Osmotic fragility test

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

Overview

The osmotic fragility test measures the ability of red blood cells to maintain integrity under osmotic stress. Red blood cells are placed in solutions of decreasing salinity (or osmolarity), starting from isotonic (similar to plasma) to increasingly hypotonic (less saline than plasma). Healthy red blood cells can swell in less saline solutions up to a point without bursting, due to their elastic cell membranes. However, cells that are more fragile may hemolyze (burst) at higher concentrations of saline than normal, indicating increased osmotic fragility.

Procedure

The procedure involves mixing a small sample of the patient's blood with a series of saline solutions of decreasing concentration. After incubation, the samples are centrifuged, and the extent of hemolysis is assessed. The degree of red blood cell destruction is measured either visually or using a spectrophotometer to determine the concentration of hemoglobin in the supernatant. The results are plotted on a graph, with the saline concentration on the x-axis and the percentage of hemolysis on the y-axis. A curve is then drawn to determine the osmotic fragility of the red blood cells.

Clinical Significance

Increased osmotic fragility is observed in conditions where the red blood cells are inherently weaker or more prone to rupture. This is characteristic of:

• Hereditary spherocytosis: A genetic disorder causing red blood cells to be sphere-shaped rather than the typical donut shape, making them more susceptible to osmotic stress.
• Certain types of thalassemia: A group of inherited blood disorders that affect the production of hemoglobin, leading to fragile red blood cells.

Decreased osmotic fragility, though less common, can occur in conditions such as:

• Sickle cell anemia: A genetic disorder where red blood cells are shaped like sickles, which may be less prone to osmotic lysis in certain conditions.

Limitations

While the osmotic fragility test is useful in diagnosing certain conditions, it is not a definitive test for all types of hemolytic anemia. Other diagnostic tests and clinical evaluations are necessary to accurately diagnose and manage hemolytic disorders.

See Also

• Hemolytic anemia
• Red blood cell indices
• Complete blood count