Osmotic fragility test is used to identify disorders of red cell membrane. It is widely used to diagnose hereditary spherocytosis. Hereditary spherocytosis is a disease where the red cells become spherical in shape. This is due to abnormalities in the red cell membrane proteins.
Osmotic fragility test is designed according to the principle of osmosis. The osmolarity of human blood is around 300 mOsms. The osmolarity of red blood cell plasma is also the same. In this test, red blood cells are added in to a series of solutions with decreasing osmolarity. The first solution contains 0.9% sodium chloride, which is isotonic to the red cell plasma osmolarity. The last test tube contains distill water of which the osmolarity is 0. On the middle is 0.45% sodium chloride solution. Equal amounts of blood is put in to each of these test tube and kept for about an hour. Then the test tubes are centrifuged and observed.
In a normal sample of blood, almost all the red cells are destroyed at the concentration of 0.3%. This occurs as a result of osmosis. When the red cells are put in to a hypotonic solution, water molecules moves into the cell causing it to swell. At the end, the cell bursts. Almost half of the red cells are destroyed when they are inside 0.45% sodium chloride. When centrifuged, the test tube with 0.9% sodium chloride and blood will have all the red cells deposited at the bottom of the tube and the solution will be clear. The tube with distill water will have no deposition because all the red cells are destroyed. It will have a homogeneously reddish solution.
To diagnose hereditary spherocytosis, the osmotic fragility of patients blood is compared with that of a normal age and sex matched individual. As the red cells of the patients are more fragile, they tend to burst early. The diagnosis can be done if this is observed.
