Normal red cells show 50% haemolysis in a NaCl concentration of 0.40-0.445%, the median corpuscular fragility (MCF), and complete haemolysis in NaCl concentration of 0.20%. Hypochromic or thalassaemic cells show a greater resistance to hypotonicity, a decrease in osmotic fragility. Spheroid cells show less resistance to hypotonicity, an increase in osmotic fragility. After incubation at 37ºC for 24 hours, the MCF of normal red cells increases to 0.465-0.59% NaCl. The osmotic fragility of abnormal red cells may be reduced or abnormally increased after incubation at 37ºC.

The test reflects the ability of red cells to take up water without lysis. Red cells are placed in graded dilutions of NaCl solution; swelling and haemolysis in the lower dilutions provide an index of the resistance of the cells in hypotonic saline. Percent haemolysis is determined using optical density measurements

Lorne Osmotest Kit consists of two sets of 12 tubes containing buffered solutions of various NaCl concentrations. All the reagents are supplied at the optimal dilution for use with all the recommended techniques stated below without the need for further dilution or addition.