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Anaemia: pathophysiology, classification, and clinical features 

Anaemia: pathophysiology, classification, and clinical features
Anaemia: pathophysiology, classification, and clinical features

David J. Weatherall

, and Chris Hatton

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date: 02 March 2021

Anaemia is defined by the World Health Organization as a reduction of the haemoglobin concentration to less than 130 g/litre (males) or less than 120 g/litre (females). It is a common problem, with a prevalence around 3% for middle-aged men and 14% for middle-aged women in the United Kingdom, and is seen more frequently with age. There is a much greater prevalence in the developing world. Anaemia can be caused by the defective production of red cells or an increased rate of loss of cells, either by bleeding or premature destruction (haemolysis). The causes of defective production of red cells include (1) deficiency of key haematinics, including iron, vitamin B12, or folate; (2) anaemia of chronic disease; (3) reduced erythropoietin production typically seen in chronic kidney disease; and (4) primary diseases of the bone marrow. Haemolytic anaemias may be classified as either genetic or acquired. The key issues are to determine the degree of disability caused by the anaemia and hence how quickly treatment must be started, and the cause of the anaemia. The main causes of anaemia can usefully be classified according to the associated red cell morphological changes: (1) hypochromic, microcytic—including iron deficiency (the commonest cause of anaemia) and thalassaemia (common in some populations); (2) normochromic, macrocytic—vitamin B12 or folate deficiency, alcohol, and myelodysplasia; (3) polychromatophilic, macrocytic—haemolysis; (4) normochromic, normocytic—chronic disorders, renal failure, and diseases of the bone marrow; and (5) leucoerythroblastic—myelofibrosis, leukaemia, and metastatic carcinoma. The reticulocyte count also provides a good way of thinking about the underlying cause of anaemia—whether due to defective production or excess consumption of red cells.

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