**3.1 Anisocytosis**

Normal red cells (normocytes) are about 7–8 μm in diameter [2]. Reduced size is termed microcytosis. Increase in red cell diameter above normal is called macrocytosis. Red cell sizes form the basis for morphologic or cytometric classification of anaemia. In terms of red cell size, anaemia could be described as microcytic, normocytic or macrocytic. Typically, the normal red cell size is adjudged by comparison with the nucleus of a small lymphocyte. The reference interval for mean red cell volume (MCV) is 80–95 fl [3, 4]. MCV >95 fl is termed macrocytic. While, red cell size <6 μm and/or MCV <80 fl is termed microcytic [5]. Differentials of microcytic anaemias include iron deficiency, thalassemias, sideroblastic anaemia and anaemia of chronic inflammation (20% of cases). Further test such as serum ferritin, total iron binding capacity (TIBC), haemoglobin electrophoresis with quantification helps to differentiate microcytic anaemia [4, 6]. For instance, low serum ferritin, raised TIBC and raised RDW is expected in iron deficiency. A normal or elevated red cell counts with little red cell size variation (RDW) in the presence of microcytosis is suggestive of a thalassaemia.

Normocytic anaemia occurs in acute blood loss, marrow aplasia, anaemia of chronic disease (80% of cases) and anaemias of endocrine origin. Macrocytosis may be oval or round, with specific casual relationships. Oval macrocytes are seen in megaloblastic anaemias (folate/cobalamin deficiencies), myelodysplastic syndrome and drug therapies such as hydroxyurea [7]. Round macrocytes are seen in liver disease and excess alcohol use. MCV may appear falsely normal with the haematology analyser in combined substrate deficiency states. However, the blood picture will reveal marked anisopoikilocytosis. The red cell distribution width (RDW) is a calculated parameter and it measures the individual size variability (heterogeneity) of the red cells. RDW is the percentage coefficient of variation of the individual red cell volumes enumerated by the particle counter [8]. RDW normally ranges between 11.5 and 15.5%. For interpretation purposes, raised RDW is seen in iron deficiency anaemia, megaloblastic anaemia (folate and cobalamin deficiency), haemolytic anaemia, recent blood transfusion, hereditary spherocytosis and sickle cell syndromes [8, 9]. RDW is useful in interpreting apparently normal MCV since it will be quite high in combined micronutrient deficiency state.
