Anemia is the presence of reduced hemoglobin in the blood. Formally, the World Health Organizatiοn (WHO) defines anemia by the hemoglobin concentration in the blood according to age and sex ¹:

adult men: <130 g/L
adult women: <120 g/L

Values for pregnant women and children are different.

Pathology

Etiology

The etiology of anemia is traditionally categorized by the volume of the red cell, i.e. the mean cell volume (MCV), into microcytic (MCV <82 fL), normocytic (MCV = 82-98 fL) and macrocytic (MCV >98 fL) anemias.

Microcytic (MCV <82 fL)

iron deficiency anemia (IDA): commonest cause of anemia
thalassemia
anemia of chronic disease: e.g. rheumatoid arthritis, Hodgkin lymphoma, malignancy, chronic infection
sideroblastic anemia: very rare

Normocytic (MCV = 82-98 fL)

dietary anemia: mixed iron, vitamin B₁₂, and/or folate deficiencies
renal failure
hemolytic anemia
erythrocyte disease
- intrinsic: membranopathy, enzymopathy, hemoglobinopathy
- extrinsic: immune-mediated, microangiopathic, associated with infection, chemical agent (spider venoms), metabolic
anemia of chronic disease
bone marrow disease:
- aplasia
  - idiopathic
  - paroxysmal nocturnal hemoglobinuria (PNH)
  - Fanconi syndrome
- red cell aplasia
  - congenital
  - acquired
  - Diamond-Blackfan syndrome
- myelodysplastic syndrome
secondary bone marrow disease
- infiltrative disease: metastases, lymphoma
- drugs
- toxins e.g. selenium
- irradiation
- infective: viral
- immunological