Approach to an adult patient with a Macrocytic Anaemia

Compiled by Dr Rafiq Dhansay, Dr Trisha Moodley & Dr Jennifer Lines 3rd Quarter 2015

Macrocytosis is described as an increase in the volume or size of a red blood cell (RBC). The parameter on the
full blood count used to measure this, is the Mean Corpuscular Volume (MCV). An MCV >100 femtolitres (fL) is
considered macrocytic. On a peripheral blood smear, a red blood cell is considered macrocytic if it is larger than
the nucleus of a small mature lymphocyte. Macrocytosis may or may not be accompanied by anaemia.

The causes of a macrocytosis are broadly divided into five groups based on the mechanism:

  1. Abnormalities of DNA metabolism e.g. vitamin B12 and folate deficiencies and drugs that interfere with DNA
  2. Increase in circulating immature red cells or “stressed” red cells, e.g. reticulocytosis in response to
    haemorrhage or haemolysis, aplastic anaemia, pure red cell aplasia, erythropoietin (EPO) effect.
  3. Primary bone marrow disorders, e.g. myelodysplastic syndromes, leukaemia.
  4. Lipid abnormalities, e.g. liver disease, hypothyroidism.
  5. Unknown mechanism.
The following table extract outlines conditions which may cause a macrocytic anaemia (CLICK HERE for the complete table).





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