Osmotic fragility tests are based on the measure of red blood cell lysis as a function of osmotic
stress. When erythrocytes are placed in hypotonic solutions, they begin to take on water
osmotically. This results in swelling of the cell until the critical volume is reached, afterward the
membrane at first leaks and then bursts releasing hemoglobin. The susceptibility to osmotic lysis is
primarily determined by the surface area to volume ratio of erythrocytes and increased osmotic
fragility typically occurs in hereditary spherocytosis. Because of their reduced surface membrane
area respect to the cell volume, spherocytes cannot expand as much as normal discoid erythrocytes
and result osmotically more fragile. Increased osmotic fragility is also seen in acquired causes of
spherocytosis such as autoimmune hemolytic anemia. On the contrary, increased resistance to
hemolysis is characteristic of thalassemia, in both the homozygous and heterozygous forms, in iron
deficiency and in any other condition in which an increase of the surface area to volume ratio of
the red cells is present.