Microcytosis the term given to describe red blood cells which are smaller than their normal counterparts. The normal RBC diameter is approximately 8µm. Iron deficiency is one of the most common causes of microcytosis. The approach to microcytosis is shown below (full-size image available here).

Approach to Microcytosis. The first-line test for a microcytic anaemia is ferritin, with or without other iron studies. This will identify iron deficiency, one of the most common causes of microcytosis. If the serum ferritin is normal, consider haemoglobin electrophoresis to identify thalassaemias or variant haemoglobins. If the ferritin is high, the anaemia is usually the result of a chronic inflammatory process. In very rare instances, it may be the result of an iron-loading anaemia (e.g. congenital sideroblastic anaemias, thalassaemia major or sickle cell disease). IRIDA: iron-refractory iron-deficiency anaemia, an extremely rare condition characterised by an inability to absorb enteric iron.