The osmotic fragility test determines the ability of the RCB membrane to resist rupturing in a hypotonic saline solution. Normal disk-shaped cells can imbibe water and swell significantly before membrane capacity is exceeded, but spherocytes (RBCs that lack the normal biconcave shape) and cells with damaged membranes burst in saline solutions only slightly less concentrated than normal saline. Conversely, in thalassemia, sickle cell disease, and other disorders, RBCs are more than normally resistant to osmotic damage (Table 1-9).

The test is performed by exposing RBCs to increasingly dilute saline solutions. The percentage of the solution at which the cells swell and rupture is then noted.

• Normal erythrocytes rupture in saline solutions of 0.30 to 0.45 percent. RBC rupture in solutions of greater than 0.50 percent saline indicates increased fragility. Lack of rupture in solutions of less than 0.30 percent saline indicates decreased RBC fragility.

• Confirmation of disorders that alter RBC fragility, including hereditary anemias (see Table 1-9)
• Evaluation of the extent of extrinsic damage to RBCs from burns, inadvertent instillation of hypotonic intravenous fluids, microorganisms, and excessive exercise

Nursing Care Before the Procedure

Client preparation is the same as that for any study involving the collection of a peripheral blood sample (see Appendix I).

A venipuncture is performed and the sample collected in a green-topped tube. A capillary sample may be obtained in infants and children as well as in adults for whom a venipuncture may not be feasible.

Nursing Care After the Procedure

Care and assessment after the procedure are the same as for any study involving the collection of a peripheral blood sample (see Appendix I).

• Abnormal test results, complications, and precautions: Respond as for any laboratory analysis to determine RBC abnormalities leading to anemia.