Type of test Blood

Normal findings

Hemolysis begins at 0.5% NaCl.

Hemolysis complete at 0.3% NaCl.

Test explanation and related physiology

 Red blood cells (RBCs) are bound by a membrane that allows water to pass through while generally restricting the solutes. This process, called osmosis, causes RBCs to absorb water when in a hypotonic medium. This results in swelling and, ultimately, hemolysis when the cell bursts. The osmotic fragility (OF) test uses this fact to determine the concentration of solute inside the cell by subjecting it to salt solutions of different concentrations. The ability of the normal RBC to withstand hypotonicity results from its biconcave shape, which allows the cell to increase its volume by 70% before the surface membrane is stretched. When this limit is reached, lysis occurs. When intravascular hemolysis is identified, OF is used to determine whether the RBCs have increased fragility (tend to burst open when exposed to a higher concentrated NaCl solution) or decreased fragility (tend to burst open in lower concentrated, and thus more hypotonic, NaCl solution).

This test is performed to detect hereditary spherocytosis and thalassemia when intravascular hemolysis is identified. Round cells (spherocytes) have increased OF compared with normal indented RBCs. In hereditary spherocytosis, there is abnormal morphology because of a lack of spectrin, a key RBC cytoskeletal membrane protein. This produces membrane instability, which forces the cell to the smallest volume—that of a sphere. This common disorder is associated with intravascular hemolysis. This is shown by increased osmotic fragility. Thalassemia, on the other hand, is associated with thinner leptocytes whose OF is decreased.

Interfering factors

 • Acute hemolysis. The osmotically labile cells are already hemolyzed and therefore are not found in the blood specimen.

* Dapsone can increase OF.

Procedure and patient care

• See inside front cover for Routine Blood Testing.

• Fasting: no

 • Blood tube commonly used: green

 Abnormal findings

 Increased erythrocyte

- Acquired hemolytic anemia

- Hereditary spherocytosis

- Hemolytic disease of the newborn

- Pyruvate kinase deficiency

- Malaria

Decreased erythrocyte fragility

- Thalassemia

- Hemoglobinopathies (C and S disease)

- Iron deficiency anemia

- Reticulocytosis

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