Osmotic fragility is a test to detect abnormal fragility of red blood cells.

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and aan elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins to fill with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

For an infant or young child:

The area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

In the laboratory, red blood cells are tested with a solution that makes them swell in order to determine how fragile they are (see special considerations).

No special preparation is necessary for this test.

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

This test is performed to detect hereditary spherocytosis and thalassemia.

Hereditary spherocytosis is a relatively common disorder characterized by red blood cells that are intrinsically defective because of their sphere-like shape. These cells have increased osmotic fragility -- they are more fragile than normal.

In thalassemia, some red blood cells are more fragile than normal, but a larger fraction are less fragile than normal.

A negative test is normal.

• Thalassemia
• Hereditary spherocytosis

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)
• Multiple punctures to locate veins

Cells that are spherical have a limited capacity to expand and can withstand saltier water without rupturing than can normal red cells (which are indented or concave on both sides). Spherical cells are said to have increased osmotic fragility. Conversely, cells that are flatter than normal have a greater capacity to expand and thus have decreased osmotic fragility.

Veins and arteries vary in size from one patient to another, and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.