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Osmotic Fragility Test

Osmotic fragility measures red blood cell (RBC) resistance to hemolysis when exposed to a series of increasingly dilute saline solutions. The sooner hemolysis occurs, the greater the osmotic fragility of the cells.

Purpose

• To aid diagnosis of hereditary spherocytosis
• To supplement a stained cell examination to detect morphologic RBC abnormalities

Patient preparation

• Explain to the patient that this test is used to identify the cause of anemia.
• Tell him that a blood sample will be taken. Explain who will perform the venipuncture and when.
• Reassure him that drawing a blood sample will take less than 3 minutes.
• Explain that he may feel slight discomfort from the tourniquet pressure and the needle puncture.
• Inform him that food or fluids need not be restricted before the test.

Procedure and posttest care

• Perform a venipuncture, collecting the sample in a 7-ml green-top (heparinized) tube, or secure a special heparinized tube for collecting defibrinated blood
• If a hematoma develops at the venipuncture site, apply warm soaks.

Precautions

• Because this test isn't routinely performed, notify the laboratory before drawing the sample.
• Completely fill the collection tube and invert it gently several times to mix the sample and anticoagulant thoroughly.
• Handle the sample gently to prevent accidental hemolysis.
• In some cases, RBCs don't hemolyze immediately; incubation in solution for 141 hours improves test sensitivity.

Reference values

Osmotic fragility values (percentage of RBCs hemolyzed) are determined by the tonicity of the saline. Reference values for the different tonicities are as follows:

• - Males: 0.5% to 24.7% hemolysis
• - Females: 0% to 23.1% hemolysis

0.6 g/dl NaCI solution (incubated)

• - Males: 18% to 55.2% hemolysis
• - Females: 2.2% to 59.3% hemolysis

0.65 g/dl NaCI solution (incubated)

• - Males: 4% to 24.8% hemolysis
• - Females: 0.5% to 28.9% hemolysis.

0.75 g/dl NaCI solution (incubated)

• - Males: 0.5% to 8.5% hemolysis
• - Females: 0.1% to 9.3% hemolysis

Abnormal findings

Low osmotic fragility (increased resistance to hemolysis) is characteristic of thalassemia, iron deficiency anemia, sickle cell anemia, and other red cell disorders in which codocytes (target cells) and leptocytes are found. Low osmotic fragility also occurs after splenectomy.

High osmotic fragility (increased tendency to hemolysis) occurs in hereditary spherocytosis, in spherocytosis associated with autoinunune hemolytic anemia, severe burns, or chemical poisoning, or in hemolytic disease of the newborn (erythroblastosis fetalis).

Interfering factors

• Failure to fill the tube completely, to use the proper anticoagulant, or to adequately mix the sample and the anticoagulant
• Hemolysis due to rough handling of the sample
• Presence of hemolytic organisms in the sample
• Severe anemia or other conditions with fewer RBCs available for testing . Recent blood transfusion

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