Osmotic fragility is a test to detect red blood cells that are more fragile than normal.

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 an elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes the vein 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 normal blood flow. 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 small glass tube (pipette), 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.

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 somewhat common disorder in which red blood cells are defective because of their round, ball-like (spherical) shape. These cells are more fragile than normal.

Spherical cells are said to have increased osmotic fragility because they are less likely to expand and break open in saltier water than normal red blood cells (which are indented or curved inward on both sides). Cells that are flatter than normal are more likely to expand, and thus have decreased osmotic fragility.

Thalassemia is an inherited condition that affects the portion of blood (hemoglobin) that carries oxygen. Some red blood cells are more fragile than normal, but a larger number are less fragile than normal.

A negative test is normal.

• Thalassemia
• Hereditary spherocytosis

• Excessive bleeding
• Fainting or feeling light-headed
• Blood accumulating under the skin (hematoma)
• Infection
• Multiple punctures to find veins

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