- Osmotic fragility test assesses the ability of RBCs to withstand hypotonic saline without bursting
- Normal red cells can remain suspended in normal saline (0.9% NaCl soln.) for hours without rupturing or any change in the size or shape. But when they are placed in decreasing strengths of hypotonic saline, they imbibe water (due to osmosis) and finally burst.
- The ability of RBCs to resist this type of hemolysis can be determined quantitatively.
Procedure
- Number the test tubes from 1 to 12 with l and place in the rack
- Using the glass dropper, place the varying number of drops of 1% saline and distilled water n each of the 12 test tubes as shown in Table
- Mix the contents of each test tube
- Tube l contains normal saline, which is isotonic with plasma
- Tube 12 contains only distilled water which has no tonicity
- Gently add one drop of blood into each of the 12 tubes
- Leave the test tubes undisturbed for one hour
- Observe the extent of hemolysis in each tube by holding the rack at eye level
Report
- When RBCs are suspended in varying strengths of saline Hemolysis begins in 0.48% saline (Tube 6) and is completed at 0.36% saline (Tube 9)
- No hemolysis in solutions of 0.5% saline and above
- Hemolysis complete in Tubes 10, 11, and 12
Question & Answers
Q1. Define osmotic fragility. What is the normal range?
A1. Osmotic fragility of red ells is defined as the ease
with which the RBC’s are ruptured (hemolysed) when they are exposed to
hypotonic solution. It assess the integrity of membrane of red cells
Q2. What are the conditions in which there is alteration in
osmotic fragility of red cells?
A2. Conditions that alter fragility of RBCs
Diminished fragility
- Iron deficiency anaemia
- Thalassemia
- Sicle cell anaemia
- Obstructive jaundice
- After spleenectomy
- Variety of anaemias where target cells are seen in the peripheral blood
Increased fragility
- Heriditary spherocytosis
- Congenital hemolytic anemia
- Other condition in which spherocytes are found in blood
Q3. What is the physiological basis of the increased
fragility of RBCs in hereditary spherocytosis
A3. Hereditary spherocytosis is a disorder of the surface,
i.e. the membrane of RBCs. It causes RBCs to be shaped like spheres instead of
flattened discs that are curved inward.
Spherocytic red blood cells have a decreased capacity to
expand and will rupture in mildly hypotonic conditions which fail to lyse
normal red blood cells.
Hence they exhibit increased osmotic fragility.
Osmotirc fragility is determined by measuring the degree of
hemolysis in hypotonic saline solution