What is the WHO classification of anemia?

WHO Classification of Anemia

According to the World Health Organization (WHO), anemia is defined as a hemoglobin concentration below 13.0 g/dL in men and below 12.0 g/dL in women. 1, 2

Classification by Hemoglobin Level

The WHO classification of anemia by severity is based on hemoglobin concentration:

Population Group	Mild Anemia	Moderate Anemia	Severe Anemia
Children 6-59 months	10.0-10.9 g/dL	7.0-9.9 g/dL	<7.0 g/dL
Children 5-11 years	11.0-11.4 g/dL	8.0-10.9 g/dL	<8.0 g/dL
Children 12-14 years	11.0-11.9 g/dL	8.0-10.9 g/dL	<8.0 g/dL
Non-pregnant women (≥15 years)	11.0-11.9 g/dL	8.0-10.9 g/dL	<8.0 g/dL
Pregnant women	10.0-10.9 g/dL	7.0-9.9 g/dL	<7.0 g/dL
Men (≥15 years)	11.0-12.9 g/dL	8.0-10.9 g/dL	<8.0 g/dL

Classification by Morphology

Anemia can be classified based on red blood cell morphology using Mean Corpuscular Volume (MCV):

1. Microcytic Anemia (MCV < 80 fL)

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

2. Normocytic Anemia (MCV 80-100 fL)

   • Anemia of chronic disease/inflammation
   • Acute blood loss
   • Hemolytic anemia
   • Renal disease
   • Endocrine disorders
   • Mixed deficiencies

3. Macrocytic Anemia (MCV > 100 fL)

   • Vitamin B12 deficiency
   • Folate deficiency
   • Liver disease
   • Alcoholism
   • Myelodysplastic syndrome
   • Hypothyroidism

Classification by Pathophysiology

Anemia can also be classified based on reticulocyte count, which reflects bone marrow response:

1. Regenerative Anemia (Increased Reticulocytes > 10 × 10^9/L)

   • Hemolytic anemia
   • Acute blood loss

2. Non-regenerative Anemia (Normal or Low Reticulocytes)

   • Iron deficiency
   • Anemia of chronic disease
   • Vitamin deficiencies
   • Bone marrow failure

Diagnostic Approach

The diagnostic algorithm for anemia should follow these steps:

1. Confirm anemia based on WHO criteria (Hb < 13 g/dL in men, < 12 g/dL in women)
2. Classify by MCV (microcytic, normocytic, macrocytic)
3. Check reticulocyte count to determine if regenerative or non-regenerative
4. Perform specific tests based on initial classification:

For Microcytic Anemia:

• Iron profile (serum ferritin, transferrin saturation)
• Hemoglobin electrophoresis if thalassemia suspected

For Normocytic Anemia:

• Creatinine, CRP
• Consider bone marrow biopsy in selected cases

For Macrocytic Anemia:

• Vitamin B12 and folate levels
• Thyroid function tests
• Consider bone marrow biopsy if myelodysplasia suspected

Common Pitfalls in Anemia Classification

1. Relying solely on MCV - Mixed deficiencies can mask typical MCV patterns
2. Misinterpreting ferritin levels - Ferritin is an acute phase reactant and may be elevated despite iron deficiency in inflammatory states (values up to 100 μg/L may still be consistent with iron deficiency in inflammation) 1
3. Overlooking combined deficiencies - B12 and folate deficiencies can coexist with iron deficiency
4. Failure to consider chronic disease - Anemia of chronic disease is common and can overlap with other types

Special Considerations

• In patients with inflammatory conditions, serum ferritin up to 100 μg/L may still be consistent with iron deficiency 1
• Vitamin B12 deficiency should be ruled out before treating folate deficiency to avoid masking B12 deficiency and precipitating neurological complications 2
• Anemia in elderly patients is often multifactorial and requires comprehensive evaluation

By following this classification system and diagnostic approach, clinicians can efficiently identify the underlying cause of anemia and implement appropriate treatment strategies to improve patient outcomes.