Spherocytes of any origin (including conditions other than hereditary spherocytosis) are more susceptible than normal RBCs to hemolysis in dilute (hypotonic) saline and show increased osmotic fragility. Generally, fully expanded cells (spheroidal cells or spherocytes) have increased osmotic fragility, whereas cells with higher surface area-to-volume ratios (e.g., thin cells, hypochromic cells, tart cells) have decreased osmotic fragility.
In hereditary spherocytosis, the osmotic fragility test may be normal initially. Therefore, the test is incubated at 37 °C for 24 hours, at which time the test is positive for hereditary spherocytosis.
Immediate test:
Hemolysis begins at 0.5% NaCl.
Hemolysis completes at 0.3% NaCl.
24-hour incubation:
Hemolysis begins at 0.7% NaCl.
Hemolysis completes at 0.4% NaCl.
Obtain a 7-mL venous blood sample using a green-topped tube (heparin) or lavender-topped tube (EDTA). Label the specimen with the patients name, date and time of collection, and test(s) ordered. Place the specimen in a biohazard bag.
Expose erythrocytes to varying dilutions of sodium chloride. Read hemolysis on a spectrophotometer (optical density measurement). Perform studies and measure both before and after 24-hour incubation of the RBCs.
Increased osmotic fragility is found in:
Hemolytic anemia (acquired immune)
Hereditary spherocytosis (stomatocytosis)
Hemolytic disease of the newborn
Malaria
Severe PK deficiency
Chronic lymphocytic leukemia
Decreased osmotic fragility occurs in:
Iron-deficiency anemia (macrocytic hypochromic)
Thalassemias
Asplenia (postsplenectomy)
Liver disease (obstructive jaundice)
Reticulocytosis
Hemoglobinopathies, especially Hb C, Hb S
Pretest Patient Care
Explain test purpose and procedure. There should be no exercising before tests.
Withhold transfusion until after blood samples are drawn (especially with osmotic fragility).
Follow guidelines in Chapter 1 for safe, effective, informed pretest care.
Posttest Patient Care
Follow guidelines in Chapter 1 for safe, effective, informed posttest care.
Be aware that the usual treatment for hereditary spherocytosis is splenectomy, which removes the agent of RBC destruction and prevents complications such as aplastic anemia.