Click image to open full size
Anemia Classification
Microcytic, Normocytic, Macrocytic
Related Tools
Related Labs
Related Worksheets
Related Cheat Sheets
Anemia is a reduction in the blood's oxygen carrying capacity, usually reflected by low hemoglobin or a reduced number of functional red blood cells. A practical first step in classifying anemia is to look at red blood cell size, measured by the mean corpuscular volume or MCV. This divides anemia into microcytic, normocytic, and macrocytic patterns. Size based classification helps narrow the differential diagnosis quickly and guides the next laboratory tests.
Microcytic anemia usually results from impaired hemoglobin synthesis, normocytic anemia often reflects blood loss, hemolysis, or reduced production without major size change, and macrocytic anemia commonly arises from impaired DNA synthesis or abnormal marrow response. Typical workup includes CBC indices, reticulocyte count, peripheral smear, iron studies, vitamin B12 and folate levels, and sometimes hemolysis labs or bone marrow evaluation. Linking MCV to mechanism helps students connect cell appearance with underlying pathology. This approach is clinically useful because similar symptoms such as fatigue and pallor can come from very different causes.
Key Facts
- Anemia is commonly defined by low hemoglobin, hematocrit, or red blood cell mass.
- MCV = hematocrit / RBC count, and it classifies anemia by average red blood cell size.
- Microcytic anemia: MCV < 80 fL.
- Normocytic anemia: MCV 80 to 100 fL.
- Macrocytic anemia: MCV > 100 fL.
- Reticulocyte production index helps separate decreased production from increased loss or destruction.
Vocabulary
- Mean corpuscular volume
- Mean corpuscular volume, or MCV, is the average volume of red blood cells and is used to classify anemia by cell size.
- Reticulocyte
- A reticulocyte is an immature red blood cell released from the bone marrow, and its count shows how strongly the marrow is responding.
- Hemolysis
- Hemolysis is the premature destruction of red blood cells in the circulation or spleen.
- Iron deficiency anemia
- Iron deficiency anemia is a microcytic anemia caused by inadequate iron for normal hemoglobin synthesis.
- Megaloblastic anemia
- Megaloblastic anemia is a macrocytic anemia caused by impaired DNA synthesis, most often from vitamin B12 or folate deficiency.
Common Mistakes to Avoid
- Assuming all microcytic anemia is iron deficiency, which is wrong because thalassemia, anemia of chronic disease, and sideroblastic anemia can also produce small red blood cells.
- Using MCV alone to make the final diagnosis, which is wrong because reticulocyte count, smear findings, and targeted labs are needed to identify the actual cause.
- Forgetting that acute blood loss can initially be normocytic, which is wrong because cell size may stay normal even when hemoglobin is falling rapidly.
- Treating macrocytic anemia with folate before checking vitamin B12 status, which is wrong because folate can improve the anemia while allowing neurologic injury from B12 deficiency to continue.
Practice Questions
- 1 A patient has hemoglobin 9.0 g/dL and MCV 72 fL. Into which anemia category does this patient fall, and name two likely causes.
- 2 A CBC shows hematocrit 30% and RBC count 3.0 x 10^6 per microliter. Calculate the MCV in fL and classify the anemia as microcytic, normocytic, or macrocytic.
- 3 A patient has anemia with MCV 108 fL, glossitis, and numbness in the feet. Explain why vitamin B12 deficiency is more likely than isolated folate deficiency.