osmotic fragility, an assay in which whole blood is pipetted to each of a series of saline solutions of graduated concentration. The series begins with water and increases in concentration to normal (0.85%) saline. The osmotic fragility test is used to detect spherocytosis because spherocytes rupture (lyse) in saline concentrations near the normal level. The assay also detects target cells, which, owing to their reduced hemoglobin content, are able to withstand osmotic stress and rupture only at very dilute saline concentrations. Also called erythrocyte fragility.
