alert

MCV alone doesn't diagnose the cause — it only narrows the category. A microcytic MCV still requires iron studies to distinguish iron deficiency from thalassemia, since both can look identical on MCV alone but call for opposite treatments (iron deficiency needs supplemental iron; thalassemia does not, and unnecessary iron can cause harm).

🚧 Common Trap

Don't assume every macrocytic anemia is B12 deficiency. Alcohol use and hypothyroidism are common macrocytic causes that get overlooked because B12/folate are the "textbook" answer — a full picture (diet, alcohol history, thyroid labs) is needed before assuming B12 or folate deficiency.

✅ Quick Check

A patient's MCV comes back at 108 fL. Which two vitamin deficiencies should be checked first, and why do both of them affect cell size in the same direction?

📝 Exam Prep

Common Exam Questions

❓ What is the single most common cause of microcytic anemia?

✅ Iron deficiency — it's dramatically more common than thalassemia, sideroblastic anemia, or lead poisoning, which is why iron studies are the standard first follow-up test for any microcytic result.

❓ Why do B12 and folate deficiency both cause macrocytic anemia?

✅ Both vitamins are required for DNA synthesis during red cell division. Without them, the developing red cell divides fewer times before being released into circulation, so it ends up larger than normal.