The cellular geometry, that is, the biconcave disc shape of red cells, is critical for the cells' survival. This cell surface shape provides a high ratio of surface area to cellular volume. The normal volume of the erythrocyte is approximately 90 m3. The minimum surface area that could encase this volume is a sphere of approximately 98 m3. The surface area of a biconcave disc enclosing this volume is approximately 140 m3. Thus, shape alone provides the red cell with a considerable amount of redundant membrane and cytoskeleton. This feature provides the extra membrane surface area needed when red cells swell. More importantly, this geometric arrangement allows red cells to stretch as they undergo deformation and distortion in response to the mechanical stress of the circulation. The consequent reduction in tolerance of these cells to osmotic stress explains why anaemias resulting from membrane defects often are accompanied by osmotic fragility, the basis for the clinical laboratory test. Similarly, if erythrocytes are engorged with water, they become macrospherocytic and less deformable (Dacie, J. V., Lewis, S. M., and Luzzatto, L., 1981).
Red Cell Membrane Permeability: The normal red cell membrane is nearly impermeable to monovalent and divalent cations, thereby maintaining a high potassium, low sodium, and very low calcium content. In contrast, the red cell is highly permeable to water and anions, which are readily exchanged. As a result, erythrocytes behave as nearly perfect osmometers. Water and ion transport pathways in the red cell membrane include energy-driven membrane pumps, gradient-driven systems, and various channels. An important feature of the normal red cell is its ability to maintain a constant volume. The mechanisms by which red cells "sense" changes in cell volume and activate appropriate volume regulatory pathways are unknown. The effects of disruption of the red cell permeability barrier are illustrated by complement-mediated hemolysis. Complement activation on the red cell surface leads to formation of the membrane attack complex, which is composed of terminal complement components embedded in the lipid bilayer. This multimolecular complex acts as a cation channel, allowing passive movements of sodium, potassium, and calcium across the membrane according to their concentration gradients. Attracted by fixed anions, such as hemoglobin, ATP, and 2,3-BPG, sodium accumulates in the cell in excess of potassium loss and of the compensatory efforts of the Na+-K+ pump. The resulting increase in intracellular monovalent cations and water is followed by cell swelling and ultimately colloid osmotic hemolysis (Dacie, J. V., Lewis, S. M., and Luzzatto, L., 1981).
Rationale of the Test: Osmotic activity in the red cells is tested by adding increasingly hypotonic concentrations of saline solution to red cells. As a result of osmosis, more and more water from the increasingly hypotonic solution will enter the red cells leading to increased volume of red cells by swelling. If the concentration goes beyond threshold, more water will enter into the cells which already are at maximum volume for surface area, and will burst at the most hypotonic normal saline concentrations. However, after incubation at 37C (98.6F) for 30 mins, these red cells will lose membrane surface area more readily than normal because their