What is osmotic fragility all about?
Hereditary spherocytosis is a genetically inherited condition in which the RBCs have an abnormal spherical shape. This abnormal shape leads to increased RBC destruction by the spleen, resulting in chronic anemia. In addition, the abnormal shape causes the RBCs to be particularly sensitive to changes in low plasma osmolality. The test for osmotic fragility places RBCs in increasingly hypotonic saline solutions. Spherocytes will exhibit increased fragility (they will lyse sooner or hemolyze) compared to normal RBCs. So how do you test for this increased fragility? Heparinized blood is added to commercially prepared (or manually prepared) hypotonic saline solutions. Through a process, the supernatant of this solution is is measured at a wavelength of 540nm (hemoglobin’s) after being “zeroed” with distilled water. The percent hemolysis for each tube is calculated using the following formula: % hemolysis = A(test) – A (o.85% tube) divided by A (0.00% tube) – A (o.
