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:
- Abnormalities of DNA metabolism e.g. vitamin B12 and folate deficiencies and drugs that interfere with DNA
- 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.
- Primary bone marrow disorders, e.g. myelodysplastic syndromes, leukaemia.
- Lipid abnormalities, e.g. liver disease, hypothyroidism.
- Unknown mechanism.
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