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Blood Film

Red Blood Cell Size



  • Assessment of red blood cell size - the presence of normocytosis, microcytosis or macrocytosis - is the first step in the assessment of patients with anaemia as it can rapidly be used to narrow down the likely causes.
  • On a blood film, the red cell size can be assessed by comparing red cells to small lymphocytes. The nucleus of a small mature lymphocyte is slightly larger than a normal red blood cell.


  • Microcytes are small red blood cells with mean corpuscular volume (MCV) <80 fL. On a blood film, they are significantly smaller than small mature lymphocytes.
    • Microcytosis
    • Causes of Microcytosis

    • Iron deficiency anaemia - nutritional deficiency, blood loss, malabsorption, pregnancy
    • Anaemia of chronic disease (chronic infection / inflammation / malignancy)
    • Thalassaemia (alpha / beta)
    • Congenital sideroblastic anaemia
    • Hyperthyroidism
    • Lead poisoning
    • Copper deficiency
    • Practical Points

    • Microcytosis is often associated with anaemia, but not always.
    • In thalassaemia, the MCV tends to be inappropriately low compared to the degree of anaemia.


  • Macrocytes are large red blood cells with mean corpuscular volume (MCV) >100 fL. On a blood film, they are larger than small mature lymphocytes.
    • Causes of Macrocytosis

    • Megaloblastic

    • B12 deficiency
    • Folate deficiency
    • Drugs - methotrexate, chemotherapy, reverse transcriptase inhibitors, valproate, phenytoin, nitrous oxide, trimethoprim
    • Non-Megaloblastic

    • Chronic liver disease
    • Chronic alcohol abuse
    • Post splenectomy
    • Aplastic anaemia
    • Myelodysplastic syndrome
    • Hypothyroidism
    • Reticulocytosis (haemolysis, haemorrhage, recovery post anaemia)
    • Practical Points

    • Macrocytosis without anaemia is often normal.
    • The classical findings on blood film for megaloblastic anaemia are macroovalocytes and hypersegmented neutrophils.
    • Macrocytosis with polychromasia is suggestive of reticulocytosis, and haemorrhage / haemolysis should be considered.


  • Anisocytosis refers to increased variability of red blood cell size. Red cells are normally of uniform size, and significant variation
    • Look For

    • Elevated Red cell distribution width (RDW) on full blood count
    • Variability in red blood cell size on blood film
  • Anisocytosis may be caused by any condition that results in microcytosis or macrocytosis.
  • For example, a patient with early megaloblastic anaemia may have a mix of macrocytes and normally sized cells, resulting in an overall normal mean red cell volume (MCV) with anisocytosis.
    • Interpretation

    • More microcytic - suggests iron deficiency or sideroblastic anaemia
    • More normocytic - suggests early iron deficiency or megaloblastic anaemia, reticulocytosis (haemolysis, recovery following haemorrhage or bone suppression) or sickle cell anaemia
    • More macrocytic - suggests megaloblastic anaemia, myelodysplastic syndrome or chronic liver disease
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