Erythrocyte fragility refers to the propensity of erythrocytes (red blood cells, RBC) to hemolyse (rupture) under stress. It can be thought of as the degree or proportion of hemolysis that occurs when a sample of red blood cells are subjected to stress (typically physical stress, and most commonly osmotic and/or mechanical stress). Depending on the application as well as the kind of fragility involved, the amount of stress applied and/or the significance of the resultant hemolysis may vary.
When multiple levels of stress are applied to a given population/sample of cells, a fragility profile can be obtained by measuring the relative or absolute extent of hemolysis existing at each such level, in addition to finding one or more single-number indexes (either measured directly or interpolated) associated with particular respective levels of hemolysis and/or corresponding stress. Fragility testing can be useful to assess cells' ability (or lack thereof) to withstand sustained or repeated stress. Moreover, it can be used to assess how fragility itself varies under different or changing environmental or stress conditions, during or prior to the inducement of the hemolysis. Low fragility is often termed "stability," though technically stability refers to cells' resistance to both stress-induced lysis and spontaneous auto-lysis.