Osmotic fragility |
| Definition: |
| Osmotic fragility is a test to detect abnormal fragility of red blood cells. |
| How the test is performed: |
| Adult or child: Blood is drawn from a vein (venipuncture), usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet (an elastic band) or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to distend (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 tourniquet 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. 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 by exposure to hypotonic (low ionic concentration) solutions, making them swell, in order to determine the relative fragility of the cells (see special considerations). |
| How to prepare for the test: |
| Adult: No special preparation is necessary for this test. Infants and children: The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age:
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| 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 relatively common disorder characterized by red blood cells which are intrinsically defective because of their sphere-like shape. These cells have increased osmotic fragility. Red blood cells from people with thalassemia show some cells with increased osmotic fragility, but a larger fraction with decreased fragility. |
| Normal Values: |
| A negative test is normal. |
| What abnormal results mean: |
| What the risks are: |
| Special considerations: |
| Cells that are more spherical, with a decreased surface/volume ratio, have a limited capacity to expand in low osmolality solutions and lyse (rupture) at a higher concentration of sodium chloride than do normal biconcave red cells. These spherical cells are said to have increased osmotic fragility. Conversely, cells that are hypochromic and flatter 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. |
Reviewed By: Rebecca Elstrom, M.D., Division of Hematology-Oncology, University of Pennsylvania Medical Center, Philadelphia, PA. Review provided by VeriMed Healthcare Network.
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