Types of Anemia
Anemia is classified into three main morphological categories based on mean corpuscular volume (MCV): microcytic (MCV <80 fL), normocytic (MCV 80-98 fL), and macrocytic (MCV >98 fL), with each category encompassing distinct etiologies that require specific diagnostic and therapeutic approaches 1.
Microcytic Anemias (MCV <80 fL)
The microcytic anemias represent conditions where red blood cells are smaller than normal:
- Iron deficiency anemia - The most common cause globally, resulting from nutritional deficiency, gastrointestinal blood loss, malabsorption, or increased physiological demands 1
- Anemia of chronic disease - Particularly when severe, can present with microcytosis 1
- Thalassemia syndromes - Genetic hemoglobinopathies affecting hemoglobin synthesis 1
- Sideroblastic anemias - Genetic defects in heme synthesis or iron metabolism, where iron overload often causes greater morbidity and mortality than the anemia itself 1
- Iron malabsorption disorders - Including genetic defects in iron transport proteins 1
A critical diagnostic pitfall: elevated ferritin and/or transferrin saturation in a microcytic anemia patient should immediately raise suspicion for genetic disorders of iron metabolism or heme synthesis rather than simple iron deficiency 1. These patients often have anemia refractory to iron supplementation and may develop life-threatening iron overload if misdiagnosed.
Normocytic Anemias (MCV 80-98 fL)
Normocytic anemias maintain normal red blood cell size and include:
Hemolytic anemias - Characterized by increased red blood cell destruction:
- Hereditary spherocytosis
- Enzyme deficiencies (G6PD deficiency, pyruvate kinase deficiency)
- Hemoglobinopathies (sickle cell disease)
- Autoimmune hemolytic anemia (AIHA)
- Paroxysmal nocturnal hemoglobinuria (PNH) 2
Anemia of chronic disease - Most commonly presents as normocytic, normochromic 1, 3
Bone marrow pathology:
Chronic kidney disease - Due to erythropoietin deficiency 3, 2
Acute blood loss - Before iron stores become depleted 2
Macrocytic Anemias (MCV >98 fL)
Macrocytic anemias feature enlarged red blood cells:
Megaloblastic anemias:
Non-megaloblastic causes:
Pathophysiological Classification
Beyond morphology, anemias can be classified by mechanism:
Deficient Erythropoiesis (Hypoproliferative)
- Aplastic anemia
- Myelodysplastic syndromes
- Iron deficiency anemia
- Anemia of chronic disease
- Thalassemia
- Megaloblastic anemia
- Chronic renal failure
- Hematological malignancies 2
Excessive RBC Destruction (Hemolytic)
- Hereditary spherocytosis
- Enzyme deficiencies
- Hemoglobinopathies
- Autoimmune hemolytic anemia
- Paroxysmal nocturnal hemoglobinuria 2
Blood Loss (Hemorrhagic)
Diagnostic Approach
The reticulocyte count is essential for distinguishing regenerative (elevated reticulocytes indicating appropriate bone marrow response to hemolysis or bleeding) from hyporegenerative anemias (low reticulocytes indicating production defects) 3, 5.
Key laboratory tests to differentiate anemia types:
- Complete blood count with MCV - Primary classification tool
- Reticulocyte count - Assesses bone marrow response
- Iron studies (serum iron, ferritin, transferrin saturation) - Evaluates iron status
- Vitamin B12 and folate levels - For macrocytic anemias
- Peripheral blood smear - Identifies cell morphology abnormalities
- Coombs test - For suspected hemolytic anemia
- Bone marrow examination - When diagnosis remains unclear or malignancy suspected 3, 2, 5
Clinical Context Considerations
In chronic kidney disease patients, anemia is typically normochromic and normocytic, making it indistinguishable from other chronic disease anemias without additional testing 3. The presence of anemia in CKD does not automatically indicate erythropoietin deficiency—other causes must be systematically excluded.
In primary care settings, iron deficiency anemia (48%), anemia of chronic illness (26.3%), and post-hemorrhage anemia (6.6%) account for the vast majority of cases 4. Approximately 13.7% of anemia cases require specialist referral.