Osmotic fragility

Osmotic Fragility Test

The Osmotic Fragility Test is a diagnostic procedure used in hematology to assess the red blood cell's resistance to hemolysis when exposed to varying concentrations of hypotonic saline solutions. This test is significant in diagnosing conditions that affect the durability of red blood cells, such as hereditary spherocytosis, thalassemia, and other hemolytic anemias.

Overview

Osmotic fragility refers to the susceptibility of red blood cells to rupture (hemolysis) when placed in a hypotonic solution, which is a solution with a lower concentration of solutes compared to the cell's interior. Red blood cells in such an environment absorb water, swell, and can eventually burst if the membrane's elasticity is compromised. The osmotic fragility test measures this phenomenon by subjecting the cells to increasingly dilute saline solutions and determining the concentration at which hemolysis begins and the concentration at which it is complete.

Indications

The test is primarily indicated for patients suspected of having hereditary spherocytosis, a condition characterized by the production of spherical (rather than the normal biconcave) red blood cells that are more prone to hemolysis. It may also be used to evaluate other conditions that lead to increased red blood cell fragility, including:

• Autoimmune hemolytic anemia
• Thalassemia
• Liver disease
• Vitamin E deficiency

Procedure

The osmotic fragility test involves mixing the patient's blood with solutions of sodium chloride (NaCl) at decreasing concentrations. The test starts with a solution concentration similar to that of the body (isotonic), gradually decreasing to hypotonic solutions. After incubation, the degree of hemolysis is assessed. The point at which red blood cells begin to hemolyze (minimum osmotic fragility) and the point at which all cells have hemolyzed (maximum osmotic fragility) are determined.

Interpretation

• Normal Results: Hemolysis begins at 0.45% NaCl concentration and is complete at 0.30% NaCl.
• Increased Osmotic Fragility: Hemolysis starts at higher NaCl concentrations than normal. This is typically seen in hereditary spherocytosis.
• Decreased Osmotic Fragility: Hemolysis begins at lower NaCl concentrations. This can occur in conditions such as sickle cell anemia and thalassemia.

Clinical Significance

The osmotic fragility test is a valuable tool in the diagnosis and differentiation of various types of hemolytic anemia. Increased osmotic fragility is indicative of hereditary spherocytosis, while decreased fragility can point towards conditions like thalassemia or sickle cell anemia. However, it is important to note that the test is not specific and should be interpreted in conjunction with other clinical findings and laboratory tests.

Limitations

While the osmotic fragility test is useful, it has limitations. It may not be definitive in all cases of hereditary spherocytosis, and false positives can occur. Other diagnostic tests, such as the Eosin-5'-maleimide (EMA) binding test, may be required for a conclusive diagnosis.