Red blood cell Osmotic deformability

The Laser-assisted Optical Rotational Cell Analyzer Osmoscan measures RBC deformability under defined shear stress as a function of suspending medium osmolality.  RBC deformation is defined by the Elongation Index (EI) calculated from the shape of the laser diffraction pattern. 

The maximal deformability of normal RBC is found at isotonic conditions and decreases when the cell swells (decreased osmolarity) or shrink (increased osmolarity). 

The figure shows the diffraction pattern and the derived EI as a function of osmolarity. 

The curve can be described by 3 indices: 

• O_min is the osmolality at the minimum EI point on the curve in the hypotonic range (below 290 mosmol) and correlates to an osmolality at which 50% of RBCs would hemolyze in the conventional Osmotic Fragility test.
• EI_max is the maximal EI around isotonicity.
• O_hyp is the osmolality, in the hypertonic range at which EI is 50% of EI_max

A shift in the curve is related to RBC membrane abnormalities such as hereditary spherocytosis, elliptocytosis or ovalocytosis, as well as hemoglobinopathies including Sickle cell disease, and thalassemia.

The figure shows a link between the shift in osmotic deformability and clinical severity of the sickle cell patient.  The goal of any clincal intervention is a move towards normal.