a test that measures the resistance of erythrocytes to hemolysis in hypotonic saline solutions; erythrocytes to be tested are added to varying concentrations of saline (usually ranging from 0.85-0.10% sodium chloride with 0.05% increments), and beginning and complete hemolysis are measured; normal erythrocytes show initial hemolysis at concentrations of 0.45-0.39% and complete hemolysis at 0.33-0.30%; in hereditary spherocytosis, the fragility of the erythrocytes is markedly increased, whereas in thalassemia, sickle cell anemia, and obstructive jaundice the fragility of the erythrocytes is usually reduced. SYN: erythrocyte fragility test.