Point of Sickling (POS):

RBC  Deformability under different oxygen tensions 

When sickle blood is exposed to lower oxygen tensions, hemoglobin will polymerize, starting the “sickling” process. The presence of hemoglobin polymers in the cell will decrease the ability of the cell to deform. In the ektacytometer this will result in a decrease of the elongation index (EI).  The figure shows an example of blood from a sickle cell patient exposed to different oxygen tensions (PO2, mmHg) under a 30Pa shear stress at isotonicity (290 mosmol). The oxygen tension at which the EI drops to 95% of the original EI is defined as the point pf sickling (POS). A decrease in POS relates to a lower propensity to form hemoglobin polymers and an improvement (lowering) of sickling kinetics.