This document describes the osmotic fragility test, which determines the resistance of red blood cells to hemolysis from osmotic stress. Red blood cells are exposed to decreasing concentrations of saline solutions, and hemolysis is measured colorimetrically. The results are plotted as a sigmoidal curve showing the percentage of hemolysis against saline concentration. This curve indicates the varying resistance of red blood cells to hypotonic solutions depending on their age. The osmotic fragility test is used as a diagnostic tool for red blood cell membrane disorders and research into hematology. The test results are affected by the relative volumes of blood and saline used, the pH, and the temperature during testing.