**Abstract**

The red blood cells (RBCs) carry oxygen from the lungs to the tissues, and for this, they must be able to deform. Accordingly, an impairment of RBC deformability is the cause of RBCs trapping and removal by the spleen and haemolysis. The most common causes for the decline in red cell deformability are the RBC membrane defects (abnormal shape or ionic transport imbalance), haemoglobinopathies (increased rigidity), or enzyme deficiencies (decreased anti-oxidant defences or ATP content). The most common cause of hereditary anaemia in childhood is hereditary spherocytosis (HS), characterised by a marked RBC deformabiity. A decreased RBC deformability has been found in hereditary haemolytic anaemias (HHAs) using the new-generation osmotic gradient ektacytometry (OGE), probably due to a combination of membrane protein defects and ionic imbalance. Therefore, OGE is currently considered the gold standard for the measurement of RBC deformability and the most useful complementary tool for the differential diagnosis of HHAs. Moreover, since several new forms of treatment are currently developed for hereditary RBC defects, the clinical interest of OGE is increasing. The aim of this chapter is to provide further information about the use of RBC deformability in clinical diagnosis and the OGE as a new challenge to decrease the frequency of undiagnosed rare anaemias.

**Keywords:** red cell, hereditary, anaemia, deformability, haemolysis, osmotic gradient, ektacytometry
