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Osmotic Fragility of Red Blood Cells (Unincubated And Incubated)

The normal red blood cell is a relatively impermeable biconcave disc which maintains osmotic equilibrium with the surrounding medium. As the surrounding medium becomes hypotonic, fluid will be taken into the cell to maintain stability. Eventually under very hypotonic conditions the cell will fill to capacity and rupture. Spherocytic red blood cells have a decreased capacity to expand, and will rupture in mildly hypotonic conditions that fail to lyse normal red cells. They thus exhibit increased osmotic fragility.

Osmotic fragility is determined by measuring the degree of hemolysis in hypotonic saline solution.With the unincubated test, red cell osmotic fragility is considered to be increased if hemolysis occurs in a sodium chloride concentration > 0.5%. Although increased osmotic fragility is characteristically associated with hereditary spherocytosis, it may also be increased in other types of hemolytic anemia associated with the presence of spherocytes, particularly auto-immune hemolytic anemia, and in hypernatermia (Na > 155 meq/L).

Often the increased osmotic fragility in spherocytosis is limited to a small fraction of cells that are unusually susceptible to lysis. After incubation, the defect is magnified, and a striking increase in fragility will be seen in hereditary spherocytosis.

Osmotic fragility is considered to be decreased if hemolysis is not complete in a 0.30% NaCl solution. Decreased osmotic fragility is associated with chronic liver disease, iron deficiency anemia, thalassemia, hyponatremia (Na < 130 meq/L), polycythemia vera, and sickle cell anemia after splenectomy.

Specimen requirement is one 10 mL green top (sodium heparin) tube.

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