How is anaemia (anemia) classified?

Anemia Classification

Anemia is primarily classified based on mean corpuscular volume (MCV), reticulocyte count, and underlying pathophysiology, with MCV categorizing anemia as microcytic (<80 fL), normocytic (80-100 fL), or macrocytic (>100 fL). 1, 2

Classification Based on MCV

Microcytic Anemia (MCV <80 fL)

• Most commonly caused by iron deficiency anemia 1, 2
• Other causes include:
  • Anemia of chronic disease 1
  • Thalassemia and other hereditary microcytic anemias 1, 2
  • Lead poisoning (rare) 1
  • Sideroblastic anemia 2

Normocytic Anemia (MCV 80-100 fL)

• Most common type, representing approximately 85% of anemia cases 3
• Causes include:
  • Acute hemorrhage 1, 4
  • Anemia of chronic disease/inflammation 1, 4
  • Renal anemia (chronic kidney disease) 1, 4
  • Bone marrow failure 1
  • Hemolytic anemia 4
  • Aplastic anemia 4

Macrocytic Anemia (MCV >100 fL)

• Megaloblastic causes:
  • Vitamin B12 deficiency 1, 2, 5
  • Folate deficiency 1, 2, 5
• Non-megaloblastic causes:
  • Medication-induced 1, 5
  • Alcoholism 2, 5
  • Liver dysfunction 5
  • Myelodysplastic syndrome 2, 5
  • Hypothyroidism 2, 5

Classification Based on Reticulocyte Count

Low or Normal Reticulocyte Count

• Indicates decreased production of red blood cells 1
• Causes include:
  • Nutritional deficiencies (iron, B12, folate) 1
  • Bone marrow failure 1
  • Anemia of chronic disease 1
  • Renal anemia 1

High Reticulocyte Count

• Indicates increased destruction or loss of red blood cells 1
• Causes include:
  • Hemolysis 1
  • Acute blood loss 1
  • Hemoglobinopathies like thalassemia 1

Combined Classification Approach

Microcytic Anemia

• With low/normal reticulocytes:
  • Iron deficiency 1
  • Anemia of chronic disease 1
  • Lead poisoning 1
  • Hereditary anemias 1
• With high reticulocytes:
  • Thalassemia and other hemoglobinopathies 1

Normocytic Anemia

• With low/normal reticulocytes:
  • Acute hemorrhage 1
  • Renal anemia 1
  • Anemia of chronic disease 1
  • Bone marrow failure 1
• With high reticulocytes:
  • Hemolytic anemia 4

Macrocytic Anemia

• With low/normal reticulocytes:
  • Vitamin B12 deficiency 1
  • Folate deficiency 1
  • Myelodysplastic syndrome 2
  • Medication effects 1
  • Hypothyroidism 2

Additional Diagnostic Parameters

Red Cell Distribution Width (RDW)

• RDW >14% with low MCV suggests iron deficiency anemia 2
• RDW ≤14% with low MCV suggests thalassemia minor 2

Iron Studies

• Serum ferritin <30 μg/L indicates iron deficiency in the absence of inflammation 2
• Transferrin saturation <16% suggests iron deficiency 2
• Elevated ferritin with low transferrin saturation suggests anemia of chronic disease 2

Important Clinical Considerations

• Mixed anemias can present with normal MCV as microcytosis and macrocytosis neutralize each other 2
• The initial diagnostic workup should include complete blood count with MCV, reticulocyte count, serum ferritin, transferrin saturation, and C-reactive protein 1
• MCV-guided classification alone may lead to incorrect exclusion of certain etiologies; a broader set of laboratory tests is recommended 3
• The proportion of anemia types can vary by population; for example, in people with HIV, normocytic anemia is most common (74%), followed by macrocytic (15%) and microcytic (11%) 6

Diagnostic Algorithm

1. Determine MCV from complete blood count 2
2. Calculate reticulocyte index to assess bone marrow response 2
3. For microcytic anemia:
   • Check iron studies (ferritin, iron, TIBC, transferrin saturation) 2
   • Consider hemoglobin electrophoresis if thalassemia is suspected 2
4. For normocytic anemia:
   • Evaluate for signs of hemolysis (jaundice, unconjugated hyperbilirubinemia, decreased haptoglobin) 4
   • Assess kidney function 4
   • Check for signs of acute blood loss 4
5. For macrocytic anemia:
   • Check vitamin B12 and folate levels 2
   • Review medications and alcohol intake 2
   • Check liver and thyroid function tests 2