(A) Osmotic deformability profile of normal human red blood cells showing deformability index (DI) as a continuous function of suspending medium osmolality. The maximum DI, which is attained in near isotonic osmolality, is a measure of membrane surface area. Omin in the hypotonic arm corresponds to osmolality in which 50% of the red cells lyse in an osmotic fragility test, and O′ in the hypertonic arm corresponds to the state of cell hydration and hence MCHC. (B) Osmotic deformability profile of 3 patients with HS. Decreased maximum DI is the consequence of decreased membrane surface area, increase in Omin is the result of increased osmotic fragility, and decreased O′ reflects cell dehydration.