Health Encyclopedia

Osmotic fragility

Definition

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

How the test is performed

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.

How to prepare for the test

No special preparation is necessary for this test.

How the test will feel

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.

Why the test is performed

This test is performed to detect hereditary spherocytosis and thalassemia.

Hereditary spherocytosis is a somewhat common disorder. In this condition, red blood cells are defective because of their round, ball-like (spherical) shape. These cells are more fragile than normal.

Round cells are more likely to break open when placed in water in which salt is gradually removed. Normal blood cells are stronger because they are indented or curved inward on both sides.

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.

Normal Values

A negative test is normal. Normal value ranges may vary slightly among different laboratories. Talk to your doctor about what your specific test results mean.

What abnormal results mean

Thalassemia
Hereditary spherocytosis

What the risks are

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

Special considerations

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.

Review Date:4/15/2008
Reviewed By:A.D.A.M. Editorial Team: David Zieve, MD, MHA, Greg Juhn, MTPW, David R. Eltz. Previously reviewed by Rita Nanda, MD, Department of Medicine, Section of Hematology/Oncology, University of Chicago Medical Center, Chicago, IL. Review provided by VeriMed Healthcare Network (4/27/2007).

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